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Jelly-Fish, Star-Fish, and Sea-Urchins: Being a Research on Primitive Nervous Systems

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Title: Jelly-Fish, Star-Fish, and Sea-Urchins: Being a Research on Primitive Nervous Systems

Author: George John Romanes


        
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    THE INTERNATIONAL SCIENTIFIC SERIES.

    VOLUME XLIX.




    THE INTERNATIONAL SCIENTIFIC SERIES.

    JELLY-FISH, STAR-FISH

    AND SEA-URCHINS

    BEING

    _A RESEARCH ON PRIMITIVE NERVOUS SYSTEMS_

    BY

    G. J. ROMANES, M.A., LL.D., F.R.S.

    ZOÖLOGICAL SECRETARY OF THE LINNEAN SOCIETY

    NEW YORK

    D. APPLETON AND COMPANY

    72 FIFTH AVENUE

    1898




PREFACE.


When I first accepted the invitation of the editors of the
International Scientific Series to supply a book upon Primitive
Nervous Systems, I intended to have supplemented the description of
my own work on the physiology of the _Medusæ_ and _Echinodermata_
with a tolerably full exposition of the results which have been
obtained by other inquirers concerning the morphology and development
of these animals. But it soon became apparent that it would be
impossible, within the limits assigned to me, to do justice to the
more important investigations upon these matters; and therefore I
eventually decided upon restricting this essay to an account of my
own researches.

With the exception of a few woodcuts in the last chapter (for the
loan of which I am indebted to the kindness of Messrs. Cassell),
all the illustrations are either original or copies of those in my
Royal Society papers. In the letter-press also I have not scrupled
to draw upon these papers, wherever it seemed to me that the
passages would be sufficiently intelligible to a general reader. I
may observe, however, that although I have throughout kept in view
the requirements of a general reader, I have also sought to render
the book of service to the working physiologist, by bringing together
in one consecutive account all the more important observations and
results which have been yielded by this research.

    G. J. R.

   LONDON, 1884.




CONTENTS.


    CHAPTER                                                PAGE

    INTRODUCTION                                              1

       I. STRUCTURE OF THE MEDUSÆ                            10

      II. FUNDAMENTAL EXPERIMENTS                            26

     III. EXPERIMENTS IN STIMULATION                         37

      IV. EXPERIMENTS IN SECTION OF COVERED-EYED MEDUSÆ      65

       V. EXPERIMENTS IN SECTION OF NAKED-EYED MEDUSÆ       104

      VI. CO-ORDINATION                                     130

     VII. NATURAL RHYTHM                                    146

    VIII. ARTIFICIAL RHYTHM                                 175

      IX. POISONS                                           213

       X. STAR-FISH AND SEA-URCHINS                         254




JELLY-FISH, STAR-FISH, AND SEA-URCHINS.

INTRODUCTION.


Among the most beautiful, as well as the most common, of the
marine animals which are to be met with upon our coasts are the
jelly-fish and the star-fish. Scarcely any one is so devoid of the
instincts either of the artist or of the naturalist as not to have
watched these animals with blended emotions of the æsthetic and the
scientific--feeling the beauty while wondering at the organization.
How many of us who live for most of the year in the fog and dust
of large towns enjoy with the greater zest our summer's holiday
at the seaside? And in the memories of most of us is there not
associated with the picture of breaking waves and sea-birds floating
indifferently in the blue sky or on the water still more blue, the
thoughts of many a ramble among the weedy rocks and living pools,
where for the time being we all become naturalists, and where those
who least know what they are likely to find in their search are
most likely to approach the keen happiness of childhood? If so, the
image of the red sea-stars bespangling a mile of shining sand, or
decorating the darkness of a thousand grottoes, must be joined with
the image, no less vivid, of those crystal globes pulsating with life
and gleaming with all the colours of the rainbow, which are perhaps
the most strange, and certainly in my estimation the most delicately
lovely creatures in the world.

It is with these two kinds of creatures that the present work is
concerned, and if it seems almost impious to lay the "forced fingers
rude" of science upon living things of such exquisite beauty, let
it be remembered that our human nature is not so much out of joint
that the rational desire to know is incompatible with the emotional
impulse to admire. Speaking for myself, I can testify that my
admiration of the extreme beauty of these animals has been greatly
enhanced--or rather I should say that this extreme beauty has been,
so to speak, revealed--by the continuous and close observation which
many of my experiments required: both with the unassisted eye and
with the microscope numberless points of detail, unnoticed before,
became familiar to the mind; the forms as a whole were impressed upon
the memory; and, by constantly watching their movements and changes
of appearance, I have grown, like an artist studying a face or a
landscape, to appreciate a fulness of beauty, the _esse_ of which
is only rendered possible by the _per cipi_ of such attention as is
demanded by scientific research. Moreover, association, if not the
sole creator, is at least a most important factor of the beautiful;
and therefore the sight of one of these animals is now much more to
me, in the respects which we are considering, than it can be to any
one in whose memory it is not connected with many days of that purest
form of enjoyment which can only be experienced in the pursuit of
science.

And here I may observe that the worker in marine zoology has one
great advantage over his other scientific brethren. Apart from the
intrinsic beauty of most of the creatures with which he has to deal,
all the accompaniments of his work are æsthetic, and removed from
those more or less offensive features which are so often necessarily
incidental to the study of anatomy and physiology in the higher
animals. When, for instance, I contrast my own work in a town
laboratory on vertebrated animals with that which I am now about
to describe upon the invertebrated in a laboratory set up upon the
sea-beach, it is impossible not to feel that the contrast in point
of enjoyment is considerable. In the latter case, a summer's work
resembles the pleasure-making of a picnic prolonged for months, with
the sense of feeling all the while that no time is being profitlessly
spent. Whether one is sailing about upon the sunny sea, fishing with
muslin nets for the surface fauna, or steaming away far from shore to
dredge for other material, or, again, carrying on observations in the
cool sea-water tanks and bell-jars of a neat little wooden workshop
thrown open to the sea-breezes, it alike requires some effort to
persuade one's self that the occupation is really something more than
that of finding amusement.

It is now twelve years since I first took to this kind of summer
recreation, and during that time most of my attention while at the
seaside has been devoted to the two classes of animals already
mentioned--viz. the jelly-fish and star-fish, or, as naturalists have
named them, the Medusæ and Echinodermata. The present volume contains
a tolerably full account of the results which during six of these
summers I have succeeded in obtaining. If any of my readers should
think that the harvest appears to be a small one in relation to the
time and labour spent in gathering it, I shall feel pretty confident
that those readers are not themselves working physiologists, and,
therefore, that they are really ignorant of the time and labour
required to devise and execute even apparently simple experiments,
to hunt down a physiological question to its only possible answer,
and to verify each step in the process of an experimental proof.
Moreover, the difficulties in all these respects are increased
tenfold in a seaside laboratory without adequate equipments or
attendance, and where, in consequence, more time is usually lost in
devising makeshifts for apparatus, and teaching unskilled hands how
to help, than is consumed in all other parts of a research. From the
picnic point of view, however, there is no real loss in this; such
incidental difficulties add to the enjoyment (else why choose to
make an extemporized grate and boil a kettle in the wood, when a
much more efficient grate, full of lighted coals, is already boiling
some other kettle at home?); and if they somewhat unduly prolong a
research, the full meaning of life is, after all, not exhausted by
the experiences of a mill-horse, and it is well to remember that so
soon as we cease to take pleasure in our work, we are most likely
sacrificing one part of our humanity to the altar of some other, and
probably less worthy, constituent.

I may now say a few words on the scope of the investigations which
are to be described in the present treatise. To some extent this is
conveyed by the title; but I may observe that, as the "primitive
nervous systems" whose physiology I have sought to advance are mainly
subservient to the office of locomotion, in my Royal Society papers
upon these researches I have adopted the title of "Observations on
the Locomotor System" of each of the classes of animals in question.
It is of interest to notice in this connection that the plan or
mechanism of locomotion is completely different in the two classes,
and that in the case of each class the plan or mechanism is unique,
_i.e._ is not to be met with elsewhere in the animal kingdom. It is
curious, however, that, in the case of one family of star-fish (the
_Comatulæ_), owing to an extreme modification of form and function
presented by the constituent parts of the locomotor organs, the
method of progression has come closely to resemble that which is
characteristic of jelly-fish.

There is still one preliminary topic on which I feel that it is
desirable to touch before proceeding to give an account of my
experiments, and this has reference to the vivisection which many
of these experiments have entailed. But in saying what I have to
say in this connection I can afford to be brief, inasmuch as it is
not needful to discuss the so-called vivisection question. I have
merely to make it plain that, so far as the experiments which I am
about to describe are concerned, there is not any reasonable ground
for supposing that pain can have been suffered by the animals. And
this it is easy to show; for the animals in question are so low
in the scale of life, that to suppose them capable of conscious
suffering would be in the highest degree unreasonable. Thus, for
instance, they are considerably lower in the scale of organization
than an oyster, and in none of the experiments which I have performed
upon them has so much laceration of living tissue been entailed
as that which is caused by opening an oyster and eating it alive,
after due application of pepper and vinegar. Therefore, if any one
should be foolish enough to object to my experiments on the score of
vivisection, _a fortiori_ they are bound to object to the culinary
use of oysters. Of course, it may be answered to this that two
blacks do not make a white, and that I have not by this illustration
succeeded in proving my negative. To this, however, I may in turn
reply that, for the purpose of morally justifying my experiments
on the ground which I have adopted, it is not incumbent on me to
prove any negative; it is rather for my critics to prove a positive.
That is to say, before convincing me of sin, it must be shown that
there is some reasonable ground for supposing that a jelly-fish or a
star-fish is capable of feeling pain. I submit that there is no such
ground. The mere fact that the animals are alive constitutes no such
ground; for the insectivorous plants are also alive, and exhibit even
more physiological "sensitiveness" and capability of rapid response
to stimulation than is the case with the animals which we are about
to consider. And if anyone should go so far as to object to Mr.
Darwin's experiments on these plants on account of its not being
demonstrable that the tissues did not suffer under his operations,
such a person is logically bound to go still further, and to object
on similar grounds to the horrible cruelty of skinning potatoes and
boiling them alive.

Thus, before any rational scruples can arise with regard to the
vivisection of a living organism, some reasonable ground must be
shown for supposing that the organism, besides being living, is also
capable of suffering. But no such reasonable ground can be shown in
the case of these low animals. We only know of such capability in
any case through the analogy based upon our own experience, and,
if we trust to this analogy, we must conclude that the capability
in question vanishes long before we come to animals so low in the
scale as the jelly-fish or star-fish. For within the limits of
our own organism we have direct evidence that nervous mechanisms,
much more highly elaborated than any of those which we are about
to consider, are incapable of suffering. Thus, for instance, when
the nervous continuity of the spinal cord is interrupted, so that a
stimulus applied to the lower extremities is unable to pass upwards
to the brain, the feet will be actively drawn away from a source of
irritation without the man being conscious of any pain; the lower
nervous centres in the spinal cord respond to the stimulation, but
they do so without _feeling_ the stimulus. In order to feel there
must be consciousness, and, so far as our evidence goes, it appears
that consciousness only arises when a nerve-centre attains to some
such degree of complexity and elaboration as are to be met with in
the brain. Whether or not there is a dawning consciousness in any
nerve-centres considerably lower in the scale of nervous evolution,
is a question which we cannot answer; but we may be quite certain
that, if such is the case, the consciousness which is present must
be of a commensurately dim and unsuffering kind. Consequently, even
on this positive aspect of the question, we may be quite sure that
by the time we come to the jelly-fish--where the object of the
experiments in the first instance was to obtain evidence of the very
existence of nerve-tissue--all question of pain must have vanished.
Whatever opinions, therefore, we may severally entertain on the
vexed question of vivisection as a whole, and with whatever feelings
we may regard the "blind Fury" who, in the person of the modern
physiologist, "comes with the abhorred shears and slits the thin-spun
life," we should be all agreed that in the case of these animals the
life is so very thin-spun that any suggestion of abhorrence is on the
face of it absurd.[1]

  [1] The relation of consciousness to the elaboration of
  nerve-centres throughout the animal kingdom is more fully
  considered in my work on "Mental Evolution in Animals" (Kegan
  Paul, Trench & Co.: 1883).




CHAPTER I.

STRUCTURE OF THE MEDUSÆ.


To give a full account of the morphology, development, and
classification of the Medusæ would be both unnecessary for our
present purposes and impracticable within the space which is allotted
to the present work.[2] But, for the sake of clearness in what
follows, I shall begin by briefly describing such features in the
anatomy of the jelly-fish as will afterwards be found especially to
concern us.

  [2] Those who may desire to read an excellent epitome of our
  most recent knowledge on these subjects, may refer to Professor
  E. Ray Lankester's article in the "Encyclopædia Britannica" on
  "Hydrozoa," together with Professor Haeckel's Report on the
  Medusæ of the _Challenger_ Expedition.

  [Illustration: Fig. 1.--Sarsia (natural size).]

In size, the different species of Medusæ vary from that of a small
pea to that of a large umbrella having streamers a hundred feet
long. The general form of these animals varies in different species
from that of a thimble (Fig. 1) to that of a bowl, a parasol, or a
saucer (see figures in subsequent chapters). Or we may say that the
form of the animals always resembles that of a mushroom, and that
the resemblance extends to a tolerably close imitation by different
species of the various forms which are characteristic of different
species of mushrooms, from the thimble-like kinds to the saucer-like
kinds. Moreover, this accidental resemblance to a mushroom is
increased by the presence of a central organ, occupying the position
of, and more or leas resembling in form, the stalk of a mushroom.
This organ is called the "manubrium," on account of its looking like
the "handle" of an umbrella, and the term "umbrella" is applied to
the other portion of the animal. The manubrium, like the umbrella,
varies much in size and shape in different species, as a glance
at any figures of these animals will show. Both the manubrium and
umbrella are almost entirely composed of a thick, transparent, and
non-contractile jelly; but the whole surface of the manubrium and the
whole _concave_ surface of the umbrella are overlayed by a thin layer
or sheet of contractile tissue. This tissue constitutes the earliest
appearance in the animal kingdom of true muscular fibres, and its
thickness, which is pretty uniform, is nowhere greater than that of
very thin paper.

The manubrium is the mouth and stomach of the animal, and at the
point where it is attached to or suspended from the umbrella its
central cavity opens into a tube-system, which radiates through the
lower or concave aspect of the umbrella. This tube-system, which
serves to convey digested material and may therefore be regarded as
intestinal in function, presents two different forms in the two
main groups into which the Medusæ are divided. In the "naked-eyed"
group, the tubes are unbranched and run in a straight course to the
margin of the umbrella, where they open into a common circular tube
which runs all the way round the margin (see Figs. 1 and 22). In
the "covered-eyed" group, on the other hand, the tubes are strongly
branched (see Fig. 8), although they likewise all eventually
terminate in a single circular tube. This circular or marginal tube
in both cases communicates by minute apertures with the external
medium.

The margin of the umbrella, both in the naked and covered eyed
Medusæ, supports a series of contractile tentacles, which vary
greatly in size and number in different species (see Figs. 1 and
8). The margin also supports another series of bodies which will
presently be found to be of much importance for us. These are
the so-called "marginal bodies," which vary in number, size, and
structure in different species. In all the covered-eyed species
these marginal bodies occur in the form of little bags of crystals
(therefore they are called "lithocysts"), which are protected by
curiously formed "hoods" or "covers" of gelatinous tissue; and it
is on this account that the group is called "covered-eyed," in
contradistinction to the "naked-eyed," where these little hoods or
coverings are invariably absent (compare Fig. 1 with Fig. 22), and
the crystals frequently so. In nearly all cases these marginal bodies
contain more or less brightly coloured pigments.

The question whether any nervous tissue is present in the Medusæ is
one which has long occupied the more or less arduous labours of many
naturalists. The question attracted so much investigation on account
of its being one of unusual interest in biology. Nerve-tissue had
been clearly shown to occur in all animals higher in the zoological
scale than the Medusæ, so that it was of much importance to ascertain
whether or not the first occurrence of this tissue was to be met
with in this class. But, notwithstanding the diligent application of
so much skilled labour, up to the time when my own researches began
there had been so little agreement in the results obtained by the
numerous investigators, that Professor Huxley--himself one of the
greatest authorities upon the group--thus defined the position of the
matter in his "Classification of Animals" (p. 22): "No nervous system
has yet been discovered in any of these animals."

The following is a list of the more important researches on this
topic up to the time which I have just named:--Ehrenberg, "Die
Acalephen des rothen Meeres und der Organismvs der Medusen der
Ostsee," Berlin, 1836; Kölliker, "Ueber die Randkörper der Quallen,
Polypen und Strahlthiere," Froriep's neue Notizen, bd. xxv., 1843;
Von Beneden, "Mémoire sur les Campanulaires de la côte d'Ostende,"
"Mémoires de l'Académie de Bruxelles," vol. xvii., 1843; Desor, "Sur
la Génération Medusipare des Polypes hydraires," "Annales d. Scienc.
Natur. Zool.," ser. iii. t. xii. p. 204; Krohn, "Ueber Podocoryna
carnea," "Archiv. f. Naturgeschichte," 1851, b. i.; McCrady,
"Descriptions of Oceania, etc.," "Proceedings of the Elliot Society
of Natural History," vol. i., 1859; L. Agassiz, "Contributions to the
Acaliphæ of North America," "Memoirs of the American Academy of Arts
and Sciences," vol. iii., 1860, vol. iv., 1862; Leuckart, "Archiv.
f. Naturgeschichte," Jahrg. 38, b. ii., 1872; Hensen, "Studien
über das Gehörorgan der Decapoden," "Zeitchr. f. wiss. Zool.," bd.
xiii., 1863; Semper, "Reisebericht," "Zeitschr. f. wiss. Zool.," bd.
xiii. vol. xiv.; Claus, "Bemerkungen über Clenophoren und Medusen,"
"Zeitschr. f. wiss. Zool.," bd. xiv., 1864; Allman, "Note on the
Structure of Certain Hydroid Medusæ," "Brit. Assoc. Rep.," 1867;
Fritz Müller, "Polypen und Quallen von S. Catherina," "Archiv. f.
Naturgesch.," Jahrg. 25, bd. i., 1859; also "Ueber die Randbläschen
der Hydroidquallen," "Archiv. f. Anatomie und Physiologie," 1852;
Haeckel, "Beiträge zur Naturgesch. der Hydromedusen," 1865; Eimer,
"Zoologische Untersuchungen," Würzburg, "Verhandlungen der Phys.-med.
Gesellschaft," N.F. vi. bd., 1874.

The most important of these memoirs for us to consider are the two
last. I shall subsequently consider the work of Dr. Eimer, which
up to this date was of a purely physiological character. Professor
Haeckel, who made his microscopical observations chiefly upon the
Geryonidæ, described the nervous elements as forming a continuous
circle all round the margin of the umbrella, following the course
of the radial or nutrient tubes throughout their entire length,
and proceeding also to the tentacles and marginal bodies. At the
base of each tentacle there is a ganglionic swelling, and it is from
these ganglionic swellings that the nerves just mentioned take their
origin. The most conspicuous of these nerves are those that proceed
to the radial canals and marginal bodies, while the least conspicuous
are those that proceed to the tentacles. Cells, as a rule, can
only be observed in the ganglionic swellings, where they appear as
fusiform and distinctly nucleated bodies of great transparency and
high refractive power. On the other hand, the nerves that emanate
from the ganglia are composed of a delicate and transparent tissue,
in which no cellular elements can be distinguished, but which is
longitudinally striated in a manner very suggestive of fibrillation.
Treatment with acetic acid, however, brings out distinct nuclei in
the case of the nerves that are situated in the marginal vesicles,
while in those that accompany the radial canals ganglion-cells are
sometimes met with.

A brief sketch of the contents of these and other memoirs on the
histology of the Medusæ is given by Drs. Hertwig in their more
recently published work on the nervous system and sense-organs of the
Medusæ, and these authors point to the important fact that before
the appearance of Haeckel's memoir, Leuckart was the only observer
who spoke for the fibrillar character of the so-called marginal
ring-nerve; so that in Haeckel's researches on Geryonia, whereby both
true ganglion-cells and true nerve-fibres were first demonstrated
as occurring in the Medusæ, we have a most important step in the
histology of these animals. Haeckel's results in these respects
have since been confirmed by Claus, "Grundzüge der Zoologie," 1872;
Allman, "A Monograph of the Gymnoblastic or Tubularian Hydroids,"
1871; Harting, "Notices Zoologiques," Niedlandisches "Archiv. f.
Zool.," bd. ii., Heft 3, 1873; F. E. Schulze, "Ueber den Bau von
Syncorzne Sarsii"; O. and R. Hertwig, "Das Nervensystem und die
Sinnesorgane der Medusen."

The last-named monograph is much the most important that has appeared
upon the histology of the Medusæ. I shall, therefore, give a
condensed epitome of the leading results which it has established.

There is so great a difference between the nervous system of the
naked and of the covered eyed Medusæ, that a simultaneous description
of the nervous system in both groups is not by these authors
considered practicable. Beginning, therefore, with the naked-eyed
division, they describe the nervous system as here consisting of two
parts, a central and a peripheral. The central part is localized in
the margin of the swimming-bell, and there forms a "nerve-ring,"
which is divided by the insertion of the "veil"[3] into an upper and
a lower nerve-ring. In many species the upper nerve-ring is spread
out in the form of a flattish layer, which is somewhat thickened
where it is in contact with the veil. In these species the nerve-ring
is only indistinctly marked off from the surrounding tissues. But
in other species the crowding together of the nerve-fibres at the
insertion of the veil gives rise to a considerable concentration
of nervous structures; while in others, again, this concentration
proceeds to the extent of causing a well-defined swelling of nervous
tissue against the epithelium of the veil and umbrella. In the
Geryonidæ this swelling is still further strengthened by a peculiar
modification of the other tissues in the neighbourhood, which had
been previously described by Professor Haeckel. In all species the
upper nerve-ring lies entirely in the ectoderm. Its principal mass
is composed of nerve-fibres of wonderful tenuity, among which are
to be found sparsely scattered ganglion-cells. The latter are for
the most part bi-polar, more seldom multi-polar. The fibres which
emanate from them are very delicate, and, becoming mixed with others,
do not admit of being further traced. Where the nervous tissue meets
the enveloping epithelium it is connected with the latter from
within, but differs widely from it; for the nerve-cells contain a
longitudinally striated cylindrical or thread-like nucleus which
carries on its peripheral end a delicate hair, while its central end
is prolonged into a fine nerve-fibre. There are, besides these, two
other kinds of cells which form a transition between the ganglion and
the epithelium cells. The first kind are of a long and cylindrical
form, the free ends of which reach as far as the upper surface of the
epithelium The second kind lie for the most part under the upper
surface. They are of a large size, and present, coursing towards
the upper surface, a long continuation, which at its free extremity
supports a hair. In some cases this continuation is smaller, and
stops short before reaching the outer surface. Drs. Hertwig observe
that in these peculiar cells we have tissue elements which become
more and more like the ordinary ganglion-cells of the nerve-ring the
more that their long continuation towards the surface epithelium
is shortened or lost, and these authors are thus led to conclude
that the upper nerve-ring was originally constituted only by such
prolongations of the epithelium-cells, and that afterwards these
prolongations gradually disappeared, leaving only their remnants to
develop into the ordinary ganglion-cells already described.

  [3] This is the name given to a small annular sheet of tissue
  which forms a kind of floor to the orifice of the swimming-bell,
  through the central opening of which floor the manubrium passes.
  The structure is shown in Fig. 1.

Beneath the upper nerve-ring lies the lower nerve-ring. It is
inserted between the muscle-tissue of the veil and umbrella, in
the midst of a broad strand wherein muscle-fibres are entirely
absent. It here constitutes a thin though broad layer which, like
the upper nerve-ring, belongs to the ectoderm. It also consists of
the same elements as the upper nerve-ring, viz. of nerve-fibres and
ganglion-cells. Yet there is so distinct a difference of character
between the elements composing the two nerve-rings, that even in
an isolated portion it is easy to tell from which ring the portion
has been taken. That is to say, in the lower nerve-ring there are
numerous nerve-fibres of considerable thickness, which contrast in
a striking manner with the almost immeasurably slender fibres of
the upper nerve-ring. A second point of difference consists in the
surprising wealth of ganglion-cells in the one ring as compared with
the other. Thus, on the whole, there is no doubt that the lower
nerve-ring presents a higher grade of structure than does the upper,
as shown not only by the greater multiplicity of nerve-cells and
fibres, but also by the relation in which these elements stand to the
epithelium. For in the case of the lower nerve-ring, the presumably
primitive connections of the nervous elements with the epithelium is
well-nigh dissolved--this nerve-ring having thus separated itself
from its parent structure, and formed for itself an independent
layer beneath the epithelium. The two nerve-rings are separated from
one another by a very thin membrane, which, in some species at all
events, is bored through by strands of nerve-fibres which serve to
connect the two nerve-rings with one another.

The peripheral nervous system is also situated in the ectoderm,
and springs from the central nervous system, not by any observable
nerve-trunks, but directly as a nervous plexus composed both of
cells and fibres. Such a nervous plexus admits of being detected
in the sub-umbrella of all Medusæ, and in some species may be
traced also into the tentacles. It invariably lies between the
layer of muscle-fibre and that of the epithelium. The processes of
neighbouring ganglion-cells in the plexus either coalesce or dwindle
in their course to small fibres: at the margin of the umbrella
these unite themselves with the elements of the nerve-rings. There
are also described several peculiar tissue elements, such as, in
the umbrella, nerve-fibres which probably stand in connection with
epithelium-cells; nerve-cells which pass into muscle-fibres, similar
to those which Kleinenberg has called neuro-muscular cells; and, in
the tentacles, neuro-muscular cells joined with cells of special
sensation (_Sinneszellen_).

No nervous elements could be detected in the convex surface of the
umbrella, and it is doubtful whether they occur in the veil.

In some species the nerve-fibres become aggregated in the region of
the generative organs, and in that of the radial canals, thus giving
rise in these localities to what may be called nerve-trunks. But in
other species no such aggregations are apparent, the nervous plexus
spreading out in the form of an even trellis-work.

In the covered-eyed Medusæ the central nervous system consists
of a series of separate centres which are not connected by any
commissures. These nerve-centres are situated in the margin of the
umbrella, and are generally eight in number, more rarely twelve,
and in some species sixteen. They are thickenings of the ectoderm,
which either enclose the bases of the sense-organs, or only cover
the ventral side of the same. Histologically they consist of cells
of special sensation, together with a thick layer of slender
nerve-fibres. Ganglion-cells, however, are absent, so that the
nerve-fibres are merely processes of epithelium-cells.

Drs. Hertwig made no observations on the peripheral nervous system
of the covered-eyed Medusæ; but they do not doubt that such a system
would admit of being demonstrated, and in this connection they cite
the observations of Claus, who describes numerous ganglion-cells as
occurring in the sub-umbrella of Chrysaora. Here I may appropriately
state that before Drs. Hertwig had published their results, Professor
Schäfer, F.R.S., conducted in my laboratory a careful research upon
the histology of the Medusæ, and succeeded in showing an intricate
plexus of cells and fibres overspreading the sub-umbrella tissue of
another covered-eyed Medusa (Aurelia aurita).[4] He also found that
the marginal bodies present a peculiar modification of epithelium
tissue, which is on its way, so to speak, towards becoming fully
differentiated into ganglionic cells.

  [4] See "Observations on the Nervous System of _Aurelia aurita_,"
  _Phil. Trans._, pt. ii., 1878.

Lastly, returning to the researches of Drs. Hertwig, these authors
compare the nervous system of the naked-eyed with that of the
covered-eyed Medusæ, with the view of indicating the points which
show the latter to be less developed than the former. These points
are, that in the nerve-centres of the covered-eyed Medusæ there
are no true ganglion-cells, or only very few; that the mass of the
central nervous system is very small; and that the centralization
of the nervous system is less complete in the one group than in the
other. In their memoir these authors further supply much interesting
information touching the structure of the sense-organs in various
species of Medusæ; but it seems scarcely necessary to extend the
present _résumé_ of their work by entering into this division of
their subject.

In a later publication, entitled "Der Organismus der Medusen und
seine Stellung zur Keimblättertheorie," Drs. Hertwig treat of
sundry features in the morphology of the Medusæ which are of great
theoretical importance; but here again it would unduly extend the
limits of the present treatise if I were to include all the ground
which has been so ably cultivated by these industrious workers.

It will presently be seen in how striking a manner all the
microscopical observations to which I have now briefly alluded are
confirmed by the physiological observations--or, more correctly, I
might say that the microscopical observations, in so far as they
were concerned with demonstrating the existence of nerve-tissue in
the Medusæ, were forestalled by these physiological experiments;
for, with the exception of Professor Haeckel's work on Geryonidæ,
they were all of later publication. But in matters of scientific
inquiry mere priority is not of so much importance as it is too often
supposed to be. Thus, in the present instance, no one of the workers
was in any way assisted by the publications of another. In each case
the work was independent and almost simultaneous.

The remark just made applies also to the only research which still
remains to be mentioned. This is the investigation undertaken and
published by Professor Eimer.[5] He began, like myself, by what in
the next chapter I call the "fundamental observation" on the effects
of excising the nerve-centres, and from this basis he worked both at
the physiology and the morphology of the neuro-muscular tissues. In
point of time, I was the first to make the fundamental observation,
and he was the first to publish it. The sundry features in which our
subsequent investigations agreed, and those in which they differed, I
shall mention throughout the course of the following pages.

  [5] "Die Medusen physiologisch und morphologisch auf ihr
  Nervensystem untersucht" (Tübingen, 1878).

       *       *       *       *       *

I shall now conclude this chapter by giving a brief account of those
general principles of the physiology of nerve and muscle with which
it is necessary to be fully acquainted, in order to understand the
course of the following experiments.

Nerve-tissue, then, universally consists of two elementary
structures, viz. very minute nerve-cells and very minute
nerve-fibres. The fibres proceed to and from the cells, so in some
cases serving to unite the cells with one another, and in other cases
with distant parts of the animal body. Nerve-cells are usually found
collected together in aggregates, which are called nerve-centres or
ganglia, to and from which large bundles of nerve-fibres come and go.

To explain the _function_ of nerve-tissue, it is necessary to begin
by explaining what physiologists mean by the term "excitability."
Suppose that a muscle has been cut from the body of a freshly killed
animal; so long as it is not interfered with in any way, so long
will it remain quite passive. But every time a stimulus is supplied
to it, either by means of a pinch, a burn, an electrical shock, or
a chemical irritant, the muscle will give a single contraction in
response to every stimulation. And it is this readiness of organic
tissues to respond to a suitable stimulus that physiologists
designate by the term "excitability."

Nerves, no less than muscles, present the property of being
excitable. If, together with the excised muscle, there had been
removed from the animal's body an attached nerve, every time any
part of this nerve is stimulated the attached muscle will contract
as before. But it must be carefully observed that there is this
great difference between these two cases of response on the part of
the muscle--that while in the former case the muscle responded to
a stimulus _applied directly to its own substance_, in the latter
case the muscle responded to a stimulus applied at a _distance from
its own substance_, which stimulus was then _conducted_ to the
muscle by the nerve. And in this we perceive the characteristic
function of _nerve-fibres_, viz. that of conducting stimuli to a
distance. The function of nerve-_cells_ is different, viz. that of
accumulating nervous energy, and, at fitting times, of discharging
this energy into the attached nerve-fibres. The nervous energy,
when thus discharged, acts as a stimulus to the nerve-fibre; so
that if a muscle is attached to the end of a fibre, it contracts
on receiving this stimulus. I may add that when nerve-cells are
collected into ganglia, they often appear to discharge their energy
spontaneously; so that in all but the very lowest animals, whenever
we see apparently _spontaneous_ action, we infer that ganglia are
probably present. Lastly, another important distinction must be borne
in mind--the distinction, namely, which is to be drawn between muscle
and nerve. A stimulus applied to a nerveless muscle can only course
through the muscle by giving rise to a visible wave of contraction,
which spreads in all directions from the seat of disturbance as from
a centre. A nerve, on the other hand, conducts the stimulus without
sensibly moving or undergoing any change of shape. Now, in order not
to forget this distinction, I shall always speak of muscle-fibres
as conveying a _visible_ wave of _contraction_, and of nerve-fibres
as conveying an _invisible_, or _molecular_, wave of _stimulation_.
Nerve-fibres, then, are functionally distinguished from
muscle-fibres--and also from protoplasm--by displaying the property
of conducting invisible, or molecular, waves of stimulation from
one part of an organism to another, so establishing physiological
continuity between such parts, _without the necessary passage of
waves of contraction_.




CHAPTER II.

FUNDAMENTAL EXPERIMENTS.


The naked-eyed Medusæ are very much smaller in size than the
covered-eyed, and as we shall find that the distribution of their
nervous elements is somewhat different, it will be convenient to use
different names for the large umbrella-shaped part of a covered-eyed
Medusæa and the much smaller though corresponding part of a
naked-eyed Medusa. The former, therefore, I shall call the umbrella,
and the latter the swimming-bell, or nectocalyx. In each case alike
this portion of the animal performs the office of locomotion,
and it does so in the same way. I have already said that this
mushroom-like organ, which constitutes the main bulk of the animal,
is itself mainly constituted of thick transparent and non-contractile
jelly, but that the whole of its concave surface is lined with a
thin sheet of muscular tissue. Such being the structure of the
organ, the mechanism whereby it effects locomotion is very simple,
consisting merely of an alternate contraction and relaxation of the
entire muscular sheet which lines the cavity of the bell. At each
contraction of this muscular sheet the gelatinous walls of the bell
are drawn together; the capacity of the bell being thus diminished,
water is ejected from the open mouth of the bell backwards, and the
consequent reaction propels the animal forwards. In these swimming
movements, systole and diastole follow one another with as perfect a
rhythm as they do in the beating of a heart.


_Effects of excising the entire Margins of Nectocalyces._

Confining our attention under this heading to the naked-eyed Medusæ,
I find that the following proposition applies to every species of the
group which I have as yet had the opportunity of examining: _Excision
of the extreme margin of a nectocalyx causes immediate, total, and
permanent paralysis of the entire organ_. Nothing can possibly be
more definite than in this highly remarkable effect. I have made
hundreds of observations upon various species of the naked-eyed
Medusæ, of all ages and conditions of freshness, vigour, etc.; and I
have constantly found that if the experiment be made with ordinary
care, so as to avoid certain sources of error presently to be named,
the result is as striking and decided as it is possible to desire.[6]
Indeed, I do not know of any case in the animal kingdom where the
removal of a centre of spontaneity causes so sudden and so complete
a paralysis of the muscular system, there being no subsequent
movements or twitchings of a reflex kind to disturb the absolute
quiescence of the mutilated organism. The experiment is particularly
beautiful if performed on Sarsia; for the members of this genus
being remarkably active, the death-like stillness which results from
the loss of so minute a portion of their substance is rendered by
contrast the more surprising.

  [6] I have only met with one individual exception. This occurred
  in a specimen of _Staurophora laciniata_, where, after removal
  of the entire margin, three centres of spontaneity were found to
  remain in the sheet of contractile tissue lining the nectocalyx.

From this experiment, therefore, I conclude that in the margin of
all the species of naked-eyed Medusæ which I have as yet had the
opportunity of examining, there is situated an intensely localized
system of centres of spontaneity, having at least for one of its
functions the origination of impulses, to which the contractions of
the nectocalyx, under ordinary circumstances, are exclusively due.
And this obvious deduction is confirmed (if it can be conceived to
require confirmation) by the behaviour of the severed margin. This
continues its rhythmical contractions with a vigour and a pertinacity
not in the least impaired by its severance from the main organism,
so that the contrast between the perfectly motionless swimming-bell
and the active contractions of the thread-like portion which has
just been removed from its margin is as striking a contrast as it is
possible to conceive. Hence it is not surprising that if the margin
be left _in situ_, while other portions of the swimming-bell are
mutilated to any extent, the spontaneity of the animal is not at
all interfered with. For instance, if the equator of any individual
belonging to the genus Sarsia (Fig. 1) be cut completely through,
so that the swimming-bell instead of being closed at the top is
converted into an open tube, this open tube continues its rhythmical
contractions for an indefinitely long time, notwithstanding that
the organism so mutilated is, of course, unable to progress. Thus
it is a matter of no consequence how small or how large a portion
of contractile tissue is left adhering to the severed margin of
the swimming-bell; for whether this portion be large or small, the
locomotor centres contained in the margin are alike sufficient to
supply the stimulus to contraction. Indeed, if only the tiniest piece
of contractile tissue be left adhering to a single marginal body
cut out of the bell of Sarsia, this tiny piece of tissue, in this
isolated state, will continue its contractions for hours, or even for
days.


_Effects of excising the entire Margins of Umbrellas._

Turning now to the covered-eyed division of the Medusæ, I find, in
all the species I have come across, that excision of the margins of
umbrellas produces an effect analogous to that which is produced by
excision of the margins of swimming-bells. There is an important
difference, however, between the two cases, in that the paralyzing
effect of the operation on umbrellas is neither so certain nor so
complete as it is on swimming-bells. That is to say, although in the
majority of experiments such mutilation of umbrellas is followed
by immediate paralysis, this is not invariably the case; so that
one cannot here, as with the naked-eyed Medusæ, predict with any
great confidence what will be the immediate result of any particular
experiment. Further, although such mutilation of an umbrella is
usually followed by a paralysis as sudden and marked as that which
follows such mutilation of a swimming-bell, the paralysis of the
former differs from the paralysis of the latter, in that it is very
seldom _permanent_. After periods varying from a few seconds to half
an hour or more, occasional weak and unrhythmical contractions begin
to manifest themselves, or the contractions may even be resumed with
but little apparent change in their character and frequency. The
condition of the animal before the operation, as to general vigour,
etc., appears to be one factor in determining the effect of the
operation; but this is very far from being the only factor.

Upon the whole, then, although in the species of covered-eyed Medusæ
which I have as yet had the opportunity of examining, the effects
which result from excising the margins of umbrellas are such as
to warrant me in saying that the main supply of locomotor centres
appears to be usually situated in that part of these organs, these
effects are nevertheless such as to compel me at the same time to
conclude that the locomotor centres of the covered-eyed Medusæ are
more diffused or segregated than are those of the naked-eyed Medusæ.
Lastly, it should be stated that all the species of covered-eyed
Medusæ resemble all the species of naked-eyed Medusæ, in that their
members will endure any amount of section it is possible to make
upon any of their parts other than their margins without their
spontaneity being in the smallest degree affected.


_Effects of excising Certain Portions of the Margins of Nectocalyces._

The next question which naturally presents itself is as to whether
the locomotor centres are equally distributed all round the margin
of a swimming organ, or situated only, or chiefly, in the so-called
marginal bodies. To take the case of the naked-eyed Medusæ first,
it is evident that in most of the genera, in consequence of the
intertentacular spaces being so small, it is impossible to cut
out the marginal bodies (which are situated at the bases of the
tentacles) without at the same time cutting out the intervening
portions of the margin. The genus Sarsia, however, is admirably
adapted (as a glance at Fig. 1 will show) for trying the effects of
removing the marginal bodies without injuring the rest of the margin,
and _vice versâ_. The results of such experiments upon members of
this genus are as follow.

Whatever be the condition of the individual operated upon as to
freshness, vigour, etc., it endures excision of three of its marginal
bodies without suffering any apparent detriment; but in most cases,
as soon as the last marginal body is cut out, the animal falls to
the bottom of the water quite motionless. If the subject of the
experiment happens to be a weakly specimen, it will, perhaps, never
move again: it has been killed by something very much resembling
nervous shock. On the other hand, if the specimen operated upon be
one which is in a fresh and vigorous state, its period of quiescence
will probably be but short; the nervous shock, if we may so term
it, although evidently considerable at the time, soon passes away,
and the animal resumes its motions as before. In the great majority
of cases, however, the activity of these motions is conspicuously
diminished.

The effect of excising all the marginal tissue from between the
marginal bodies and leaving the latter untouched, is not so definite
as is the effect of the converse experiment just described. Moreover,
allowance must here be made for the fact that in this experiment the
principal portion of the "veil"[7] is of necessity removed, so that
it becomes impossible to decide how much of the enfeebling effect
of the section is due to the removal of locomotor centres from the
swimming-bell, and how much to a change in the merely mechanical
conditions of the organ. From the fact, however, that excision of the
entire margin of Sarsia produces total paralysis, while excision of
the marginal bodies alone produces merely partial paralysis, there
can be no doubt that both causes are combined. Indeed, it has been
a matter of the greatest surprise to me how very minute a portion
of the intertentacular marginal tissue is sufficient, in case of
this genus, to animate the entire swimming-bell. Choosing vigorous
specimens of Sarsia, I have tried, by cutting out all the margin
besides, to ascertain how minute a portion of intertentacular tissue
is sufficient to perform this function, and I find that this portion
may be so small as to be quite invisible without the aid of a lens.

  [7] See Fig. 1.

From numerous observations, then, upon Sarsia, I conclude that in
this genus (and so, from analogy, probably in all the other genera of
the true Medusæ) locomotor centres are situated in every part of the
extreme margin of a nectocalyx, but that there is a greater supply of
such centres in the marginal bodies than elsewhere.


_Effects of excising Certain Portions of the Margin of Umbrellas._

Coming now to the covered-eyed Medusæ, I find that the concentration
of the locomotor centres of the margin into the marginal bodies, or
lithocysts, is still more decided than it is in the case of Sarsia.
Taking Aurelia aurita as a type of the group, I cannot say that,
either by excising the lithocysts alone or by leaving the lithocysts
_in situ_ and excising all the rest of the marginal tissue, I have
ever detected the slightest indications of locomotor centres being
present in any part of the margin of the umbrella other than the
eight lithocysts; so that all the remarks previously made upon this
species, while we were dealing with the effects of excising the
entire margin of umbrellas, are equally applicable to the experiment
we are now considering, viz. that of excising the lithocysts alone.
In other words, but for the sake of symmetry, I might as well have
stated at the first that in the case of the covered-eyed Medusæ all
the remarkable paralyzing effects which are obtained by excising
the entire margin of an umbrella are obtained in exactly the same
degree by excising the eight lithocysts alone; the intermediate
marginal tissue, in the case of these Medusæ, is totally destitute of
locomotor centres.


_Effects upon the Manubrium of excising the Margin of a Nectocalyx or
Umbrella._

Lastly, it must now be stated, and always borne in mind, that neither
in the case of naked nor covered-eyed Medusæ does excision of the
margin of a swimming organ produce the smallest effect upon the
manubrium. For hours and days after the former, in consequence of
this operation, has ceased to move, the latter continues to perform
whatever movements are characteristic of it in the unmutilated
organism--indeed, these movements are not at all interfered with even
by a complete severance of the manubrium from the rest of the animal.
In many of the experiments subsequently to be detailed, therefore, I
began by removing the manubrium, in order to afford better facilities
for manipulation.


_Summary of Chapter._

With a single exception to hundreds of observations upon six widely
divergent genera of naked-eyed Medusæ, I find it to be uniformly
true that removal of the extreme periphery of the animal causes
instantaneous, complete, and permanent paralysis of the locomotor
system. In the genus Sarsia, my observations point very decidedly to
the conclusion that the principal locomotor centres are the marginal
bodies, but that, nevertheless, every microscopical portion of the
intertentacular spaces of the margin is likewise endowed with the
property of originating locomotor impulses.

In the covered-eyed division of the Medusæ, I find that the
_principal_ seat of spontaneity is the margin, but that the latter is
not, as in the naked-eyed Medusæ, the exclusive seat of spontaneity.
Although in the vast majority of cases I have found that excision
of the margin impairs or destroys the spontaneity of the animal for
a time, I have also found that the paralysis so produced is very
seldom of a permanent nature. After a variable period occasional
contractions are usually given, or, in some cases, the contractions
may be resumed with but little apparent detriment. Considerable
differences, however, in these respects are manifested by different
species, and also by different individuals of the same species.
Hence, in comparing the covered-eyed group as a whole with the
naked-eyed group as a whole, so far as my observations extend,
I should say that the former resembles the latter in that its
representatives usually have their main supply of locomotor centres
situated in their margins, but that it differs from the latter in
that its representatives usually have a greater or less supply of
their locomotor centres scattered through the general contractile
tissue of their swimming organs. But although the locomotor centres
of a covered-eyed Medusa are thus, generally speaking, more diffused
than are those of a naked-eyed Medusæ, _if we consider the organism
as a whole_, the locomotor centres in the _margin_ of a covered-eyed
Medusa are _less_ diffused than are those in the _margin_ of a
naked-eyed Medusæ. In no case does the excision of the margin of
a swimming organ produce any effect upon the movements of the
manubrium.




CHAPTER III.

EXPERIMENTS IN STIMULATION.


_Mechanical, Chemical, and Thermal Stimulation._

So far as my observations extend, I find that all Medusæ, after
removal of their locomotor centres, invariably respond to every
kind of stimulation. To take the case of Sarsia as a type, nothing
can possibly be more definite than is the single sharp contraction
of the mutilated nectocalyx in response to every nip with the
forceps. The contraction is precisely similar to the ordinary ones
that are performed by the unmutilated animal; so that by repeating
the stimulus a number of times, the nectocalyx, with its centres
of spontaneity removed, may be made to progress by a succession of
contractions round and round the vessel in which it is contained,
just as a frog, with its cerebral hemispheres removed, may be made to
hop along the table in response to a succession of stimulations.[8]

  [8] In the case of the covered-eyed Medusæ, however, the
  paralyzed umbrella sometimes responds to a single stimulation
  with two, and more rarely with three contractions, which are
  separated from one another by an interval of the same duration as
  the normal diastole of the unmutilated animal.

Different species of Medusæ exhibit different degrees of irritability
in responding to stimuli; but in all the cases I have met with
the degree of irritability is remarkably high. Thus, I have seen
responsive contractions of the whole umbrella follow upon the
exceedingly slight stimulus caused by a single drop of sea-water
let fall upon the irritable surface from the height of one inch. As
regards chemical stimulation, dilute spirit or other irritant, when
dropped on the paralyzed swimming organ of Aurelia aurita, often
gives rise to a whole series of rhythmical pulsations, the systoles
and diastoles following one another at about the same rate as is
observable in the normal swimming motions of the unmutilated animal.

It is somewhat difficult, in the case of paralyzed swimming organs,
to prove the occurrence of a contraction in response to thermal
stimulation, from the fact that while these tissues are not nearly
so sensitive to this mode of excitation as might be anticipated,
they are, as just observed, extraordinarily sensitive to mechanical
excitation. It therefore becomes difficult to administer the
appropriate thermal stimulus without at the same time causing a
sufficient mechanical disturbance to render it doubtful to which
of the stimuli the response is due. This may be done, however, by
allowing a few drops of heated sea-water to run over the excitable
surface while it is exposed to the air. In this and in other ways I
have satisfied myself that the paralyzed tissues of swimming organs
respond to sudden elevations of temperature.


_Luminous Stimulation._

It is interesting to note that, in the case of some of the naked-eyed
Medusæ, the action of light as a stimulus is most marked and
unfailing. In the case of Sarsia, for instance, a flash of light let
fall upon a living specimen almost invariably causes it to respond
with one or more contractions. If the animal is vigorous and swimming
freely in water, the effect of a momentary flash thrown upon it
during one of the natural pauses is immediately to originate a bout
of swimming. But if the animal is non-vigorous, or if it be removed
from the water and spread flat upon an object-glass, it usually gives
only one contraction in response to every flash. There can thus be
no doubt that a sudden transition from darkness to light acts upon
Sarsia as a stimulus, and this even though the transition be but of
momentary duration. The question therefore arises as to whether the
stimulus consists in the presence of light, or in the occurrence
of the sudden transition from darkness to light and from light to
darkness. To answer this question, I tried the converse experiment
of placing a vigorous specimen in sunlight, waiting till the middle
of one of the quiescent stages in the swimming motions had come
on, and then suddenly darkening. In no case, however, under these
circumstances, did I obtain any response; so that I cannot doubt it
is the light per se, and not the sudden nature of the transition
from darkness to light, which in the former experiment acted as
the stimulus. Indeed, the effect of the converse experiment just
described is rather that of inhibiting contractions; for, if the
sunlight be suddenly shut off during the occurrence of a swimming
bout, it frequently happens that the quiescent stage immediately
sets in. Again, in a general way, it is observable that Sarsia are
more active in the light than they are in the dark, the comparative
duration of the quiescent stages being less in the former than in
the latter case. Light thus appears to act towards these animals as
a constant stimulus. Lastly, it may be stated that when the marginal
bodies of Sarsia are removed, the swimming-bell, although still able
to contract spontaneously, no longer responds to luminous stimulation
of any kind or degree. But if only one body be left _in situ_, or if
the severed margin alone be experimented upon, the same unfailing
response may be obtained to luminous stimulation as that which is
obtained from the entire animal.

The fact last mentioned indicates that the marginal bodies are organs
of special sense, adapted to respond to luminous stimulation; or,
in more simple words, that they perform the functions of sight. Now
it has long been thought more or less probable that these marginal
bodies are rudimentary or incipient "eyes," but hitherto the
supposition has not been tested by experiment, and was therefore of
no more value than a guess.[9] The guess in this instance, however,
happens to have been correct, as the results of the following
experiments will show.

  [9] As Professor Haeckel observes in his monograph already
  alluded to, "Die Deutung der Sinnesorgane niederer Thiere gehört
  ohne Zweifel zu den schwierigsten Objecten der vergleichenden
  Physiologie und ist der grössten Unsicherheit unterworfen. Wir
  sind gewohnt, die von den Wirbelthieren gewonnenen Anschauungen
  ohne Weiteres auch auf die wirbellosen Thiere der verschiedenen
  Kinese zu übertragen und bei diesen analoge Sinnesempfindungen
  anzunehmen als wir selbst besitzen ... Noch weniger freilich als
  die von den meisten Autoren angenommene Deutung der Randbläschen
  unserer Medusen als Gehörorgane kann die von Agassiz und Fritz
  Müller vertretene Ansicht befriedigen, dass dieselben Augen
  seien.... Alle diese Verhältnisse sind mit der Deutung der
  Concretion als 'Linse' und des sie umschliessenden Sinnesganglion
  als 'Sehnerv' durchaus unvereinbar."

  It may not be unnecessary to say that, although the simple
  experiment above described effectually proves that the marginal
  bodies have a visual function to subserve, we are not for this
  reason justified in concluding that these are so far specialized
  as organs of sight as to be precluded from ministering to any
  other sense.

Having put two or three hundred Sarsia into a large bell-jar, I
completely shut out the daylight from the room in which the jar
was placed. By means of a dark lantern and a concentrating lens,
I then cast a beam of light through the water in which the Sarsia
were swimming. The effect upon the latter was most decided. From all
parts of the bell-jar they crowded into the path of the beam, and
were most numerous at that side of the jar which was nearest to the
light. Indeed, close against the glass they formed an almost solid
mass, which followed the light wherever it was moved. The individuals
composing this mass dashed themselves against the glass nearest
the light with a vigour and determination closely resembling the
behaviour of moths under similar circumstances. There can thus be no
doubt about Sarsia possessing a visual sense.

The method of ascertaining whether this sense is lodged in the
marginal bodies was, of course, extremely simple. Choosing a dozen of
the most vigorous specimens, I removed all the marginal bodies from
nine, and placed these, together with the three unmutilated ones, in
another bell-jar. After a few minutes the mutilated animals recovered
from their nervous shock, and began to swim about with tolerable
vigour. I now darkened the room, and threw the concentrated beam of
light into the water as before. The difference in the behaviour of
the mutilated and of the unmutilated specimens was very marked. The
three individuals which still had their marginal bodies sought the
light as before, while the nine without their marginal bodies swam
hither and thither, without paying it any regard.

A further question, however, still remained to be determined. The
pigment spot of the marginal body in Medusæ is, as L. Agassiz
observed, placed _in front of_ the presumably nervous tissue, and
for this reason he naturally enough suggested that if the marginal
body has a visual function to perform, the probability is that the
rays by which the organ is affected are the heat-rays lying beyond
the range of the visible spectrum. Accordingly I brought a heated
iron, just ceasing to be red, close against the large bell-jar which
contained the numerous specimens of Sarsia; but not one of the latter
approached the heated metal.

From these observations, therefore, I conclude that in Sarsia
the faculty of appreciating luminous rays is present, and that
this faculty is lodged exclusively in the marginal bodies; while
from observations conducted on the covered-eyed Medusæ, I have
come to the same conclusion respecting them. But although I have
tested many species of naked-eyed Medusæ besides Sarsia, I have
obtained indications of response to luminous stimulation only
in the case of one other. This is a species which I have called
Tiaropsis polydiademata, and the response which it gives to luminous
stimulation is even more marked and decided than that which is given
by Sarsia; for a sudden exposure to sunlight causes this animal to
go into a kind of tonic spasm, the whole of the nectocalyx being
drawn together in a manner resembling cramp. Now, in one remarkable
particular this response to luminous stimulation on the part of
Tiaropsis polydiademata differs from that given by Sarsia tubulosa;
and the difference consists in the fact that, while with Sarsia the
period of latency (_i.e._ the time between the fall of the stimulus
and the occurrence of the response) is, so far as the eye can judge,
as instantaneous in the case of response to luminous stimulation as
it is in the case of response to any other kind of stimulation, such
is far from being true with Tiaropsis polydiademata. The period of
latency in the last-named species is, so far as the eye can judge,
quite as instantaneous as it is in the case of Sarsia, when the
stimulus employed is other than luminous; but in response to light,
the characteristic spasm does not take place till slightly more than
a second has elapsed after the first occurrence of the stimulus.
As this extraordinary difference in the latent period exhibited by
the same animal towards different kinds of stimuli appeared to me
a matter of considerable interest, I was led to reflect upon the
probable cause of the difference. It occurred to me that the only
respect in which luminous stimulation of the Medusæ differed from
all the other modes of stimulation I had employed consisted in
this--that, as proved by my previous experiments on Sarsia, which I
repeated on Tiaropsis, luminous stimulation directly affected the
ganglionic tissues. Now, as in Tiaropsis polydiademata luminous
stimulation differed from all the other modes of stimulation in
giving rise to an immensely longer period of latency, I seemed
here to have an index of the difference between the rapidity of
the response to stimuli by the contractile and by the ganglionic
tissues respectively. The next question, therefore, which presented
itself was as to whether the enormous length of time occupied by
the process of stimulation in the ganglia was due to any necessity
on the part of the latter to accumulate the stimulating influence
prior to originating a discharge, or to an immensely lengthened
period of latent stimulation manifested by the ganglia under the
influence of light.[10] This is an interesting question, because if
such a lengthened period of latent stimulation occurs in this case,
it would stand in curious antithesis to the very short period of
latent stimulation manifested by the contractile tissues of the same
animal under other modes of irritation. To test these alternative
hypotheses, I employed the very simple method of first allowing a
continuous flood of light to fall suddenly on the Medusa, and then
noting the time at which the responsive spasm first began. This
time, as already stated, was slightly more than one second. I next
allowed the animal to remain for a few minutes in the dark to recover
shock, and, lastly, proceeded to throw in single flashes of light
of measured duration. I found that unless the flash of light was of
slightly more than one second in its duration, no response was given;
that is to say, the minimal duration of a flash required to produce
a responsive spasm was just the same as the time during which a
continuous flood of light required to operate in order to produce a
similar spasm. From this, therefore, I conclude that the enormously
long period of latent excitation in response to luminous stimuli was
not, properly speaking, a period of latent excitation at all; but
that it represented the time during which a certain summation of
stimulating influence was taking place in the ganglia, which required
somewhat more than a second to accumulate, and which then caused
the ganglia to originate an abnormally powerful discharge. So that
in the action of light upon the ganglionic matter of this Medusa we
have some analogy to its action on certain chemical compounds in
this respect, that, just as in the case of those compounds which
light is able to split up, a more or less lengthened exposure to its
influence is necessary in order to admit of the summating influence
of its vibrations on the molecules, so in the ease of this ganglionic
material, the decomposition which is effected in it by light, and
which terminates in an explosion of nervous energy, can only be
effected by a prolonged exposure of the unstable material to the
summating influence of the luminous vibrations. Probably, therefore,
we have here the most rudimentary type of a visual organ that is
possible; for it is evident that if the ganglionic matter were a
very little more stable than it is, it would altogether fail to be
thrown down by the luminous vibrations, or would occupy so long a
time in the process that the visual sense would be of no use to its
possessor. How great is the contrast between the excitability of
such a sense-organ and that of a fully evolved eye, which is able
to effect the needful molecular changes in response to a flash as
instantaneous as that of lightning.

  [10] The period of latent stimulation merely means the time
  after the occurrence of an excitation during which a series of
  physiological processes are taking place, which terminate in
  a contraction; so that, whether the excitation is of a strong
  or of a weak intensity, the period of latent stimulation is
  not much affected. The above question, therefore, was simply
  this--Does the prolonged delay on the part of these ganglia, in
  responding to light, represent the time during which the series
  of physiological processes are taking place in response to an
  adequate stimulus, or does it represent the time during which
  light requires to act before it becomes an adequate stimulus?

With regard to luminous stimulation, it is only necessary further to
observe that responses were given equally well to direct sunlight,
diffused daylight, and to light reflected from a mirror inclined at
the polarizing angle. It must also be stated that responses are given
to any of the luminous rays of the spectrum when these are employed
separately; but that neither the non-luminous rays beyond the red,
nor those beyond the violet, appear to exert the smallest degree of
stimulating influence.


_Electrical Stimulation._

All the excitable parts of all the Medusæ which I have examined are
highly sensitive to electrical stimulation, both of the constant and
of the induced current.

Exploration with needle-point terminals and induction shocks of
graduated strength showed that certain parts or tracts of the
nectocalyx are more sensitive than others. The most sensitive
parts are those which correspond with the distribution of the main
nerve-trunks, _i.e._ round the margin of the nectocalyx and along
the course of the radial tubes. The external or convex surface of a
nectocalyx or umbrella is totally insensitive to stimulation, and
the same statement applies to the whole thickness of the gelatinous
substance to which the neuro-muscular sheet is attached.

In all other respects the excitable tissues of the Medusæ in their
behaviour towards electrical stimulation conform to the rules which
are followed by excitable tissues of other animals. Thus, closure
of the constant current acts as a stronger stimulus than does
opening of the same, while the reverse is true of the induction
shock; and exhaustion supervenes under the influence of prolonged
excitation. Moreover, I have obtained evidence of that polarization
of nerve-tissues under the influence of the constant current,
which is known to physiologists by the term "electrotonus;" but
it would be somewhat tedious to detail the evidence on this head
which I have already published elsewhere.[11] Tetanus produced by
faradaic electricity is not of the nature of an apparently single
and prolonged contraction, but that of a number of contractions
rapidly succeeding one another, as in the case of the heart under
similar excitation. This at least applies to Sarsia. In the case of
Aurelia, tolerably strong faradization does cause a more or less
well-pronounced tetanus. The continuity of the spasm is, indeed,
often interrupted by momentary and partial relaxations. These
interruptions are the more frequent the weaker the current; so that,
at a certain strength of the latter, the tetanus is of a wild and
tumultuous nature; but with strong currents the spasm is tolerably
uniform. That in all cases the tetanus is due to summation of
contractions may be very prettily shown by the following experiment.
An Aurelia is cut into a spiral strip, and all its lithocysts are
removed; single induction-shocks are then thrown in with a key at
one end of the strip--every shock, of course, giving rise to a
contraction wave. If these shocks are thrown in at a somewhat fast
rate, two contraction waves may be made at the same time to course
along the spiral strip, one behind the other; but if the shocks are
thrown in at a still faster rate, so as to diminish the distance
between any two successive waves, a point soon arrives at which
every wave mounts upon its predecessor; and if several waves be thus
made to coalesce, the whole strip becomes thrown into a state of
persistent contraction.

  [11] See "Croonian Lectures," 1875. _Philosophical Transactions_,
  vol. 166, part I. pp. 284-6.

In this way sustained tetanus, or single contraction waves, or any
intermediate phase, may be instantly produced at pleasure. In such
experiments, moreover, it is interesting to observe that, no matter
how long the strip be, whatever disturbances are set up at one end
are faithfully transmitted to the other. For instance, if an Aurelia
be cut into the longest possible strip with a remnant of the disk
left attached at one end, as represented in Fig. 11 (p. 70), then
all the peculiar time relations between successive contractions
which are intentionally caused by the experimenter at one end of the
strip, are afterwards accurately reproduced at the other end of the
strip by the remainder of the disk. Now, as this fact is observable
however complex these time relations may be, and however rapidly
the successive stimuli are thrown in, I think it is a point of some
interest that these complicated relations among rapidly succeeding
stimuli do not become blended during their passage along the thirty
or forty inches of contractile tissue. The fact, of course, shows
that the rate of transmission is so identical in the case of all
the stimuli originated, that the sum of the effects of any series
of stimuli is delivered at the distal end of the strip, with all
its constituent parts as distinct from one another as they were at
starting from the proximal end of the strip.


_Period of Latency, and Summation of Stimuli._

I shall now give an account of my experiments in the period of
latency and the summation of stimuli. To do this, I must first
describe the method which I adopted in order to obtain a graphic
record of the movements which were given in response to the stimuli
supplied. As Aurelia aurita is the only species on which I have
experimented in this connection, my remarks under this heading will
be confined to it alone.

The method by which I determined the latent period in the case of
this species was as follows. A basin containing the Medusa was
filled to its brim with sea-water, and placed close beside a smoked
cylinder, which, while it lay in a horizontal position, could be
rotated at a known rate. The Aurelia[12] was placed with its concave
aspect uppermost, and an inch or two below the surface of the water.
The animal was held firmly in this position by means of a pair of
compasses thrust through it and forced into a piece of wood, which
was fastened to the bottom of the basin. The legs of the compasses
were provided with india-rubber sliders, so that by placing these
under the Medusa, the latter might be kept at any elevation in the
water which might be desired. The manubrium and lithocysts were now
removed, and also a segment of the umbrella. A light straw was then
forced through the gelatinous substance of the umbrella in a radial
direction, and close to the gap caused by the missing segment. The
other, or free, end of this straw was firmly joined to a capillary
glass rod, which was suitably bent to avoid contact with the rim of
the basin, and also to write on the smoked cylinder. If the straw
was not itself sufficient to support the weight of the capillary
rod, a small cross-piece of cork might easily be tied to it, so as
to add to the flotation power. A part of the excitable tissue was
now raised above the surface of the water by means of a disk of cork
placed beneath it, and on the part of the tissue thus raised there
were placed a pair of platinum electrodes. These electrodes proceeded
from an electro-magnetic apparatus, which was arranged in such a
way that every time the current in it was opened or closed, it gave
an induction shock and moved a lever at the same instant of time.
This lever was therefore placed upon the cylinder immediately above
the capillary glass-writer which proceeded from the Medusa, care
being taken to place the two writers in the same line, parallel to
the axis of the cylinder. Such being the arrangement, the cylinder
was rotated, and thus two parallel lines were marked upon it by the
two writers. If the current was now closed, an induction shock was
thrown into the tissue at the same instant that the electro-magnet
writer recorded the fact, by altering its position on the cylinder.
Again, as soon as the paralyzed Medusa responded to the induction
shock, the radii of the vacant segment were drawn apart, and in
this way a curve was obtained by the other writer on the rotating
cylinder. Now, by afterwards dropping a perpendicular line from
the point at which the electro-magnet writer changed its position,
to the parallel line made by the other writer, and then measuring
the distance between the point of contact and the point on the
last-mentioned line on which the curve began, the period of latent
stimulation was determined. A glance at Figs. 3 and 4 (p. 55)
will render this description clear to any one who is not already
acquainted with the method, when it is stated that the upper line is
a record of the movements of the electro-magnet writer, and the lower
line that of the movements of the other writer. It will be observed
that the point _a_ in the upper line marks the point at which the
induction shock was thrown in; so that by first producing the
perpendicular till it meets the lower line at _b_, and then measuring
the distance between the point _b_ and the point _c_, at which the
curve in the lower line first begins, the latent period (_b c_) is
determined--the time occupied by the rotation of the cylinder from
_b_ to _c_ being known.

  [12] It may here be stated that in all the experiments on
  stimulation subsequently to be detailed, there is no difference
  to be observed between the behaviour of an entire swimming organ
  deprived of its ganglia, and that of a portion of any size which
  may be separated from it.

_Summation of Stimuli._--In this way I have been able to ascertain
the period of latent stimulation in Aurelia aurita with accuracy.
It must be stated at the outset, however, that the period is subject
to great variations under certain varying conditions, so that we can
only arrive at a just estimation of it by understanding the nature
of the modifying causes. To take the simplest cause first, suppose
that the paralyzed Aurelia has been left quiet for several minutes in
sea-water at forty-five degrees, and that it is then stimulated by
means of a single induction shock; the responsive contraction will
be comparatively feeble with a very long period of latency, viz.
five-eighths of a second. If another shock of the same intensity be
thrown in as soon as the tissue has relaxed, a somewhat stronger
contraction, with a somewhat shorter latent period, will be given.
If the process is again repeated, the response will be still more
powerful, with a still shorter period of latency; and so on, perhaps,
for eight or ten stages, when the maximum force of contraction of
which the tissue is capable will have been attained, while the period
of latency will have been reduced to its minimum. This period is
three-eighths of a second, or, in some cases, slightly less.

Now, we have here a remarkable series of phenomena, and as it is
a series which never fails to occur under the conditions named,
I append tracings to give a better idea of the very marked and
striking character of the results. The first tracing (Fig. 2) is a
record of the successive increments of the responses to successive
induction shocks of the same intensity, thrown in at three seconds'
intervals--the cylinder being stationary during each response, and
rotated a short distance with the hand during each interval of repose.

  [Illustration: Fig. 2.]

The second tracing (Figs. 3 and 4) is a record of the difference
between the lengths of the latent period, and also between the
strengths of the contraction, in the case (_a_) of the first of
such a series of responses (Fig. 3), and (_b_) of the last of such
a series (Fig. 4). From these tracings it will be manifest, without
further comment, how surprising is the effect of a series of stimuli;
first, in _arousing_ the tissue, as it were, to increased _activity_,
and, second, in developing a state of _expectancy_.

In accordance with the now customary terminology, I shall call such
a series of responses as are given in Fig. 2 a "staircase." Such a
staircase has a greater number of steps in it if caused by a weak
current (compare Figs. 2 and 5); and if the strength of the current
be suddenly increased after the maximum level of a staircase has
been reached by using a feeble current, this level admits of being
slightly raised (see Fig. 5). Moreover, I find that a stimulus,
which at the bottom of a staircase is of less than minimal intensity,
is able, at the top of a staircase, to give rise to a contraction
of very nearly maximum intensity. That is to say, by employing
an induction stimulus of slightly less than minimal intensity in
relation to the original irritability of the tissue, no response is
given to the first two or three shocks of a series; but at the third
or fourth shock a slight response is given, and from that point
onward the staircase is built up as usual. This was the case in the
experiment of which Fig. 2 is a record, no response having been given
to the first two shocks.

  [Illustration: Fig. 3.]

  [Illustration: Fig. 4.]

  [Illustration: Fig. 5.]

With regard to this interesting staircase action, two questions
naturally present themselves. In the first place, we are anxious
to know whether the arousing effect which is so conspicuous in
a staircase series is due to the occurrence of the previous
_stimulations_, or to that of previous _contractions_; and, in the
next place, we should like to know whether, during the _natural_
rhythm of the tissue, each contraction exerts a beneficial influence
on its successor, analogous to that which occurs in the case of
contractions which are due to _artificial_ stimuli. To answer the
first of these questions, therefore, I built up a staircase in the
ordinary way, and then suddenly transferred the electrodes to the
opposite side of the umbrella from that on which they rested while
constructing the staircase. On now throwing in another shock at this
part of the contractile tissue so remote from the part previously
stimulated, the response was a maximum response. Similarly, if the
electrodes were transferred in the way just described, not after the
maximum effect had been attained, but at any point during the process
of constructing a staircase, the response given to the next shock was
of an intensity to make it rank as the next step in the staircase.
Hence, shifting the position of the electrodes in no wise modifies
the peculiar effect we are considering; and this fact conclusively
proves that the effect is a general one, pervading the whole mass of
the contractile tissue, and not confined to the locality which is
the immediate seat of stimulation. Nevertheless, this fact does not
tend to prove that the staircase effect depends on the process of
contraction as distinguished from the process of stimulation, because
the wave of the former process must always precede that of the
latter. But, on the other hand, in this connection it is of the first
importance to remember the fact already stated, viz. that a current
which at the beginning of a series of stimulations is of slightly
less than minimal intensity presently becomes minimal, and eventually
of much more than minimal intensity--a staircase being thus built up
of which the first observable step (or contraction) only occurs in
response to the second, third, or even fourth shock of the series.
This fact conclusively proves that the staircase effect, at any
rate at its commencement, depends on the process of stimulation as
distinguished from that of contraction; for it is obvious that the
latter process cannot play any part in thus constructing what we may
term the invisible steps of a staircase.

To answer the second of the above questions, I placed an Aurelia with
its concave surface uppermost, and removed seven of its lithocysts;
I then observed the spontaneous discharge of the remaining one, and
found it to be conspicuous enough that, after the occurrence of one
of the natural pauses (if this were of sufficient duration), the
first contraction was feeble, the next stronger, the next still
stronger, and so on, till the maximum was attained. This natural
staircase action admits of being very prettily shown in another
way. If a tolerably large Aurelia is cut into a spiral strip of
small width and great length, and if all the lithocysts are removed
except one at one end of the strip, it may be observed that,
after the occurrence of a natural pause, the first discharge only
penetrates perhaps about a quarter of the length of the strip, the
next discharge penetrates a little further, the next further, and so
on, till finally the contraction waves pass from end to end. On now
removing the ganglion, waiting a few minutes, and then stimulating
with successive induction shocks, the same progressive penetration is
observable as that which previously took place with the ganglionic
stimulation. Lastly, the identity of natural and artificial
staircase action may be placed beyond all doubt by an experiment in
which the effects of induction shocks and of ganglionic discharges
are combined. To accomplish this, all the lithocysts save one are
removed, and a staircase is then built up in the ordinary way by
successive induction shocks. It will now occasionally happen that the
ganglion originates a discharge during the process of constructing
the staircase, which is being built up by the artificial stimuli;
when this happens the resulting contraction takes its proper rank
in the series, and this at whatever point the natural contraction
happens to come in.

Thus, then, to summarize and conclude these observations, we have
seen that if a single stimulation, whether of a natural or artificial
kind, is supplied to the excitable tissues of a jelly-fish, a short
period, called the period of latency, will elapse, and then the
jelly-fish will give a single weak contraction. If, as soon as the
tissue has relaxed, the stimulation is again repeated, the period of
latency will be somewhat shorter, and will be followed by a somewhat
stronger contraction. Similarly, if the stimulation is repeated a
third time, the period of latency will be still shorter, and the
ensuing contraction still stronger. And so on up to nine or ten
times, when the period of latency will be reduced to its _minimum_,
while the force of the contraction will be raised to its _maximum_;
so that in the jelly-fish, the effect of a series of excitations
supplied at short intervals from one another is that of both arousing
the tissue into a state of increased _activity_, and also of
producing in it a state of greater _expectancy_. We have, moreover,
seen that this effect depends upon the repetition of the process of
stimulation, and not upon that of the process of contraction.

Now, effects very similar to these have been found to occur in the
case of the excitable plants by Dr. Burdon-Sanderson; in the case of
the frog's heart by Dr. Bowditch; and in the case of reflex action
of the spinal cord by Dr. Stirling. Indeed, the only difference in
this respect between these four tissues, so widely separated from one
another in the biological scale, consists in the _time_ which may be
allowed to elapse between the occurrence of the successive stimuli,
in order to produce this so-called summating effect of one stimulus
upon its successor: the _memory_, so to speak, of the heart-tissue
for the occurrence of a former stimulus being longer than the memory
of the jelly-fish tissue; while the memory of the latter is longer
than that of the plant tissue. And I may here add that even in our
own organization we may often observe the action of this principle of
the summation of stimuli. For instance, we can tolerate for a time
the irritation caused by a crumb in the larynx, but very rapidly
the sense of irritation accumulates to a point at which it becomes
impossible to avoid coughing. And similarly with tickling generally,
the convulsive reflex movements to which it gives rise become more
and more incontrollable the longer the stimulation is continued,
until they reach a _maximum_ point, where, in persons susceptible
to this kind of stimulation, the muscular action passes completely
beyond the power of the will. Lastly, I may further observe, what I
do not think has ever been observed before, that even in the domain
of psychology the action of this principle admits of being clearly
traced. We find it, for instance, in the rhythmical waves of emotion
characteristic of grief, and at the other extreme we find it in the
case of the ludicrous. We can endure for a short time, without giving
any visible response, the psychological stimulation which is supplied
by a comical spectacle; but if the latter continues sufficiently
long in a sufficiently ludicrous manner, our appropriate emotion
rapidly runs up to a point at which it becomes uncontrollable, and
we burst into an explosion of ill-timed laughter. But in this case
of psychological tickling, as in the previous case of physiological
tickling, some persons are much more susceptible than others.
Nevertheless, there can be no doubt that from the excitable tissues
of a plant, through those of a jelly-fish and a frog, up even to the
most complex of our psychological processes, we have in this recently
discovered principle of the summation of stimuli a very remarkable
uniformity of occurrence.


_Effects of Temperature on Excitability._

I shall now conclude this chapter with a brief statement of the
effects of temperature on the excitability of the Medusæ; and before
stating my results, I may observe that in all my experiments in this
connection I changed the temperature of the Medusæ by drawing off
the water in which they floated with a siphon, while at the same
time I substituted water of a different temperature from that which
I thus abstracted. In this way, without modifying any of the other
conditions to which the animals were exposed, I was able to observe
the effects of changing the temperature alone.

With regard to the effect of temperature on the latent period of
stimulation, the following table, setting forth the results of one
among several experiments, explains itself.

Period of latent stimulation of the deganglionated tissues of Aurelia
aurita as affected by temperature:--

    -------------------------------+-------------------------------
     Temperature of water (Fahr.). | Period of latent stimulation.
    -------------------------------+-------------------------------
                  70°              |           1/5 second
                  50°              |           1/3 second
                  35°              |           2/5 second
                  20°              |           1/2 second
    -------------------------------+-------------------------------

In the case of each observation, several shocks were administered
before the latent period was taken, in order to decrease this period
to its _minimum_ by the staircase action. When this is not done, the
latent period at 20° may be as long as 1-1/5 seconds; but soon after
this irritability disappears.

The extraordinary sluggishness of the latent period at very low
temperatures is fully equalled by the no less extraordinary
sluggishness of the contraction.

In order to render apparent the degree in which both these effects
are produced, I here append a pair of tracings which were procured
from the same piece of tissue when exposed to the different
temperatures named. (N.B.--The seconds are wrongly marked in Fig. 7;
they ought to be the same as in Fig. 6.)

  [Illustration: Fig. 6.]

  [Illustration: Fig. 7.]

I may as well state here that in water at all temperatures, within
the limits where responses to stimuli are given at all, the staircase
action admits of being equally well produced; but in order to
procure the _maximum_ effect for any given temperature, the rate at
which the successive stimuli are thrown in must be quicker in warm
than in cold water.




CHAPTER IV.

EXPERIMENTS IN SECTION OF COVERED-EYED MEDUSÆ.

   _Amount of Section which the Neuro-muscular Tissues of the
   Medusæ will endure without suffering Loss of their Physiological
   Continuity._


The extent to which the neuro-muscular tissues of the Medusæ may
be mutilated without undergoing destruction of their physiological
continuity is in the highest degree astonishing. For instance, to
begin with the covered-eyed Medusæ, I shall briefly state three modes
of section, the results of which serve to show in a striking manner
the fact in question.

  [Illustration: Fig. 8.]

  [Illustration: Fig. 9.]

The annexed woodcuts represent the umbrella of Aurelia aurita, with
its manubrium cut off at the base, and the under or concave surface
of the umbrella exposed to view, shewing in the centre the ovaries,
and radiating from them the branched system of nutrient tubes. The
umbrella when fully expanded, as here represented, is about the size
of a soup plate, and, as previously stated, all the marginal ganglia
are aggregated in the eight marginal bodies or lithocysts. Therefore
if the reader will imagine the first of the diagrams (Fig. 8) to
be overspread with a disc of muslin, the fibres and mesh of which
are finer than those of the finest and closest cobweb, and if he
will imagine the mesh of these fibres to start from these marginal
ganglia, he will gain a tolerably correct idea of the lowest nervous
system in the animal kingdom. Now, suppose that seven of these eight
ganglia are cut out, the remaining one then continues to supply its
rhythmical discharges to the muscular sheet of the bell, the result
being, at each discharge, two contraction waves, which start at the
same instant, one on each side of the ganglion, and which then course
with equal rapidity in opposite directions, and so meet at the point
of the disc which is opposite to the ganglion. Suppose, now, a number
of radial cuts are made in the disc according to such a plan as this
(Fig. 9), wherein every radial cut deeply overlaps those on either
side of it. The contraction waves which now originate from the
ganglion must either become blocked and cease to pass round the disc,
or they must zigzag round and round the tops of these overlapping
cuts. Now, remembering that the passage of these contraction waves
is presumably dependent on the nervous network progressively
distributing the ganglionic impulse to the muscular fibres, surely
we should expect that two or three overlapping cuts, by completely
severing all the nerve-fibres lying between them, ought to destroy
the functional continuity of these fibres, and so to block the
passage of the contraction wave. Yet this is not the case; for even
in a specimen of Aurelia so severely cut as the one here represented,
the contraction waves, starting from the ganglion, continued to
zigzag round and round the entire series of sections.

  [Illustration: Fig. 10.]

The second mode of section to which I have alluded is as follows
(Fig. 10). The central circle (_x_) stands for an open space cut out
of the umbrella; the outer circle indicates the margin of the animal,
with all lithocysts save one (_l_) removed; and the median circular
line represents a cut. It will be seen that the effect of this cut is
almost completely to sever the mass of tissue at _z_ from the rest of
the umbrella, the only connection between them being the narrow neck
of tissue at _z_. Yet, in the case to which I refer, the contraction
waves emanating from _l_ passed in the directions represented by
the arrows without undergoing any appreciable loss of vigour. Upon
completing the circular cut at _z_, the ring of tissue (_y z_) became
totally paralyzed, while the outer circle, of course, continued
its contractions as before. Now, the neck of tissue at _z_ measured
only one-eighth of an inch across, while the ring of tissue (_y z_),
when cut through and straightened out upon the table, measured one
inch across and sixteen inches in length; that is to say, sixteen
square inches of tissue derived its impulse to vigorous contractions
through a channel one-eighth of an inch wide, notwithstanding that
the latter was situated at the furthest point of the circle from the
discharging lithocyst which the form of the section rendered possible.

  [Illustration: Fig. 11.]

Lastly, the third mode of section is represented in the next cut.
Here seven of the marginal ganglia having been removed as before,
the eighth one was made the point of origin of a circumferential
section, which was then carried round and round the bell in the
form of a continuous spiral--the result, of course, being this long
ribbon-shaped strip of tissue with the ganglion at one end and the
remainder of the swimming-bell at the other. Well, as before, the
contraction-waves always originated at the ganglion; but now they
had to course all the way along the strip until they arrived at its
other extremity; and, as each wave arrived at that extremity, it
delivered its influence into the remainder of the swimming-bell,
which thereupon contracted. Now, in this experiment, when the spiral
strip is only made about half an inch broad, it may be made more than
a yard long before all the bell is used up in making the strip; and
as nothing can well be imagined as more destructive of the continuity
of a nerve-plexus than this spiral mode of section must be, we cannot
but regard it as a very remarkable fact that the nerve-plexus should
still continue to discharge its function. Indeed, so remarkable
does this fact appear, that to avoid accepting it we may well
feel inclined to resort to another hypothesis, namely, that these
contraction-waves do not depend for their passage on the nervous
network at all, but that they are of the nature of the muscle-waves,
or of the waves which we see in undifferentiated protoplasm,
where all parts of the mass being equally excitable and equally
contractile, however severely we cut the mass, as long as we do not
actually divide it, contraction-waves will pass throughout the whole
mass. But this very reasonable hypothesis of the contraction-waves in
the Medusæ being possibly nothing more than muscle-waves is negatived
by other facts, which I shall now proceed to state.

In the first place, if a number of experiments be tried in any of
the three modes of section above described, it will be found that
extreme variations are manifested as regards the degree of tolerance.
In the spiral mode of section, for instance, it will sometimes happen
that the contraction-wave will become blocked when the contractile
strip is only an inch long, while in other cases (as in the one
represented) the wave will continue to pass through a strip more
than thirty times that length; and between these two extremes there
are all possible grades of tolerance. Now it seems to me that if the
tissue through which these contraction-waves pass is supposed (as far
as they are concerned) to be of a functionally homogeneous nature,
no reason can be assigned why there should be such great differences
in the endurance of the tissue in different individual cases; while,
if we suppose that the passage of the contraction-waves is more or
less dependent on the functional activity of the nervous plexus which
we know from microscopical examination to be present, we encounter
no such difficulty; for it is almost to be expected that in some
cases it would happen that important nerves would soon be encountered
by the section, while in other cases it would happen that such
nerves would escape the section for a longer distance. It is indeed
incredible that any one nerve should happen to pursue a spiral course
twice or thrice round the umbrella, and at the same time happen to be
concentric with the course pursued by the section; but, as we shall
presently see, such an hypothesis as this is not necessary to account
for the facts.

Again, in the second place, strong evidence that the passage of
the contraction-waves is dependent on the functional activity
of the nervous plexus, and therefore that they are not merely
muscle-waves, is furnished by the fact that at whatever point in
a spiral strip which is being progressively elongated by section
the contraction-wave becomes blocked, the blocking is sure to
take place _completely_ and _exclusively_ at that point. Now, as
I have tried this experiment a great number of times, and always
tried it by carefully feeling the way round (_i.e._ only making a
very short continuation of the cut after the occurrence of each
contraction-wave, and so very precisely localizing the spot at
which the contraction-wave ceased to pass), I can scarcely doubt
that in every case the blocking is caused by the cutting through of
nerves.[13]

  [13] In a highly interesting paper recently published by Dr. W.
  H. Gaskell, F.R.S., on "The Innervation of the Heart" (_Journ.
  of Physiol._, vol. iv. p. 43, _et seq._), it is shown that the
  experiments in section thus far described yield strikingly
  similar results when performed upon the heart of the tortoise and
  the heart of the skate. Dr. Gaskell inclines to the belief that
  in these cases the contraction-waves are merely muscle-waves.
  There is one important fact, however, which even here seems to
  me to indicate that the propagation of the wave is at least in
  some measure dependent on nervous conduction. This fact is, that
  after a contraction-wave has been blocked by the activity of a
  spiral or other form of section, it may again be made to force a
  passage under the influence of vagus stimulation. Moreover, in
  a paper still more recently published by Drs. Brunton and Cash
  on "Electrical Stimulation of the Frog's Heart" (_Proc. Roy.
  Soc._, vol. xxxv., No. 227, p. 455, _et seq._) it is remarked,
  "Another interesting consideration is, whether the stimulus
  which each cavity of the heart transmits to the succeeding one
  consists in the propagation of an actual muscular wave, or in
  the propagation of an impulse along the nerves. The observations
  of Gaskell have given very great importance to the muscular wave
  occurring in each cavity of the heart of cold-blooded animals
  as a stimulus to the contraction of the next succeeding cavity.
  Our observations appear to us to show that, while this in an
  important factor, it is not the only one in the transmission of
  stimuli.... We consider that stimuli are also propagated from one
  chamber of the heart to another through nervous channels: thus we
  find that irritation of the venus sinus will sometimes produce
  simultaneous contractions of the auricle and ventricle, instead
  of the ventricular beat succeeding the auricular in the ordinary
  way. This we think is hardly consistent with the hypothesis, that
  a stimulus consists of the propagation of a muscular wave only
  from the auricle to the ventricle."

  [Illustration: Fig. 12.]

But, lastly, the strongest evidence in favour of this view as
afforded by the following observations. At the beginning of this
treatise I stated that the distinguishing function of nerve consists
in its power of conducting stimuli to a distance, irrespective of
the passage of a contraction-wave; and I may here add that when
a stimulus so conducted reaches a ganglion, or nerve-centre, it
causes the ganglion to discharge by so-called "reflex action."
Now, this distinguishing function of nerve can plainly be proved
to be present in the Medusæ. For instance, take such a section of
Aurelia as this one (Fig. 12), wherein the bell has been cut into
the form of a continuous parallelogram of tissue with the ovaries
and a single remaining ganglion at one end. (The cuts interposed in
the parallelogram may, for the present, be disregarded.) Now, if
the end marked _a_ of the neuro-muscular sheet most remote from
the ganglion be gently brushed with a camel's hair brush--_i.e._
too gently to start a responsive contraction-wave--the ganglion at
the other end will shortly afterwards discharge, as shown by its
starting a contraction-wave at its own end of the parallelogram,
_b_; thus proving that the stimulus caused by brushing the tissue at
the other end, _a_, must have been conducted all the way along the
parallelogram to the terminal ganglion, _b_, so causing the terminal
ganglion to discharge by reflex action. Indeed, in many cases, the
passage of this nervous wave of stimulation admits of being _seen_.
For the numberless tentacles which fringe the margin of Aurelia are
more highly excitable than is the general contractile tissue of the
bell; so that on brushing the end _a_ of the parallelogram remote
from the ganglion, the tentacles at this end respond to the stimulus
by a contraction, then those next in the series do the same, and so
on--a wave of contraction being thus set up in the tentacular fringe,
the passage of which is determined by the passage of the nervous wave
of stimulation in the superjacent nervous network. This tentacular
wave is in the illustration represented as having traversed nearly
half the whole distance to the terminal ganglion, and when it reaches
that ganglion it will cause it to discharge by reflex action, so
giving rise to a visible wave of muscular contraction passing in the
direction _b a_, opposite to that which the nervous or tentacular
wave had previously pursued. Now this tentacular wave, being an
optical expression of a passage of a wave of stimulation, is a sight
as beautiful as it is unique; and it affords a first-rate opportunity
of settling this all-important question, namely, Will this conductile
or nervous function prove itself as tolerant towards a section of
the tissue as the contractile or muscular function has already
proved itself to be? For, if so, we shall gain nothing on the side
of simplicity by assuming that the _contraction_-waves are merely
muscle-waves, so long as the _conduction_ or _undoubtedly nervous_
waves are equally able to pass round sections interposed in their
path. Briefly, then, I find that the nervous waves of stimulation
are quite as able to pass round these interposed sections as are the
waves of contraction. Thus, for instance, in this specimen (Fig. 12),
the tentacular wave of stimulation continued to pass as before, even
after I had submitted the parallelogram of tissue to the tremendously
severe form of section which is represented in the illustration;
and this fact, in my opinion, is one of the most important that has
been brought to light in the whole range of invertebrate physiology.
For what does it prove? It proves that the distinguishing function
of nerve, where it first appears upon the scene of life, admits of
being performed vicariously to almost any extent by all parts of
the same tissue-mass. If we revert to our old illustration of the
muslin as representing the nerve-plexus, it is clear that, however
much we choose to cut the sheet of muslin with such radial or spiral
sections as are represented in the illustrations, one could always
trace the threads of the muslin with a needle round and round the
disc, without once interrupting the continuity of the tracing; for
on coming to the end of a divided thread, one could always double
back on it and choose another thread which might be running in the
required direction. And this is what we are now compelled to believe
takes place in the fibres of this nervous network, if we assume that
these visible fibres are the only conductile elements which are
present. Whenever a stimulus wave reaches a cut, we must conclude
that it doubles back and passes into the neighbouring fibres, and so
on, time after time, till it succeeds in passing round and round any
number of overlapping cuts.

This is, no doubt, as I have already observed, a very remarkable
fact; but it becomes still more so when we have regard to the
histological researches of Professor Schäfer on the structural
character of this nerve-plexus. For these researches have shown that
the nerve-fibres which so thickly overspread the muscular sheet of
Aurelia do not constitute a true plexus, but that each fibre is
comparatively short and nowhere joins with any of the other fibres;
that is to say, although the constituent fibres of the network
cross and recross one another in all directions--sometimes, indeed,
twisting round one another like the strands of a rope--they can
never be actually seen to join, but remain anatomically insulated
throughout their length. So that the simile by which I have
represented this nervous network--the simile, namely, of a sheet
of muslin overspreading the whole of the muscular sheet--is, as a
simile, even more accurate than has hitherto appeared; for just as
in a piece of muslin the constituent threads, although frequently
meeting one another, never actually coalesce, so in the nervous
network of Aurelia, the constituent fibres, although frequently in
contact, never actually unite.

Now, if it is a remarkable fact that in a fully differentiated
nervous network the constituent fibres are not improbably capable
of vicarious action to almost any extent, much more remarkable does
this fact become when we find that no two of these constituent
nerve-fibres are histologically continuous with one another. Indeed,
it seems to me we have here a fact as startling as it is novel. There
can scarcely be any doubt that _some_ influence is communicated from
a stimulated fibre _a_ to the adjacent fibre _b_ at the point where
these fibres come into close apposition. But what the nature of the
process may be whereby a disturbance in the excitable protoplasm of
_a_ sets up a sympathetic disturbance in the anatomically separate
protoplasm of _b_, supposing it to be really such--this is a question
concerning which it would as yet be premature to speculate. But I
think it may be well for physiologists to keep awake to the fact that
a process of this kind probably takes place in the case of these
nerve-fibres. For it thus becomes a possibility which ought not to
be overlooked, that in the fibres of the spinal cord, and in ganglia
generally, where histologists have hitherto been unable to trace any
anatomical or structural continuity between cells and fibres, which
must nevertheless be supposed to possess physiological or functional
continuity--it thus becomes a possibility that in these cases no such
anatomical continuity exists, but that the physiological continuity
is maintained by some such process of physiological induction as
probably takes place among the nerve-fibres of Aurelia.[14]

  [14] That it can scarcely be _electrical induction_ would seem
  to be shown by the fact that such effects can only be produced
  on nerves by strong currents, and also by the fact that the
  saline tissues of the swimming-bell must short-circuit any feeble
  electrical currents as soon as they are generated.

I have now to detail another fact of a very puzzling nature, but
one which is certainly of importance. When the spiral section
is performed on Aurelia aurita, and when, as a consequence, the
contraction-waves which traverse the elongating strip become at some
point suddenly blocked, if the section be stopped at this point it
not unfrequently happens that after a time the blocking suddenly
ceases, the contraction-waves again passing from the strip into
the umbrella as freely as they did before the section reached the
point at which the blocking occurred. The time required for this
restoration of physiological continuity is very variable, the limits
being from a few seconds to an hour or more; usually, however, it
is from two to four minutes. This process of re-establishing the
physiological connections, although rapid, is not so instantaneous
as is that of their destruction by section. In general it requires
the passage of several contraction-waves before the barrier to the
passage of succeeding waves is completely thrown down. The first
wave which effects a passage appears to have nearly all its force
expended in overcoming the barrier, the residue being only sufficient
to cause a very feeble, and sometimes almost imperceptible,
contraction of the umbrella. The next wave, however, passes across
the barrier with more facility, so that the resulting contraction
of the umbrella is more decided. The third wave, again, causes a
still more pronounced contraction of the umbrella; and so on with
all succeeding waves, until every trace of the previous blocking has
disappeared. When this is the case, it generally happens that the
strip will again admit of being elongated for a short distance before
a blocking of the contraction-waves again supervenes. Sometimes it
will be found that this second blockage will also be overcome, and
that the strip will then admit of being still further elongated
without the passage of the waves being obstructed; and so on
occasionally for three or four stages.

The same series of phenomena may be shown in another way. If
a contractile strip of tolerable length be obtained, with the
waves passing freely from one end to the other, and if a series
of parallel and equidistant cuts be made along one side of the
strip, in a direction at right angles to the length, and each cut
extending two-thirds of the breadth of the strip, the chances are
in favour of the contraction-waves being wholly unaffected by the
sections, however numerous these may be. But now, if another series
of parallel and equidistant cuts of the same length as the first
ones, and alternating with them, be made along the other side
of the contractile strip, the result is, of course, a number of
interdigitating cuts; and it is easy to see that by beginning with a
few such cuts and progressively increasing their number, a point must
somewhere be reached at which one portion will become physiologically
separated from the rest. The amount of such section, however, which
contractile strips will sometimes endure is truly surprising. I
have seen such a strip twenty inches long by one and a half inches
wide with ten such cuts along each side, and the contraction-waves
passing without impediment from end to end. But what I wish more
especially to observe just now is, that by progressively increasing
the number of such interdigitating cuts up to the point at which the
contraction-wave is blocked, and then leaving the tissue to recover
itself, in many cases it will be observed that the blocking is sooner
or later overcome; that on then adding more interdigitating cuts the
blocking again supervenes; but that in time it may again be overcome,
and so on. It is, however, comparatively rare to find cases in which
blocking is overcome twice or thrice in succession.

Section is not the only way in which blocking of waves may be caused
in contractile strips. I find that pressure, even though very gentle,
exerted on any part of a strip causes a blocking of the waves at that
part, even after the pressure has been removed. If the pressure has
been long continued, after its removal the blocking will probably
be permanent; but if the pressure has been only of short duration,
the blocking will most likely be transitory. Even the slight strains
caused by handling a contractile strip in the air are generally
followed by a decrease in the rate of the waves, and sometimes by
their being completely blocked. Other methods by which the passage of
waves in contractile strips admits of being blocked will be alluded
to farther on.

Now, in all these cases of temporary blocking we must conclude that
when the contraction-waves succeed in at last forcing a passage,
some structural change has taken place in the tissue at the
region of injury, corresponding with the functional change of the
re-establishment of physiological continuity. The waves previously
stopped at a certain point of section or otherwise, after beating for
a time on the physiological barrier, are at last able to throw down
the barrier, and thenceforward to proceed on their way unhindered.
What, then, is the nature of the structural change which has taken
place?

In the early days of this research, before the presence of a
nerve-plexus had been proved histologically, I argued in favour of
such a plexus on the grounds furnished by many of the foregoing
experiments; and at a lecture given in the Royal Institution I
ventured to say that if a careful investigation of the histology of
these tissues should fail to show the plexus which the result of
those experiments required me to assume, we should still be compelled
to suppose that the plexus was present, although not sufficiently
differentiated to admit of being seen. I further ventured to
suggest that in this event the facts just stated might be taken to
substantiate the theory of Mr. Herbert Spencer on the genesis of
nerve-tissue in general. This theory is that which supposes incipient
conductile tissues, or rudimentary nerve-fibres, to be differentiated
from the surrounding contractile tissues, or homogeneous protoplasm,
by a process of integration which is due simply to use; so that just
as water continually widens and deepens the channel through which
it flows, so molecular or nervous waves of stimulation, by always
flowing through the same tissue-tracts, tend ever more and more to
excavate for themselves functionally differentiated lines of passage.

Such being Mr. Spencer's theory, I applied it hypothetically to the
above facts in the words which I may here quote.

"As the successive waves beat rhythmically on the area of
obstruction, more or less of the molecular disturbances must every
time be equalized through these lines of discharge, which from the
first have been almost sufficient to maintain the physiological
continuity of the tissue. Therefore, according to the hypothesis,
every wave that is blocked imposes upon these particular lines of
discharge a much higher degree of functional activity than they were
ever before required to exercise; and this greater activity causing
in its turn greater permeability, a point will sooner or later arrive
at which these lines of discharge, from having been _almost_, become
_quite_ able to draft off sufficient molecular motion, or stimulating
influence, to carry on the contraction-waves beyond the areas of
previous blocking. In such instances, of course, we should expect
to find what I always observed to be the case, viz. that the first
contraction-wave which passes the barrier is only very feeble, the
next stronger, the next still stronger, and so on, according as the
new passage becomes more and more permeable by use, until at last
the contraction-waves pour over the original barrier without any
perceptible diminution of their force. In some cases, by exploring
with graduated stimuli and needle-point terminals, I was able to
ascertain the precise line through which this eruption of stimulating
influence had taken place."

I have now to state the effect upon this hypothesis which in my
opinion has been produced by the histological proof that the plexus
in question is composed of fully differentiated nerves. Briefly,
then, I think that the hypothesis still holds to the extent of
being the only one available whereby to explain the facts; but
there is this great difference, viz. that the hypothesis need not
now be applied to the genesis of nerve-tissue out of comparatively
undifferentiated contractile tissue, but rather to the increasing of
the functional activity of already well-differentiated nerve-tissue.
In other words, we have not now to suppose that nerve-tissue is
formed _de novo_ in the region of blocking; but, in my opinion,
we still have to suppose that the nerve-fibres which were already
there have their functional capabilities so far improved by the
greater demand imposed upon them, that whereas at first they were
not able, eventually they became able to draft off enough molecular
disturbance to carry on a stimulus adequate to cause a muscular
contraction. It will be observed that the difference thus expressed
is one of considerable importance; for now the facts cease to lend
any countenance to Mr. Spencer's theory touching the formation
of nerves out of protoplasm, or other contractile material. They
continue, however, to countenance his views touching the improvement
of functional capacity which nerve-fibres, when already formed,
undergo by use; and this, which is in itself an important matter, is
the point with which I was mainly concerned in the lecture of the
Royal Institution just alluded to. For, as I then observed, in this
theory of nerve-fibres becoming more and more functionally developed
by use, we probably have a physical explanation, which is as full and
complete as such an explanation can ever be, of the genesis of mind.
"For from the time that intelligence first dawned upon the scene of
life, whenever a new relation had to be established in the region of
mind, it could only be so established in virtue of some new line of
discharge being excavated through the substance of the brain. The
more often this relation had to be repeated in the mind, the more
often would this discharge require to take place in the brain, and
so the more easy would every repetition of the process become....
Thus it is, according to the theory, that there is always a precise
proportion between the constancy with which any relations have
been joined together during the history of intelligence, and the
difficulty which intelligence now experiences in trying to conceive
of such relations as disjoined. Thus it is that, even during the
history of an individual intelligence, 'practice makes perfect,' by
frequently repeating the needful stimulations along the same lines
of cerebral discharge, so rendering the latter ever more and more
permeable by use. Thus it is that a child learns its lessons by
frequently repeating them; and thus it is that all our knowledge is
accumulated."[15]

  [15] I have associated the above theory of nerve-genesis with the
  name of Mr. Spencer, because it occupies so prominent a place in
  his "Principles of Psychology." But from what I have said in the
  text, I think it is clear that the theory, as presented by Mr.
  Spencer, consists of two essentially distinct hypotheses--the
  one relating to the formation of nerve-tissue out of protoplasm,
  and the other to the increase of functional capacity in a
  nerve-fibre by use (a third hypothesis of Mr. Spencer relating to
  the formation of ganglion-tissue does not here concern us). The
  latter hypothesis, however, ought not to be associated with Mr.
  Spencer's name without explaining that it has likewise occurred
  to other writers, the first of which, so far as I can ascertain,
  was Lamarck, who says, "Dans toute action, le fluide des nerfs
  qui la provoque, subit un mouvement de déplacement qui y donne
  lieu. Or, lorsque cette action a été plusieurs fois répétée, il
  n'est pas douteux que le fluide qui l'a exécutée, ne se soit
  frayé une route, qui lui devient alors d'autant plus facile à
  parcourir, qu'il l'a effectivement plus souvent franchie, et
  qu'il n'ait lui-même une aptitude plus grand à suivre cette route
  frayée que celles qui le sont moins." ("Phil. Zool.," tom ii. pp.
  318-19.)


_Rate of Transmission of Stimuli._

The rate at which contraction-waves traverse spiral strips of Aurelia
is variable. It is largely determined by the length and width of
the strip; so that the best form of strip to use for the purpose
of ascertaining the _maximum_ rate is one which I shall call the
circular strip. A circular strip is obtained by first cutting out
the central bodies (_i.e._ manubrium and ovaries), and then, with a
single radial cut, converting the animal from the form of an open
ring to that of a continuous band. I distinguish this by the name
"circular" band or strip, because the two ends tend to preserve their
original relative positions, so giving the strip more or less of a
circular form. Such a strip has the advantage of presenting all the
contractile tissue of the swimming-bell in one continuous band of
the greatest possible width, and is therefore the form of strip that
yields the _maximum_ rate at which contraction-waves are able to
pass. The reason why the _maximum_ rate should be the one sought for
is because this is the rate which must most nearly approximate the
natural rate of contraction-waves in the unmutilated animal. This
rate, at the temperature of the sea and with vigorous specimens, I
find to be eighteen inches per second.

In a circular strip the rate of the waves is uniform over the whole
extent of the strip; so that the time of their transit from one
point to another varies directly as the length of the strip. But
on now narrowing such a strip, although the rate is thus slowed,
the relation between the narrowing and the slowing is not nearly so
precise as to admit of our saying that the rate varies inversely as
the width. The following figure will serve to show the proportional
extent to which the passage of contraction-waves is retarded by
narrowing the area through which they pass:--

  [Illustration: Fig. 13.

  Time from end to end of a circular strip ...

  Time after width has been reduced to one-half ...

  Time after width has been reduced to one-quarter ...

  Time after width has been reduced to one-eighth ...

  Fig. 14.

  In such experiments it generally happens, as here represented,
  that reducing the width of a circular strip by one-half produces
  no effect, or only a slight effect, on the rate, while further
  narrowing to the degree mentioned produces a conspicuous
  effect. I may also state that if, as occasionally happens, the
  immediate effect of narrowing a circular strip to one-half is to
  temporarily block the contraction-waves, when the latter again
  force their passage, their rate is slower than it was before. It
  seems as if the more pervious tissue tracts having been destroyed
  by the section, the less pervious ones, though still able to
  convey the contraction-wave, are not able to convey it so rapidly
  as were the more pervious tracts.]

  [Illustration: Fig. 14.]

In order to ascertain whether certain zones of the circular
contractile sheet in all individuals habitually convey more of
the contractile influence than do other zones, I tried a number
of experiments in the following form of section. Having made a
circular strip, I removed all the lithocysts save one, and then cut
the strip as represented in Fig. 14. On now stimulating the end
_a_, or on watching the lithocyst there discharge, the resulting
contraction-wave would be observed to bifurcate at _b_, and then pass
on as two separate waves through the zones, _b_ _c_, _b_ _d_. Now,
as these two waves were started at the same instant of time, they
ran, as it were, a race in the two zones, and in this way the eye
could judge with perfect ease which wave occupied the shortest time
in reaching its destination. This experiment could be varied by again
bisecting each of these two zones, thus making four zones in all, and
four waves to run in each race. A number of experiments of this kind
showed me that there is no constancy in the relative conductivity
of the same zones in different individuals. In some instances, the
waves occupy less time in passing through the zone _b_ _c_ than in
passing through the zone _b_ _d_; in other instances, the time in the
two zones is equal; and, lastly, the converse of the first-mentioned
case is of equally frequent occurrence. Very often the waves become
blocked in _b_ _c_, while they continue to pass in _b_ _d_, and
_vice versâ_. Now, all these various cases are what we might expect
to occur, in view of the variable points at which contraction-waves
become blocked in spiral strips, etc.; for if the contractile tissues
are not functionally homogeneous, and if the relatively pervious
conductile tracts are not constant as to their position in different
individuals, the results I have just described are the only ones
that could be yielded by the experiments in question. Considering,
however, that in these experiments the central zones are not so long
as the peripheral zones, I think it may fairly be said that the
conductile power of the latter is greater than that of the former;
for, otherwise, the above experiments ought to yield a large majority
of races won by the waves that course through the central zones,
and this is not the case. Indeed, it is surprising how often the
race is, as it were, neck and neck, thus showing that the relative
conductivity of all the zones is precisely adjusted to their relative
lengths; and forasmuch as in the unmutilated animal this adjustment
must clearly serve the purpose of securing to the contraction-wave
a passage of uniform rate over the whole radius of the umbrella,
I doubt not that, if it were possible to perform the race-course
section without interrupting any of the lines of conduction-tissue,
neck and neck races would be of invariable occurrence.

Interdigitating cuts, as might be expected, prolong the time of
contraction-waves in their passage through the tissue in which
the cuts are interposed. For example, in a spiral strip measuring
twenty-six inches in length, the time required for the passage of
a contraction-wave from one end to the other is represented by the
line _a_ _b_ in the annexed woodcut. But after twenty interdigitating
cuts had been interposed, ten on each side of the strip, the time
increased to _c_ _d_, the line _e_ _f_ representing one second. And
more severe forms of section are, of course, attended with a still
more retarding influence.

  [Illustration: Fig. 15.]

The effects of temperature on the rate of contraction-waves are
very striking. For instance, in a rather narrow strip measuring
twenty-eight inches long and one and a half inches wide, the
following variations in rate occurred:--

    -------------------------+-------------------------------
      Temperature of water.  |  Time occupied in passage of
                             |    contractile waves.
    -------------------------+-------------------------------
              26°            |          4 seconds.
              32°            |          3 seconds.
              42°            |          2-2/5 seconds.
              65°            |          2 seconds.
              75°            |          1-3/5 seconds.
              85°            |          Blocked.


Or, adopting again the graphic method of statement, these variations
may be represented as follows:--

  [Illustration: Fig. 16.

  26°......

  32°......

  42°......

  65°......

  75°......

  85°......

  One second ...]

Submitting a contractile strip to slight strains has also the
effect of retarding the rate of the waves while they pass through
the portions of the strip which have been submitted to strain. The
method of straining which I adopted was to pass my finger below
the strip, and then, by raising my hand, to bring a portion of
the strip slightly above the level of the water. The irritable or
contractile surface was kept uppermost, and therefore suffered a
gentle strain; for the weight of the tissue on either side of the
finger made the upper surface somewhat convex. By passing the finger
all the way along the strip in this way, the latter might be gently
strained throughout its entire length, the degree of straining
being determined by the height out of the water to which the tissue
was raised. Of course, if the strip is too greatly strained, the
contraction-waves become blocked altogether, as described above;
but shortly before this degree of straining was reached, I could
generally observe that the rate of the waves was diminished. To give
one instance, a contractile strip measuring twenty-two inches had
the rate of its waves taken before and after straining of the kind
described. The result was as follows:--

  [Illustration: Fig. 17. Before straining ...

  After straining ...

  One second ... ]

Immediately after severe handling of this kind, the retardation
of contraction-waves, is sometimes even more marked than here
represented; but I think this may be partly due to shock, for on
giving the tissue a little while to recover, the rate of the waves
becomes slightly increased.

Anæsthetics likewise have the effect of slowing the rate of
contraction-waves before blocking them. Taking, for instance, the
case of chloroform, a narrow spiral strip between one and two feet
long was immersed in sea-water containing a large dose of the
anæsthetic; the observations being taken at six seconds' intervals,
the following were the results:--

  [Illustration: Fig 18.

  Normal water ...

  Six seconds after transference to chloroform

  Six seconds later ...

  Six seconds later ...

  Six seconds later ...

  Six seconds later ...

  One second ... ]

In such experiments, the recovery of the normal rate in unpoisoned
water is gradual. Taking, for instance, the case of a spiral strip
in morphia (Fig. 19), it will be seen that the original rate did not
fully return. Some substances, however, exert a more marked permanent
effect of this kind than do weak solutions of morphia. Here, for
instance, is an experiment with alcohol (see Fig. 20).

  [Illustration: Fig. 19

  In normal water ...

  Quarter of an hour after exposed to morphia ...

  One minute after strengthening dose ...

  Four minutes later, and just before blocking of wave ...

  Fifteen seconds later, wave continuing blocked ...

  Immediately after passage of wave on restoration to normal
  sea-water ...

  Four minutes later ...

  Quarter of an hour later ...

  An hour later ...

  One second ... ]

  [Illustration: Fig. 20

  In normal water ...

  Quarter of an hour after exposure to weak dose ...

  Two minutes after strengthening of dose ...

  Five minutes later, and just before blocking of wave ...

  Fifteen seconds later, wave continuing blocked ...

  Immediately after passage of wave on restoration to normal
  sea-water ...

  An hour later ...

  One second ... ]

From these experiments, however, it must not be definitely concluded
that it is the anæsthesiating property of such substances which
exerts this slowing and blocking influence on contraction-waves,
for I find that almost any foreign substance, whether or not an
anæsthetic, will do the same. That nitrite of amyl, caffein, etc.,
should do so, one would not be very surprised to hear; but it might
not so readily be expected that strychnine, for instance, should
block contraction-waves; yet it does so, even in doses so small as
only just to taste bitter. Nay, even fresh water completely blocks
contraction-waves after the strip has been exposed to its influence
for about half an hour, and exerts a permanently slowing effect after
the tissue is restored to sea-water. These facts show the extreme
sensitiveness of the neuro-muscular tissues of the Medusæ to any
change in the character of their surrounding medium, a sensitiveness
which we shall again have occasion to comment upon when treating of
the effects of poisons.

In conclusion, I may mention an interesting fact which is probably
connected with the summation of stimuli before explained. When a
contractile strip is allowed to rest for a minute or more, and
when a wave is then made to traverse it, careful observation will
show that the passage of the first wave is slower than that of its
successor, provided the latter follows the former after not too great
an interval of time. The difference, however, is exceedingly slight,
so that to render it apparent at all the longest possible strips
must be used, and even then the experimenter may fail to detect the
difference, unless he has been accustomed to signalling, by which
method all these observations on rate have to be made.


_Stimulus-waves._

The rate of transmission of tentacular waves is only one-half that of
contraction-waves, viz. nine inches a second. This fact appeared to
me very remarkable in view of the consideration that the tentacular
wave is the optical expression of a stimulus-wave, and that there can
be no conceivable use in a stimulus-wave being able to pass through
contractile tissue independently of a contraction-wave, unless the
former is able to travel more rapidly than the latter; for the only
conceivable use of the stimulus-wave is to establish physiological
harmony between different parts of the organism, and if this wave
cannot travel more rapidly than a contraction-wave which starts from
the same point, it would clearly fail to perform this function.

In view of this anomaly, I was led to think that if the rate of the
stimulus-wave is dependent in a large degree on the strength of the
stimulus that starts it, the slow rate of nine inches a second might
be more than doubled, if, instead of using a stimulus so gentle as
not to start a contraction-wave, I used a stimulus sufficiently
strong to do this. Accordingly I chose a specimen of Aurelia wherein
the occurrence of tentacular waves was very conspicuous, and found,
as I had hoped, that every time I stimulated too gently to start a
contraction-wave, the tentacular wave travelled only at the rate
of nine inches a second; whereas, if I stimulated with greater
intensity, I could always observe the tentacular wave coursing an
inch or two in front of the contraction-wave.

It is remarkable, however, that in this, as in all the other
specimens of Aurelia which I experimented upon, the reflex response
of the manubrium was equally long, whatever strength of stimulus
I applied to the umbrella; or, at any rate, the time was only
slightly less when a contraction-wave had passed than when only a
tentacular wave had done so. The loss of time, however, appears to
take place in the manubrium itself, where the rate of response is
astonishingly slow. Thus, if one lobe be irritated, it is usually
from four to eight seconds before the other lobes respond. But the
time required for such sympathetic response may be even more variable
than this--the limits I have observed being as great as from three
to ten seconds. In all cases, however, the response, when it does
occur, is sudden, as if the distant lobe had then for the first
time received the stimulus. Moreover, one lobe--usually one of those
adjacent to the lobe directly irritated--responds before the other
two, and then a variable time afterwards the latter also respond.
This time is, in most cases, comparatively short, the usual limits
being from a quarter of a second to two seconds. How much of these
enormous intervals is occupied by the period of ganglionic latency,
and how much by that of transmission, it is impossible to say; but I
have determined that the rate of transmission from the end of a lobe
of the manubrium to a lithocyst (deducting a second for the double
period of latent stimulation) is the same as the rate of a tentacular
wave, viz. nine inches a second. The presumption, therefore, is that
the immense lapse of time required for reflex response on the part of
the manubrium is required by the lobular ganglia, or whatever element
it is that here performs the ganglionic function.


_Exhaustion._

In various modes of section of Aurelia I have several times observed
a fact that is worth recording. It sometimes happens that when the
connecting isthmus between two almost severed areas of excitable
tissue is very narrow, the passage of contraction-waves across the
isthmus depends upon the freshness, or freedom from exhaustion,
of the tissue which constitutes the isthmus. That is to say, on
faradizing one of the two tissue-areas which the isthmus serves
to connect, the resulting contraction-waves will at first pass
freely across the isthmus; but after a time it may happen in some
preparations that every now and then a contraction-wave fails to
pass across the isthmus. When this is the case, if the stimulation
is still continued, a greater and greater proportion of waves fail
to pass across the isthmus, until perhaps only one in every five or
six becomes propagated from the one area to the other. If single
induction-shocks be then substituted for the faradaic stimulation,
it may be found that by leaving an interval of four or five seconds
between the successive shocks, every wave that is started in the
one area will be propagated across the isthmus to the other area.
But if the interval between the successive shocks be reduced to
two or three seconds, every now and then a wave will fail to pass
across the isthmus; and if the interval be still further reduced to
one second, or half a second, comparatively few of the waves will
pass across. Now, however, if the tissue be allowed five minutes'
rest from stimulation, and the single shocks be thrown in at one
second's intervals, all the first six or ten waves will pass across
the isthmus, after which they begin to become blocked as before. It
may be observed also that when the waves are thus blocked, owing to
exhaustion of the connecting isthmus, they may again be made to force
a passage by increasing the intensity of the stimulation, and so
giving rise to stronger waves having a greater power of penetration.
Thus, on re-enforcing the electrical stimulus with the simultaneous
application of a drop of spirit, the resulting waves of contraction
are almost sure to pass across the isthmus, even though this has been
exhausted in the manner just described.


_Ganglia appearing to assert their Influence at a Distance from their
own Seat._

Another fact, which I have several times noticed during my sections
of Aurelia, also deserves to be recorded. I have observed it under
several modes of section, but it will be only necessary to describe
one particular case.

In the Aurelia of a portion of which the accompanying woodcut (p.
102) is a representation, seven of the lithocysts were removed,
while the remaining one was almost entirely isolated from the
general contractile tissue by the incisions _aa_, _bb_, _cc_. The
lithocyst continued to animate the tissue-area _xxxx_, and through
the connecting passage _y_ the contraction-waves spread over the
remainder of the sub-umbrella tissue _zzzz_. So far, of course, the
facts were normal; but very frequently it was observed that the
contraction-waves did not start from the lithocyst, or from the area
_xxxx_, but from the point o in the area _zz_. After this origination
of the contraction-waves from the point _o_ had been observed a great
number of times, I removed the lithocyst. The effect was not only
to prevent the further origination of contraction-waves in the area
_xxxx_, but also to prevent their further origination from the point
_o_, the entire umbrella thus becoming paralyzed. Hence, before the
removal of the lithocyst, the contraction-waves which originated at
the point _o_, no less than those which originated at the lithocyst
itself, must in some way or other have been due to the ganglionic
influence emanating from the lithocyst and asserting itself at the
distant point _o_.

  [Illustration: Fig. 21.]

This property, which lithocysts sometimes present, of asserting their
ganglionic influence at a distance from their own locality, can only,
I think, be explained by supposing that at the point where under
these circumstances the contractions originate, there are situated
some scattered ganglionic cells of considerable functional power,
but yet not of power enough to originate contraction-waves unless
re-enforced by some stimulating influence, which reaches them from
the lithocyst through the nervous plexus.


_Regeneration of Tissues._

The only facts which remain to be stated in the present chapter
have reference to the astonishing rapidity with which the excitable
tissues of the Medusæ regenerate themselves after injury. In this
connection I have mainly experimented on Aurelia aurita, and shall,
therefore, confine my remarks to this one species.

If with a sharp scalpel an incision be made through the tenuous
contractile sheet of the sub-umbrella of Aurelia, in a marvellously
short time the injury is repaired. Thus, for instance, if such an
incision be carried across the whole diameter of the sub-umbrella,
so as entirely to divide the excitable tissues into two parts while
the gelatinous tissues are left intact, the result of course is that
physiological continuity is destroyed between the one half of the
animal and the other, while the form of the whole animal remains
unchanged--the much greater thickness of the uninjured gelatinous
tissues serving to preserve the shape of the umbrella. But although
the contractile sheet which lines the umbrella is thus completely
severed throughout its whole diameter, it again reunites, or heals
up, in from four to eight hours after the operation.




CHAPTER V.

EXPERIMENTS IN SECTION OF NAKED-EYED MEDUSÆ.


_Distribution of Nerves in Sarsia._

My experiments have shown that the nervous system in the naked-eyed
Medusæ is more highly organized, or integrated, than it is in the
covered-eyed Medusæ; for whereas in the latter I obtained no evidence
of the gathering together of nerve-fibres into definite bundles or
trunks (the plexus being evenly distributed over the entire surface
of the neuro-muscular sheet lining the umbrella), in the former I
found abundant evidence of this advance in organization. And as the
experiments in this connection serve to substantiate the histological
researches of Professors Haeckel, Schultz, Eimer, and Hertwig, in
as far as the distribution of the main nerve-trunks is concerned, I
shall here detail at some length the character and results of these
experiments in the case of Sarsia.

The occurrence of reflex action in Sarsia is of a very marked and
unmistakable character. I may begin by stating that when any part
of the internal surface of the bell is irritated, the manubrium
responds; but as there is no evidence of ganglia occurring in the
manubrium, this cannot properly be regarded as a case of reflex
action. But now the converse of the above statement is likewise true,
viz. that when any part of the manubrium is irritated, the bell
responds; and it is in this that the unequivocal evidence of reflex
action consists, for while the sympathy of the manubrium with the
bell is not in the least impaired by removing the marginal ganglia of
the latter, the sympathy of the bell with the manubrium is by this
operation entirely destroyed.

We have thus very excellent demonstration of the occurrence of reflex
action in the Medusæ. Further experiments show that the reflex action
occurs, not between the marginal ganglia and every part of the
manubrium, but only between the marginal ganglia and the point of the
bell from which the manubrium is suspended--it being only the pull
which is exerted upon this point when the manubrium contracts that
acts as a stimulus to the marginal ganglia. But the high degree of
sensitiveness shown by the marginal ganglia to the smallest amount
of traction of this kind is quite as remarkable as their lack of
sensitiveness to disturbances going on in the manubrium.

Turning now to the physiological evidence of the distribution of
nerves in Sarsia, when one of the four tentacles is very gently
irritated, it alone contracts. If the irritation be slightly
stronger, all the four tentacles, and likewise the manubrium,
contract. If one of the four tentacles be irritated still more
strongly, the bell responds with one or more locomotor contractions.
If in the latter case the stimulus be not too strong, or, better
still, if the specimen operated on be in a non-vigorous or in a
partly anæsthesiated state, it may be observed that a short interval
elapses between the response of the tentacles and that of the bell.
Lastly, the manubrium is much more sensitive to a stimulus applied
to a tentacle, or to one of the marginal bodies, than it is to a
stimulus applied at any other part of the nectocalyx.

These facts clearly point to the inference that nervous connections
unite the tentacles with one another and also with the manubrium;
or, perhaps more precisely, that each marginal body acts as a
co-ordinating centre between nerves proceeding from it in four
directions, viz. to the attached tentacle, to the margin on either
side, and to the manubrium. This, it will be observed, is the
distribution which Haeckel describes as occurring in Geryonia, and
Schultz as occurring in Sarsia. It is, further, the distribution to
which my explorations by stimulus would certainly point. But, in
order to test the matter still more thoroughly, I tried the effects
of section in destroying the physiological relations which I have
just described. These effects, in the case of the tentacles, were
sufficiently precise. A minute radial cut (only just long enough to
sever the tissues of the extreme margin) introduced between each pair
of adjacent marginal bodies completely destroyed the physiological
connection between the tentacles. If only three marginal cuts were
introduced, the sympathy between those two adjacent tentacles
between which no cut was made continued unimpaired, while the
sympathy between them and the other tentacles was destroyed.

The nervous connections between the tentacles and the manubrium
are of a more general character than those described between the
tentacles themselves; that is to say, severing the main radial
nerve-trunks produces no appreciable effect upon the sympathy between
the tentacles and the manubrium.

The nervous connections between the whole excitable surface of the
nectocalyx and the manubrium are likewise of this general character,
so that, whether or not the radial nerve-trunks are divided, the
manubrium will respond to irritation applied anywhere over the
internal surface of the nectocalyx. The manubrium, however, shows
itself more sensitive to stimuli applied at some parts of this
surface than it is to stimuli applied at other parts, although in
different specimens there is no constancy as to the position occupied
by these excitable tracts.


_Distribution of Nerves in Tiaropsis Indicans._

  [Illustration: Fig. 22.]

We have seen that in Sarsia reflex action obtains between the
manubrium and the nectocalyx; we shall now see that in Tiaropsis
indicans something resembling reflex action obtains between the
nectocalyx and the manubrium. The last-named species is a new one,
which I have described elsewhere, and I have called it "indicans"
from a highly interesting and important peculiarity of function
which is manifested by its manubrium. The Medusa in question measures
about one and a half inches in diameter, and is provided with a
manubrium of unusual proportional size, its length being about
five-eighths of an inch, and its thickness being also considerable.
Now, if any part of the nectocalyx be irritated, the following
series of phenomena takes place. Shortly after the application of
the stimulus, the large manubrium suddenly contracts--the appearance
presented being that of an exceedingly rapid crouching movement.
The crouching attitude in which this movement terminates continues
for one or two seconds, after which the organ begins gradually to
resume its former dimensions. Concurrently with these movements on
the part of the manubrium, the portion of the nectocalyx which has
been stimulated bends inwards as far as it is able. The manubrium
now begins to deflect itself towards the bent-in portion of the
nectocalyx; and this deflection continuing with a somewhat rapid
motion, the extremity of the manubrium is eventually brought, with
unerring precision, to meet the in-bent portion of the nectocalyx.
I here introduce a drawing of more than life-size to render a
better idea of this _pointing_ action by the manubrium to a seat
of irritation located in the bell. It must further be stated that
in the unmutilated animal such action is quite invariable, the
tapered extremity of the manubrium never failing to be placed on the
exact spot in the nectocalyx where the stimulation is being, or had
previously been, applied. Moreover, if the experimenter irritates
one point of the nectocalyx, with a needle or a fine pair of forceps
for instance, and while the manubrium is applied to that point he
irritates another point, then the manubrium will leave the first
point and move over to the second. In this way the manubrium may be
made to indicate successively any number of points of irritation;
and it is interesting to observe that when, after such a series
of irritations, the animal is left to itself, the manubrium will
subsequently continue for a considerable time to visit first one and
then another of the points which have been irritated. In such cases
it usually dwells longest and most frequently on those points which
have been irritated most severely.

I think the object of these movements is probably that of stinging
the offending body by means of the urticating cells with which the
extremity of the manubrium is armed. But, be the object what it
may, the fact of these movements occurring is a highly important
one in connection with our study of the distribution of nerves in
Medusæ, and the first point to be made out with regard to these
movements is clearly as to whether or not they are truly of a reflex
character. Accordingly, I first tried cutting off the margin, and
then irritating the muscular tissue of the bell; the movements in
question were performed exactly as before. I was thus led to think
it probable that the reflex centres of which I was in search might
be seated in the manubrium. Accordingly, I cut off the manubrium,
and tried stimulating its own substance directly. I found, however,
that no matter how small a portion of this organ I used, and no
matter from what part of the organ I cut it, this portion would do
its best to bend over to the side which I irritated. Similarly,
no matter how short a stump of the manubrium I left in connection
with the nectocalyx, on irritating any part of the latter, the
stump of the manubrium would deflect itself towards that part of
the bell, although, of course, from its short length it was unable
to reach it. Hence there can be no doubt that every portion of the
manubrium--down, at least, to the size which is compatible with
conducting these experiments--is independently endowed with the
capacity of very precisely localizing a point of irritation which is
seated either in its own substance or in that of the bell.

We have here, then, a curious fact, and one which it will be well to
bear in mind during our subsequent endeavours to frame some sort of a
conception regarding the nature of these primitive nervous tissues.
The localizing function, which is so very efficiently performed
by the manubrium of this Medusa, and which if anything resembling
it occurred in the higher animals would certainly have definite
ganglionic centres for its structural co-relative, is here shared
equally by every part of the exceedingly tenuous contractile tissue
that forms the outer surface of the organ. I am not aware that such
a diffusion of ganglionic function has as yet been actually proved
to occur in the animal kingdom, but I can scarcely doubt that future
investigation will show such a state of things to be of common
occurrence among the lower members of that kingdom.[16]

  [16] The only case I know which rests on direct observation,
  and which is at all parallel to the one above described, is the
  case of the tentacles of Drosera. Mr. Darwin found, when he cut
  off the apical gland of one of these tentacles, together with
  a small portion of the apex, that the tentacle thus mutilated
  would no longer respond to stimuli applied directly to itself.
  Thus far the case differs from that of the manubrium of Tiaropsis
  indicans, and, in respect of localization of co-ordinating
  function, resembles that of ganglionic action. But Mr. Darwin
  also found that such a "headless tentacle" continued to be
  influenced by stimuli applied to the glands of neighbouring
  tentacles--the headless one in that case bending over in whatever
  direction it was needful for it to bend, in order to approach the
  seat of stimulation. This shows that the analogue of ganglionic
  function must here be situated in at least more than one part
  of a tentacle; and I think it is not improbable that, if trials
  were expressly made, this function would be found to be diffused
  throughout the whole tentacle.

I shall now proceed to consider the nature of the nervous connections
between the nectocalyx and manubrium of this Medusa.

Bearing in mind that in an unmutilated Tiaropsis indicans the
manubrium invariably localizes with the utmost precision any minute
point of irritation situated in the bell, the significance of the
following facts is unmistakable, viz. that when a cut is introduced
between the base of the manubrium and the point of irritation in the
bell, the localizing power of the former, as regards that point in
the latter, is wholly destroyed. For instance, if such a cut as that
represented at _a_ (see Fig. 22) be made in the nectocalyx of this
Medusa, the manubrium will no longer be able to localize the seat of
a stimulus applied below that cut, as, for instance, at _b_. Now,
having tried this experiment a number of times, and having always
obtained the same result, I conclude that the nervous connections
between the nectocalyx and the manubrium, which render possible the
localizing action of the latter, are connections the functions of
which are intensely specialized, and the distribution of which is
radial.

So far, then, we have highly satisfactory evidence of tissue-tracts
performing the function of afferent nerves. But another point
of interest here arises. Although, in the experiment just
described, the manubrium is no longer able to _localize_ the seat
of stimulation in the bell, it nevertheless continues able to
perceive, so to speak, that stimulation is being applied in the bell
_somewhere_; for every time any portion of tissue below the cut a
is irritated, the manubrium actively dodges about from one part of
the bell to another, applying its extremity now to this place and
now to that one, as if seeking in vain for the offending body. If
the stimulation is persistent, the manubrium will every now and
then pause for a few seconds, as if trying to decide from which
direction the stimulation is proceeding, and will then suddenly move
over and apply its extremity, perhaps to the point that is opposite
to the one which it is endeavouring to find. It will then suddenly
leave this point and try another, and then another, and another,
and so on, as long as the stimulation is continued. Moreover, it is
important to observe that there are _gradations_ between the ability
of the manubrium to localize correctly and its inability to localize
at all, these gradations being determined by the circumferential
distance from the end of the cut and the point of stimulation. For
instance, in Fig. 22, suppose a cut A B, quarter of an inch long,
to be made pretty close to the margin and concentric with it, then
a stimulus applied at the point _c_, just below the middle point of
A B, would have the effect of making the manubrium move about to
various parts of the bell, without being able in the least degree
to localize the seat of irritation. But if the stimulus be applied
at _d_, the manubrium will probably be so far able to localize the
seat of irritation as to confine its movements, in its search for the
offending body, to perhaps the _quadrant_ of the bell in which the
stimulation is being applied. If the stimulation be now supplied at
_e_, the localization on the part of the manubrium will be still more
accurate; and if applied at _f_ (that is, _almost_ beneath one end of
the cut A B), the manubrium may succeed in localizing quite correctly.

These facts may also be well brought out by another mode of section,
viz. by cutting round a greater or less extent of the marginal
tissue, leaving one end of the resulting slip free, and the other
end attached _in situ_. If this form of section be practised on
Tiaropsis indicans, as represented at _g_ _k_ in the figure, it may
also be observed that irritation of a distant point in the marginal
strip, such as _g_ or _h_, causes the manubrium to move in various
directions, without any special reference to that part of the bell
which the irritated point of the marginal strip would occupy if _in
situ_. But if the stimulation be applied only one or two millims.
from the point of attachment of the marginal strip, as at _i_, the
manubrium will confine its localizing motions to perhaps the proper
quadrant of the bell; and if the stimulus be applied still nearer
to the attachment of the severed strip, as at _j_, the localizing
motions of the manubrium may become quite accurate.

Again, with regard to _radial_ distance, if the cut A B in the figure
were situated higher up in the bell, as at A´ B´, and the arc, _c_,
_d_, _e_, _f_, of the margin irritated as before, the manubrium
would be able to localize better than if, as before, the radial
distance between A B and _c_, _d_, _e_, _f_ were less. The greater
this radial distance, the better would be the localizing power of
the manubrium; so that, for instance, if the cut A´ B´ were situated
nearly at the base of the manubrium, the latter organ might be able
to localize correctly a stimulus applied, not only as before at _f_,
but also at _e_ or _d_. In such comparative experiments, however, it
is to be understood that the higher up in the bell a cut is placed,
the shorter it must be; for a fair comparison requires that the
two ends of the cut shall always touch the same two radii of the
nectocalyx. Still, if the cut is only a very short one (say one or
two millims. long), this consideration need not practically be taken
into account; for such a cut, if situated just above the margin, as
represented at _a_, will have the effect of destroying the localizing
power of the manubrium as regards the corresponding arc of the
margin; but if situated high up in the bell, even though its length
be still the same, it will not have this effect.

From all this, then, we have seen that the connections which render
possible the _accurate_ localizing functions of the manubrium are
almost, though not quite, exclusively radial. We have also seen that
between accurate localization and mere random movements on the part
of the manubrium there are numerous gradations, the degree of decline
from one to the other depending on the topographical relations
between the point of stimulation and the end of the section (the
section being of the form represented by A B in the figure). These
relations, as we have seen, are the more favourable to correct
localization: (_a_) the greater the radial distance between the point
of stimulation and the end of the section; and (_b_) the less the
circumferential distance between the point of the stimulation and
the radius let fall from the end of the section. But we have seen
that the limits as regards severity of section within which these
gradations of localizing ability occur, are exceedingly restricted--a
cut of only a few millims. in length, even though situated at the
greatest radial distance possible, being sufficient to destroy all
localizing power of the manubrium as regards the middle point of the
corresponding arc of the margin, and a stimulus applied only a few
millims. from the attached end of a severed marginal strip entirely
failing to cause localizing action of the manubrium. Lastly, we have
seen that even after all localizing action of the manubrium has been
completely destroyed by section of the kinds described, this organ
nevertheless continues actively, though ineffectually, to search for
the seat of irritation.

  [Illustration: Fig. 23.]

The last-mentioned fact shows that after excitational continuity
of a higher order has been destroyed, excitational continuity of a
lower order nevertheless persists; or, to state the case in other
words, the fact in question shows that after severance of the almost
exclusively radial connections between the bell and the manubrium, by
which the perfect or unimpaired localizing function of the latter
is rendered possible, other connections between these organs remain
which are not in any wise radial. I therefore next tested the degree
in which these non-radial connections might be cut without causing
destruction of that excitational continuity of a lower order which
it is their function to maintain. It will here suffice to record one
mode of section which has yielded definite results. A glance at the
accompanying illustration (Fig. 23) will show the manner in which
the Medusa is prepared. The margin having been removed (in order to
prevent possible conduction by the marginal nerve-fibres), a single
deep radial cut (_a_ _a_) is first made, and then a circumferential
cut (_a_, _b_, _c_) is carried nearly all the way round the base of
the manubrium. In this way the nectocalyx, deprived of its margin,
is converted into a continuous band of tissue, one of the ends of
which supports the manubrium. Now it is obvious that this mode of
section must be very trying to nervous connections of any kind
subsisting between the bell and the manubrium. Nevertheless, in many
cases, irritating any part of the band _a_ _l_ has the effect of
causing the manubrium to perform the active random motions previously
described. In such cases, however, it is observable that the further
away from the manubrium the stimulus is applied, the less active
is the response of this organ. In very many instances, indeed, the
manubrium altogether fails to respond to stimuli applied at more than
a certain distance from itself. For example, referring to Fig. 23,
the manubrium might actively respond to irritation of any point in
the division _d_, _e_, _f_, _g_, while to irritation of any point
in the division _f_, _g_, _h_, _i_ its responses would be weaker,
and to irritation of any point in _h_, _i_, _j_, _k_, they would be
very uncertain or altogether absent. Hence in this form of section
we have reached about the limit of tolerance of which the non-radial
connections between the bell and manubrium are capable.

Another interesting fact brought out by this form of section is, that
the radial tubes are tracts of comparatively high irritability as
regards the manubrium; for the certainty and vigour with which the
manubrium responds to a stimulus applied at one of the severed radial
tubes, _f_, _g_, or _h_, _i_, or _j_, _k_, contrast strongly with the
uncertainty and feebleness with which it often responds to stimuli
applied between any of these tubes. Indeed, it frequently happens
that a specimen which will not respond at all to a stimulus applied
between two radial tubes, will respond at once to a stimulus applied
much further from the manubrium, but in the course of the radial tube
_f_ _k_.

And this leads us to another point of interest. In such a form of
section, when any part of the mutilated nectocalyx is irritated, the
manubrium shows a very marked tendency to touch some point in the
tissue-mass _a_ _a_ _d_ _e_ (Fig. 23) by which it still remains in
connection with the bell, and through which, therefore, the stimulus
must pass in order to reach the manubrium. And it is observable that
this tendency is particularly well marked if the section has been
planned as represented in Fig. 23, _i.e._ in such a way as to leave
the tissue-tract _a_ _a_ _d_ _e_ pervaded by a nutrient-tube _d_ _e_,
this tube being thus left intact. When this is done, the manubrium
most usually points to the uninjured nutrient-tube _d_ _e_ every time
any part of the tissue-band _a_ _l_ is irritated.

Let us now very briefly consider the inferences to which these
results would seem to point. The fact that the localizing power of
the manubrium is completely destroyed as regards all parts of the
bell lying beyond an incision in the latter, conclusively proves,
as already stated, that all parts of the bell are pervaded by
radial lines of differentiated tissue, which have at least for one
of their functions the conveying of impressions to the manubrium.
The fact in question also proves that the particular effect which
is produced on the manubrium by stimulating any one of these lines
cannot be so produced by stimulating any of the other lines. But
although these tracts of differentiated tissue thus far resemble
afferent nerves in their function, we soon see that in one important
particular they differ widely from such nerves; for we have seen
that, after they have been divided, stimulation of their peripheral
parts still continues to be transmitted to their central parts,
as shown by the non-localizing movements of the manubrium. Of
course this transmission cannot take place through the divided
tissue-tracts themselves; and hence the only hypothesis we can
frame to account for the fact of its occurrence is that which would
suppose these tissue-tracts, or afferent lines, to be capable of
vicarious action. Such vicarious action would probably be effected
by means of intercommunicating fibres, the directions of which would
probably be various. In this way we arrive at the hypothesis of the
whole contractile sheet being pervaded by an intimate plexus of
functionally differentiated tissue, the constituent elements of which
are capable of a vicarious action in a high degree.

Now we know from histological observation that there is a plexus of
nerve-fibres pervading the whole expanse of the contractile sheet,
and therefore we may conclude that this is the tissue through which
the effects are produced. But, if so, we must further conclude that
the fibres of this nerve-plexus are capable of vicarious action in
the high degree which I have explained.

And this hypothesis, besides being recommended by the consideration
that it is the only one available, is confirmed by the fact that
the stimuli which it supposes to escape from a severed phalanx of
nerve-fibres, and then to reach the manubrium after being diffused
through many or all of the other radial lines (such stimuli thus
converging from many directions), are responded to when they reach
the manubrium, not by any decided localizing action on the part of
the latter, but, as the hypothesis would lead us to expect, by the
tentative and apparently random motions which are actually observed.
Moreover, we must not neglect to notice that these tentative or
random movements resemble in every way the localizing movements,
save only in their want of precision. Again, this hypothesis is
rendered more probable by the occurrence of those _gradations_ in
the localizing power of the manubrium which we have seen to be
so well marked under certain conditions. The occurrence of such
gradations under the conditions I have named is what the theory
would lead us to expect, because the closer beneath a section that
a stimulus is applied, the greater must be the immediate lateral
spread of the stimulus through the plexus before it reaches the
manubrium. Similarly, the further the circumferential distance from
the nearest end of such a section that the stimulus is applied, the
greater will be its lateral spread before reaching the manubrium.
Lastly, the present hypothesis would further lead us to anticipate
the fact that when Tiaropsis indicans is prepared as represented in
Fig. 23, the manubrium refers a stimulus applied anywhere in the
mutilated nectocalyx to the band of tissue by which it is still left
in connection with that organ; for it is evident that, according to
the hypothesis, the radial fibres occupying such a band are the only
ones whose irritation the manubrium is able to perceive, and hence it
is to be expected that it should tend to refer to these particular
fibres a source of irritation occurring anywhere in the mutilated
bell.

It is not quite so easy to understand why, in the last-mentioned
experiment, the manubrium should tend to refer a seat of irritation
to the unsevered nutrient tube, or nerve-trunk, rather than to the
unsevered nerves in the general nerve-plexus on either side of
that nerve-trunk; for if this nerve-trunk at all resembles in its
functions the nerve-trunks of higher animals, the afferent elements
collected in it ought to communicate to the manubrium the impression
of having had their _distal_ terminations irritated, and therefore
the fact of a number of such elements being collected into a single
trunk ought not to cause the manubrium to refer a distant seat of
irritation to that trunk rather than to any of the parts from which
the plexus-elements may emanate. Concerning this difficulty, however,
I may observe that we seem to have in it one of those cases in which
it would be very unsafe to argue, with any confidence, from the
highly integrated nervous systems with which we are best acquainted,
to the primitive nervous systems with which we are now concerned. And
although it would occupy too much space to enter into a discussion
of this subject, I may further observe that I think it is not at
all improbable that the manubrium of Tiaropsis indicans should, in
the absence of more definite information, refer a distant seat of
injury to that tract of collected afferent elements through which it
actually receives the strongest stimulation.


_Staurophora Laciniata._

This is a Medusa about the size of a small saucer which responds to
stimulation of its marginal ganglia, or radial nerve-trunks, by a
peculiar spasmodic movement. This consists in a sudden and violent
contraction of the entire muscle-sheet, the effect of which is
to draw together all the gelatinous walls of the nectocalyx in a
far more powerful manner than occurs during ordinary swimming. In
consequence of this spasmodic action being so strong, the nectocalyx
undergoes a change in form of a very marked and distinctive
character. The corners of the four radial tubes, being occupied by
comparatively resisting tissue, are not so much affected by the
spasm as are other parts of the bell; and they therefore constitute
a sort of framework upon which the rest of the bell contracts, the
whole bell thus assuming the form of an almost perfect square, with
each side presenting a slight concavity inwards. These spasmodic
movements, however, are quite unmistakable when they occur even in
a very minute portion of detached tissue; for, however large or
small the portion may be, when in a spasm it folds upon itself with
the characteristically strong and persistent contraction. I say
_persistent_ contraction, because a spasmodic contraction, besides
being of unusual strength, is also of unusual duration; that is to
say, while an ordinary systolic movement only lasts a short time,
a spasm lasts from six to ten seconds or more, and this whether it
occurs in a large or in a small piece of tissue. Again, the diastolic
movements differ very much in the case of an ordinary locomotor
contraction and in that of a spasm; for while in the former case the
process of relaxation is rapid even to suddenness, in the latter it
is exceedingly prolonged and gradual, occupying some four or five
seconds in its execution, and, from its slow but continuous nature,
presenting a graceful appearance. Lastly, the difference between the
two kinds of contraction is shown by the fact that, while a spasm is
gradually passing off the ordinary rhythmical contractions may often
be seen to be superimposed on it--both kinds of contraction being
thus present in the same tissue at the same time.

Now the point with which we shall be especially concerned is, that
it is only stimulation of _certain parts_ of the organism which has
the effect of throwing it into a spasm. These parts are the margin
(including the tentacles) and the courses of the four radial tubes
(including the manubrium, which in this species is spread over the
radial tubes). This limitation, however, is not invariable; for I
have often seen individuals of this species respond with a spasm
to irritation of the general contractile tissue. Nevertheless,
such response to such stimulation in the case of this species is
exceptional--the usual response to muscular irritation being an
ordinary locomotor contraction, which forms a marked contrast to the
tonic spasm that _invariably_ ensues upon stimulation of the margin,
and _almost_ invariably upon the stimulation of a radial tube.

The first question I undertook to answer was the amount of section
which the excitable tissues of Staurophora laciniata would endure
without losing their power of conducting the spasmodic contraction
from one of their parts to another. This was a very interesting
question to settle, because Staurophora laciniata, like all the other
species of discophorus naked-eyed Medusæ, differs from Aurelia, etc.,
in that the ordinary contraction-waves are very easily blocked by
section. It therefore became interesting to ascertain whether or not
the wave of spasm admitted of being blocked as easily. First, then,
as regards the margin. If this be all cut off in a continuous strip,
with the exception of one end left attached _in situ_, irritation
of any part of the almost severed strip will cause a responsive
spasm of the bell, so soon as the wave of stimulation has time to
reach the latter. I next continued this form of section into the
contractile tissues themselves, carrying the incision round and round
the bell in the form of a spiral, as represented in the case of
Aurelia by Fig. 11, page 70. In this way I converted the whole Medusa
into a ribbon-shaped piece of tissue;[17] and on now stimulating
the marginal tissue at one end of the ribbon, a portion of the
latter would go into a spasm. The object of this experiment was to
ascertain how far into the ribbon-shaped tissue the wave of spasm
would penetrate. As I had expected, different specimens manifested
considerable differences in this respect, but in all cases the degree
of penetration was astonishingly great. For it was the exception to
find cases in which the wave of spasm failed to penetrate from end
to end of a spiral strip caused by a section that had been carried
twice round the nectocalyx; and this is very astonishing when we
remember that the ordinary contraction-waves, whether originated by
stimulation of the contractile tissues or arising spontaneously from
the point of attachment of the marginal strip, usually failed to
penetrate further than a quarter of the way round. Moreover, these
waves of spasm will continue to penetrate such a spiral strip even
after the latter has been submitted to a system of interdigitating
cuts of a very severe description.

  [17] It may be stated that while conducting this mode of section
  of Staurophora laciniata, the animal responds to each cut of the
  contractile tissues with a locomotor contraction (or it may not
  respond at all); but each time the section crosses one of the
  radial tubes, the whole bell in front of the section, and the
  whole strip behind it, immediately go into a spasm.

Now, we have here to deal with a class of facts which physiologists
will recognize as of a perfectly novel character. Why it should be
that the very tenuous tracts of tissue which I have named should
have the property of responding even to a feeble stimulus by issuing
an impulse of a kind which throws the contractile tissues into a
spasm; why it should be that a spasm, when so originated, should
manifest a power of penetration to which the normal contractions of
the tissues in which it occurs bear so small a proportion; why it is
that the contractile tissues should be so deficient in the power of
originating a spasm, even in response to the strongest stimulation
applied to themselves;--these and other questions at once suggest
themselves as questions of interest. At present, however, I am wholly
unable to answer them; though we may, I think, fairly assume that it
is the ganglionic element in the margin, and probably also in the
radial tubes, which responds to direct stimulation by discharging
a peculiar impulse, which has the remarkable effect in question.
For the sake of rendering the matter quite clear, let us employ a
somewhat far-fetched but convenient metaphor. We may compare the
general contractile tissues of this Medusa to a mass of gun-cotton,
which responds to ignition (direct stimulation) by burning with
a quiet flame, but to detonation (marginal stimulation) with an
explosion. In the tissue, as in the cotton, every fibre appears to
be endowed with the capacity of liberating energy in either of two
very different ways; and whenever one part of the mass is made, by
the appropriate stimulus, to liberate its energy in one of these
two ways, all other parts of the mass do the same, and this no
matter how far through the mass the liberating process may have to
extend. Now, employing this metaphor, what we find is that, while the
contractile fibres resemble the cotton fibres in the respects just
mentioned, the ganglion cells resemble detonators, when themselves
directly stimulated. In other words, the ganglion-cells of this
Medusa are able to originate two very different kinds of impulse,
according as they liberate their energy spontaneously or in answer to
direct stimulation, and the muscular tissues respond with a totally
different kind of contraction in the two cases. Possibly, indeed,
direct stimulation of the ganglia is followed by a spasm of the
muscular tissue only because a greater amount of ordinary ganglion
influence is thus liberated than in the case of a merely spontaneous
discharge. If this were the explanation, however, I should not
expect so great a contrast as there is between the facility with
which a spasm may be caused by stimulation of the margin and of the
contractile tissue respectively. The slightest nip of the margin of
Staurophora laciniata, for instance, is sufficient to cause a spasm,
whereas even crushing the contractile tissues with a large pair of
dissecting-forceps will probably fail to cause anything other than an
ordinary contraction. Nevertheless, pricking the margin with a fine
needle usually has the effect of causing only a locomotor contraction.

In conclusion, I may state that anæsthetics have the effect of
blocking the spasmodic wave in any portion of tissue that is
submitted to their influence. It is always observable, however, that
this effect is not produced till after spontaneity has been fully
suspended, and even muscular irritability destroyed as regards direct
stimulation. Up to this stage the certainty and vigour of the spasm
consequent on marginal irritation are not perceptibly impaired;
but soon after this stage the intensity of the spasm begins to
become less, and later still it assumes a _local_ character. It is
important, also, to notice that at this stage the effect of marginal
stimulation is very often that of producing a _general locomotor_
contraction, and sometimes a series of two or three such. During
recovery in normal sea-water all these phases occur in reverse order.




CHAPTER VI.

CO-ORDINATION.

_Covered-eyed Medusæ._


From the fact that in the covered-eyed Medusæ the passage of a
stimulus-wave is not more rapid than that of a contraction-wave,
we may be prepared to expect that in these animals the action of
the locomotor ganglia is not, in any proper sense of the term,
a co-ordinated action; for if a stimulus-wave cannot outrun a
contraction-wave, one ganglion cannot know that another ganglion
has discharged its influence till the contraction-wave, which
results from a discharge of the active ganglion, has reached the
passive one. And this I find to be generally the case; for it may
usually be observed that one or more of the lithocysts are either
temporarily or permanently prepotent over the others, i.e. that
contraction-waves emanate from the prepotent lithocysts, and then
spread rapidly over the swimming-bell, without there being any signs
of co-ordinated or simultaneous action on the part of the other
lithocysts. Nevertheless, in many cases such prepotency cannot, even
with the greatest care, be observed; but upon every pulsation all
parts of the swimming-bell seem to contract at the same instant.
And this apparently perfect co-ordination among the eight marginal
ganglia may continue for any length of time. I believe, however, that
such apparently complete physiological harmony is not co-ordination
properly so called, _i.e._ is not due to special nervous connections
between the ganglia; for, if such were the case, perfectly
synchronous action of this kind ought to be the rule rather than the
exception.

I am therefore inclined to account for these cases of perfectly
synchronous action by supposing that all, or most, of the ganglia
require exactly the same time for their nutrition; that they are,
further, of exactly equal potency in relation to the resistance (or
excitability) of the surrounding contractile tissues; and that,
therefore, the balance of forces being exactly equal in the case
of all, or most, of the ganglia, their rhythm, though perfectly
identical, is really independent. I confess, however, that I am by
no means certain regarding the accuracy of this conclusion, as it
is founded on negative rather than on positive considerations; that
is to say, I arrive at this conclusion regarding the cases in which
such apparent co-ordination is observable only because in other cases
such apparent co-ordination is not observable; and also, I may add,
because my experiments in section have not revealed any evidence
of nervous connections capable of conducting a stimulus-wave with
greater rapidity than a contraction-wave. I therefore consider this
conclusion an uncertain one, and its uncertainty is, perhaps, still
further increased by the result of the following experiments.

If a covered-eyed Medusa be chosen in which perfectly synchronous
action of the ganglia is observable, and if a deep radial incision be
made between each pair of adjacent ganglia--the incisions being thus
eight in number and carried either from the margin towards the centre
or _vice versâ_--it then becomes conspicuous enough that the eight
partially divided segments no longer present synchronous action; for
now one segment and now another takes the initiative in starting a
contraction-wave, which is then propagated to the other segments. And
it is evident that this fact tends to negative the above explanation,
for if the discharges of the ganglia are independently simultaneous
before section, we might expect them to continue so after section.
It must be remembered, however, that the form of section we are
considering is a severe one, and that it must therefore not only
give rise to general shock, but also greatly interfere with the
passage of contraction-waves, and, in general, disturb the delicate
conditions on which, according to the suggested explanation, the
previous harmony depended. Besides, as we shall subsequently see, for
some reason or other segmentation of a Medusa profoundly modifies the
rate of its rhythm. In view of these considerations, therefore, the
results yielded by such experiments must not be regarded as having
any conclusive bearing on the question before us; and as these or
similar objections apply to various other modes of section by which
I have endeavoured to settle this question, I will not here occupy
space in detailing them.

It seems desirable, however, in this connection again to mention
a fact briefly stated in a former chapter, namely, that section
conclusively proves a contraction-wave to have the power, when it
reaches a lithocyst, of stimulating the latter into activity; for it
is not difficult to obtain a series of lithocysts connected in such
a manner that the resistance offered to the passage of the waves by
a certain width of the junction-tissue, is such as just to allow the
residuum of the contraction-wave which emanates from one lithocyst to
reach the adjacent lithocyst, thus causing it to originate another
wave, which, in turn, is just able to pass to the next lithocyst
in the series, and so on, each lithocyst in turn acting like a
reinforcing battery to the passage of the contraction-wave. Now this
fact, I think, sufficiently explains the mechanism of ganglionic
action in those cases where one or more lithocysts are prepotent over
the others; that is to say, the prepotent lithocyst first originates
a contraction-wave, which is then successively reinforced by all
the other lithocysts during its passage round the swimming-bell.
In this way the passage of a contraction-wave is no doubt somewhat
accelerated; for I found, in marginal strips, that the rate of
transit from a terminal lithocyst to the other end of the strip was
somewhat lowered by excising the seven intermediate lithocysts.

  [Illustration: Fig. 24.]

I may here state, in passing, a point of some little interest in
connection with this reinforcing action of lithocysts. When I first
observed this action, it appeared to me a mysterious thing why its
result was always to propagate the contraction-wave in only one
direction--the direction, namely, in which the wave happened to be
passing before it reached the lithocyst. For instance, suppose we
have a strip A D, with a lithocyst at each of the equidistant points
A, B, C, D; and suppose that the lithocyst B originates a stimulus:
the resulting contraction-wave passes, of course, with equal rapidity
in the two opposite directions, B A, B C (arrows _b a_, _b c_): the
contraction-wave _b a_ therefore reaches the lithocyst A at the
same time as the contraction-wave _b c_ reaches the lithocyst C,
and so both A and C discharge simultaneously. What, then, should we
expect to be the result? I think we should expect the wave _b c_ to
continue on its course to D, after having been strengthened at C,
and a _reflex_ wave _a´ b´_ to start from A (owing to the discharge
at A), which would reach B at the same time as a similar _reflex_
wave _c´ b´_ starting from C (owing to the discharge at C); so that
by the time the original wave _b c d_ had reached D, the point B
would be the seat of a collision between the two reflex waves _a´
b´_ and _c´ d´_. And, not to push the supposed case further, it
is evident that if such reflex waves were to occur, the resulting
confusion would very soon require to end in tetanus. As a matter of
fact, these reflex waves do not occur; and the question is, why do
they not? Why is it that a wave is only reinforced in the direction
in which it happens to be travelling--so that if, for instance, it
happens to start from A in the above series, it is successively
propagated by B C in the direction A, B, C, D, and in that direction
only; whereas, if it happens to start from D, it is propagated by
the same lithocysts in the opposite direction, D, C, B, A, and in
that direction only--the wave in the one case terminating at the
lithocyst D, and in the other case at the lithocyst A? Now, although
this absence of reflex waves appears at first sight mysterious,
it admits of an exceedingly simple explanation. I find that the
contractile tissues of the covered-eyed Medusæ cannot be made to
respond to two successive stimuli of minimal, or but slightly more
than minimal intensity, unless such stimuli are separated from one
another by a certain considerable interval of time. Now, when in
the above illustration the contraction-wave starts from A, by the
time it reaches B the portion of tissue included between A and B
has just been in contraction in response to the stimulus from A,
while the portion of tissue included between B and C has not been in
contraction. Consequently, the stimulus resulting from a ganglionic
discharge being presumably of minimal, or but slightly more than
minimal intensity, the tissue included between A and B will not
respond to the discharge of B; while the tissue included between
B and C, not having been just previously in contraction, will
respond. And conversely, of course, if the contraction-wave had been
travelling in the opposite direction.

Seeing that this explanation is the only one possible, and that it
moreover follows as a deductive necessity from my experiments on
stimulation, I think there is no need to detail any of the further
experiments which I made with the view of confirming it. But the
following experiment, devised to confirm this explanation, is of
interest in itself, and on this account I shall state it. Having
prepared a contractile strip with a single remaining lithocyst
at one end, I noted the rhythm exhibited by this lithocyst, and
then imitated that rhythm by means of single induced shocks thrown
in with a key at the other end of the strip. The effect of these
shocks was, of course, to cause the contraction-waves to pass in
the direction opposite to that in which they passed when originated
by the lithocyst. Now I found, as I had expected, that so long as
I continued exactly to imitate the rate of ganglionic rhythm, so
long did the waves always pass in the direction B A--A being the
lithocyst, and B the other end of the strip. I also found that if I
allowed the rate of the artificially caused rhythm to sink slightly
below that of the natural rhythm, after every one to six waves (the
number depending on the degree in which the rate of succession of my
induction shocks approximated to the rate of the natural rhythm)
which passed from B to A, one would pass from A to B.[18]

  [18] When two such waves met, they neutralized each other at
  their line of collision; or perhaps more correctly, the tissue on
  each side of that line, having just been in contraction, was not
  able again to convey a contraction-wave passing in the opposite
  direction to the wave which it had conveyed immediately before.

Of course the only interpretation to be put on these facts is that
every time an artificially started wave reached the terminal ganglion
it caused the latter to discharge; but that the occurrence of a
discharge could not in this case be rendered apparent, because of
the inadequacy of that discharge to start a reflex wave. But that
such discharges always took place was manifest, both _à priori_
because from analogy we may be sure that if there had happened to
be any contractile tissue of appropriate width on the other side of
the ganglion, the discharge of the latter would have been rendered
apparent, and _à posteriori_ because, after the arrival of every
artificially started wave, the time required for the ganglion to
originate another wave was precisely the same as if it had itself
originated the previous wave.

In view of these results, it occurred to me as an interesting
experiment to try the effect on the natural rhythm of exhausting a
ganglion thus situated, by throwing in a great number of shocks at
the other end of the strip. I found that after five hundred single
shocks had been thrown in with a rapidity almost sufficient to
tetanize the strip, immediately after the stimulation ceased, the
natural rhythm of the ganglion, which had previously been twenty
in the minute, fell to fourteen for the first minute, eighteen for
the second, and the original rate of twenty for the third. In such
experiments the diminution of rate is most conspicuous during the
first fifteen or thirty seconds of the first minute. Sometimes
there are no contractions at all for the first fifteen seconds
after cessation of the stimulating process, and in such cases the
natural rhythm, when it first begins, may be as slow as one-half
or even one-quarter its normal rate. All these effects admit of
being produced equally well, and with less trouble, by faradizing
the strip, when it may be even better observed how prolonged may be
the stimulation, without causing anything further than such slight
exhaustion of the ganglion as the above results imply.[19]

  [19] In this description I have everywhere adopted the current
  phraseology with regard to ganglionic action--a phraseology which
  embodies the theory of ganglia supplying interrupted stimulation.
  But although I have done this for the sake of clearness, of
  course it will be seen that the facts harmonize equally well with
  the theory of continuous stimulation, to which I shall allude
  further on.


_Naked-eyed Medusæ._

It would be impossible to imagine movements on the part of so
simple an organism more indicative of physiological harmony than
are the movements of Sarsia. One may watch several hundreds of
these animals while they are swimming about in the same bell-jar
and never perceive, as in the covered-eyed Medusæ, the slightest
want of ganglionic co-ordination exhibited by any of the specimens.
Moreover, that the ganglionic co-ordination is in this case
wonderfully far advanced is proved by the fact of members of this
genus being able to steer themselves while following a light, as
previously described.[20]

  [20] Removing the manubrium does not interfere with this steering
  action; but if any considerable portion of the ma

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