Encyclopaedia Britannica, 11th Edition, "Ichthyology" to "Independence"
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Title: Encyclopaedia Britannica, 11th Edition, "Ichthyology" to "Independence"
Author: Various
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*** START OF THE PROJECT GUTENBERG EBOOK ENCYCLOPAEDIA BRITANNICA, 11TH EDITION, "ICHTHYOLOGY" TO "INDEPENDENCE" ***
Transcriber's notes:
(1) Numbers following letters (without space) like C2 were originally
printed in subscript. Letter subscripts are preceded by an
underscore, like C_n.
(2) Characters following a carat (^) were printed in superscript.
(3) Side-notes were relocated to function as titles of their respective
paragraphs.
(4) Macrons and breves above letters and dots below letters were not
inserted.
(5) [root] stands for the root symbol; [alpha], [beta], etc. for greek
letters.
(6) The following typographical errors have been corrected:
ARTICLE ICHTHYOLOGY: "Within it in Neoceratodus are a number of
longitudinal rows of pocket valves." 'longitudinal' amended from
'longtitudinal'.
ARTICLE IDEALISM: "... the systems of impious and profane persons
which exclude all freedom, intelligence, and design from the
formation of things, and instead thereof make a self-existent ..."
'freedom' amended from 'freeedom'.
ARTICLE ILLINOIS: "An anti-trust law of 1893 exempted from the
definition of trust combinations those formed by producers of
agricultural products and live stock, but the United States Supreme
Court in 1902 declared the statute unconstitutional as class
legislation." 'United' amended from 'Untied'.
ARTICLE IMAM: "... Juynboll's De Mohammedanische Wet, 316 seq. ..."
'Mohammedanische' amended from 'Mohammedaanische'.
ENCYCLOPAEDIA BRITANNICA
A DICTIONARY OF ARTS, SCIENCES, LITERATURE
AND GENERAL INFORMATION
ELEVENTH EDITION
VOLUME XIV, SLICE III
Ichthyology to Independence
ARTICLES IN THIS SLICE:
ICHTHYOLOGY ILLUSTRES
ICHTHYOPHAGI ILLYRIA
ICHTHYOSAURUS ILMENAU
ICHTHYOSIS ILMENITE
ICKNIELD STREET ILOILO
ICON ILSENBURG
ICONIUM IMAGE
ICONOCLASTS IMAGE WORSHIP
ICONOSTASIS IMAGINATION
ICOSAHEDRON IMAM
ICTERUS IMBECILE
ICTINUS IMBREX
IDA IMBROS
IDAHO IMERETIA
IDAR IMIDAZOLES
IDAS IMITATION
IDDESLEIGH, STAFFORD NORTHCOTE IMITATION OF CHRIST, THE
IDEA IMMACULATE CONCEPTION, THE
IDEALISM IMMANENCE
IDELER, CHRISTIAN LUDWIG IMMANUEL BEN SOLOMON
IDENTIFICATION IMMERMANN, KARL LEBERECHT
IDEOGRAPH IMMERSION
IDIOBLAST IMMIGRATION
IDIOM IMMORTALITY
IDIOSYNCRASY IMMUNITY
IDOLATRY IMOLA
IDOMENEUS IMP
IDRIA IMPATIENS
IDRIALIN IMPEACHMENT
IDRISI IMPERIAL CHAMBER
IDUMAEA IMPERIAL CITIES OR TOWNS
IDUN IMPEY, SIR ELIJAH
IDYL IMPHAL
IFFLAND, AUGUST WILHELM IMPLEMENT
IGLAU IMPLUVIUM
IGLESIAS IMPOSITION
IGNATIEV, NICHOLAS PAVLOVICH IMPOST
IGNATIUS IMPOTENCE
IGNORAMUS IMPRESSIONISM
IGNORANCE IMPRESSMENT
IGNORANTINES IMPROMPTU
IGUALADA IMPROVISATORE
IGUANA IN-ANTIS
IGUANODON INAUDI, JACQUES
IGUVIUM INCANTATION
IJOLITE INCE, WILLIAM
IKI INCE-IN-MAKERFIELD
ILAGAN INCENDIARISM
ILCHESTER INCENSE
ÎLE-DE-FRANCE INCEST
ILETSK INCH
ILFELD INCHBALD, MRS ELIZABETH
ILFORD INCHIQUIN, MURROUGH O'BRIEN
ILFRACOMBE INCLEDON, CHARLES BENJAMIN
ILHAVO INCLINOMETER
ILI INCLOSURE
ILION IN COENA DOMINI
ILKESTON INCOME TAX
ILKLEY INCORPORATION
ILL INCUBATION and INCUBATORS
ILLAWARRA INCUBUS
ILLE-ET-VILAINE INCUMBENT
ILLEGITIMACY INCUNABULA
ILLER INDABA
ILLINOIS INDAZOLES
ILLORIN INDEMNITY
ILLUMINATED MSS. INDENE
ILLUMINATI INDENTURE
ILLUMINATION INDEPENDENCE
ILLUSTRATION
ICHTHYOLOGY (from Gr. [Greek: ichthys], fish, and [Greek: logos],
doctrine or treatise), the branch of zoology which treats of the
internal and external structure of fishes, their mode of life, and their
distribution in space and time. According to the views now generally
adopted, all those vertebrate animals are referred to the class of
fishes which combine the following characteristics: they live in water,
and by means of gills or branchiae breathe air dissolved in water; the
heart consists of a single ventricle and single atrium; the limbs, if
present, are modified into fins, supplemented by unpaired median fins;
and the skin is either naked or covered with scales or with osseous
plates or bucklers. With few exceptions fishes are oviparous. There are,
however, not a few members of this class which show a modification of
one or more of these characteristics, and which, nevertheless, cannot be
separated from it.
I. HISTORY AND LITERATURE DOWN TO 1880
The commencement of the history of ichthyology coincides with that of
zoology generally. Aristotle (384-322 B.C.) had a perfect knowledge of
the general structure of fishes, which he clearly discriminates both
from the aquatic animals with lungs and mammae, i.e. Cetaceans, and from
the various groups of aquatic invertebrates. According to him: "the
special characteristics of the true fishes consist in the branchiae and
fins, the majority having four fins, but those of an elongate form, as
the eels, having two only. Some, as the _Muraena_, lack the fins
altogether. The rays swim with their whole body, which is spread out.
The branchiae are sometimes furnished with an operculum, sometimes they
are without one, as in the cartilaginous fishes.... No fish has hairs or
feathers; most are covered with scales, but some have only a rough or a
smooth skin. The tongue is hard, often toothed, and sometimes so much
adherent that it seems to be wanting. The eyes have no lids, nor are any
ears or nostrils visible, for what takes the place of nostrils is a
blind cavity; nevertheless they have the senses of tasting, smelling and
hearing. All have blood. All scaly fishes are oviparous, but the
cartilaginous fishes (with the exception of the sea-devil, which
Aristotle places along with them) are viviparous. All have a heart,
liver and gall-bladder; but kidneys and urinary bladder are absent. They
vary much in the structure of their intestines: for, whilst the mullet
has a fleshy stomach like a bird, others have no stomachic dilatation.
Pyloric caeca are close to the stomach, and vary in number; there are
even some, like the majority of the cartilaginous fishes, which have
none whatever. Two bodies are situated along the spine, which have the
function of testicles; they open towards the vent, and are much enlarged
in the spawning season. The scales become harder with age. Not being
provided with lungs, fishes have no voice, but several can emit grunting
sounds. They sleep like other animals. In most cases the females exceed
the males in size; and in the rays and sharks the male is distinguished
by an appendage on each side of the vent."
Aristotle's information on the habits of fishes, their migrations, mode
and time of propagation, and economic uses is, so far as it has been
tested, surprisingly correct. Unfortunately, we too often lack the means
of recognizing the species of which he gives a description. His ideas of
specific distinction were as vague as those of the fishermen whose
nomenclature he adopted; it never occurred to him that vernacular names
are subject to change, or may be entirely lost in course of time, and
the difficulty of identifying his species is further increased by the
circumstance that sometimes several popular names are applied by him to
the same fish, or different stages of growth are designated by distinct
names. The number of fishes known to Aristotle seems to have been about
one hundred and fifteen, all of which are inhabitants of the Aegean Sea.
That one man should have laid so sure a basis for future progress in
zoology is less surprising than that for about eighteen centuries a
science which seemed to offer particular attractions to men gifted with
power of observation was no further advanced. Yet such is the case.
Aristotle's successors remained satisfied to be his copiers or
commentators, and to collect fabulous stories or vague notions. With few
exceptions (such as Ausonius, who wrote a small poem, in which he
describes from his own observations the fishes of the Moselle) authors
abstained from original research; and it was not until about the middle
of the 16th century that ichthyology made a new step in advance by the
appearance of Belon, Rondelet and Salviani, who almost simultaneously
published their great works, by which the idea of species was
established.
Belon.
P. Belon travelled in the countries bordering on the eastern part of the
Mediterranean in the years 1547-1550; he collected rich stores of
positive knowledge, which he embodied in several works. The one most
important for the progress of ichthyology is that entitled _De
aquatilibus libri duo_ (Paris, 1553). Belon knew about one hundred and
ten fishes, of which he gives rude but generally recognizable figures.
Although Belon rarely gives definitions of the terms used by him, it is
not generally very difficult to ascertain the limits which he intended
to assign to each division of aquatic animals. He very properly divides
them into such as are provided with blood and those without it--two
divisions corresponding in modern language to vertebrate and
invertebrate aquatic animals. The former are classified by him according
to size, the further sub-divisions being based on the structure of the
skeleton, mode of propagation, number of limbs, form of the body and
physical character of the habitat.
Salviani.
The work of the Roman ichthyologist H. Salviani (1514-1572), bears
evidence of the high social position which the author held as physician
to three popes. Its title is _Aquatilium animalium historia_ (Rome,
1554-1557, fol.). It treats exclusively of the fishes of Italy.
Ninety-two species are figured on seventy-six plates, which, as regards
artistic execution, are masterpieces of that period, although those
specific characteristics which nowadays constitute the value of a
zoological drawing were overlooked by the author or artist. No attempt
is made at a natural classification, but the allied forms are generally
placed in close proximity. The descriptions are equal to those given by
Belon, entering much into the details of the economy and uses of the
several species, and were evidently composed with the view of collecting
in a readable form all that might prove of interest to the class of
society in which the author moved. Salviani's work is of a high order.
It could not fail to render ichthyology popular in the country to the
fauna of which it was devoted, but it was not fitted to advance
ichthyology as a science generally; in this respect Salviani is not to
be compared with Rondelet or Belon.
Rondelet.
G. Rondelet (1507-1557) had the great advantage over Belon of having
received a medical education at Paris, and especially of having gone
through a complete course of instruction in anatomy as a pupil of
Guentherus of Andernach. This is conspicuous throughout his
works--_Libri de piscibus marinis_ (Lyons, 1554); and _Universae
aquatilium historiae pars altera_ (Lyons, 1555). Nevertheless they
cannot be regarded as more than considerably enlarged editions of
Belon's work. For, although he worked independently of the latter, the
system adopted by him is characterized by the same absence of the true
principles of classification. His work is almost entirely limited to
European and chiefly to Mediterranean forms, and comprises no fewer than
one hundred and ninety-seven marine and forty-seven fresh-water fishes.
His descriptions are more complete and his figures much more accurate
than those of Belon; and the specific account is preceded by
introductory chapters, in which he treats in a general manner of the
distinctions, the external and internal parts, and the economy of
fishes. Like Belon, he had no conception of the various categories of
classification--confounding throughout his work the terms "genus" and
"species," but he had an intuitive notion of what his successors called
a "species," and his principal object was to give as much information as
possible regarding such species.
For nearly a century the works of Belon and Rondelet continued to be the
standard works on ichthyology; but the science did not remain stationary
during that period. The attention of naturalists was now directed to the
fauna of foreign countries, especially of the Spanish and Dutch
possessions in the New World; and in Europe the establishment of
anatomical schools and academies led to careful investigation of the
internal anatomy of the most remarkable European forms. Limited as these
efforts were as to their scope, they were sufficiently numerous to
enlarge the views of naturalists, and to destroy that fatal dependence
on preceding authorities which had kept in bonds even Rondelet and
Belon. The most noteworthy of those engaged in these inquiries in
tropical countries were W. Piso and G. Marcgrave, who accompanied as
physicians the Dutch governor, Count Maurice of Nassau, to Brazil
(1630-1644).
Of the men who left records of their anatomical researches, we may
mention Borelli (1608-1679), who wrote a work _De motu animalium_ (Rome,
1680, 4to), in which he explained the mechanism of swimming and the
function of the air-bladder; M. Malpighi (1628-1694), who examined the
optic nerve of the sword-fish; the celebrated J. Swammerdam (1637-1680),
who described the intestines of numerous fishes; and J. Duverney
(1648-1730), who investigated in detail the organs of respiration.
A new era in the history of ichthyology commences with Ray, Willughby
and Artedi, who were the first to recognize the true principles by which
the natural affinities of animals should be determined. Their labours
stand in so intimate a connexion with each other that they represent but
one great step in the progress of this science.
Ray and Willughby.
J. Ray (1628-1705) was the friend and guide of F. Willughby (1635-1672).
They found that a thorough reform in the method of treating the
vegetable and animal kingdoms had become necessary; that the only way of
bringing order into the existing chaos was by arranging the various
forms according to their structure. They therefore substituted facts for
speculation, and one of the first results of this change, perhaps the
most important, was that, having recognized "species" as such, they
defined the term and fixed it as the starting-point of all sound
zoological knowledge.
Although they had divided their work so that Ray attended to the plants
principally, and Willughby to the animals, the _Historia piscium_ (Oxf.,
1686), which bears Willughby's name on the title-page and was edited by
Ray, is their joint production. A great part of the observations
contained in it were collected during the journeys they made together in
Great Britain and in the various countries of Europe.
By the definition of fishes as animals with blood, breathing by gills,
provided with a single ventricle of the heart, and either covered with
scales or naked, the Cetaceans are excluded. The fishes proper are
arranged primarily according to the cartilaginous or the osseous nature
of the skeleton, and then subdivided according to the general form of
the body, the presence or the absence of ventral fins, the soft or the
spinous structure of the dorsal rays, the number of dorsal fins, &c. No
fewer than four hundred and twenty species are thus arranged and
described, of which about one hundred and eighty were known to the
authors from personal examination--a comparatively small proportion,
but descriptions and figures still formed in great measure the
substitute for our modern collections and museums. With the increasing
accumulation of forms, the want of a fixed nomenclature had become more
and more felt.
Artedi.
Peter Artedi (1705-1734) would have been a great ichthyologist if Ray or
Willughby had not preceded him. But he was fully conscious of the fact
that both had prepared the way for him, and therefore he did not fail to
reap every possible advantage from their labours. His work, edited by
Linnaeus, is divided as follows:--
(1) In the _Bibliotheca ichthyologica_ Artedi gives a very complete
list of all preceding authors who had written on fishes, with a
critical analysis of their works. (2) The _Philosophia ichthyologica_
is devoted to a description of the external and internal parts of
fishes; Artedi fixes a precise terminology for all the various
modifications of the organs, distinguishing between those characters
which determine a genus and such as indicate a species or merely a
variety; in fact he establishes the method and principles which
subsequently have guided every systematic ichthyologist. (3) The
_Genera piscium_ contains well-defined diagnoses of forty-five genera,
for which he has fixed an unchangeable nomenclature. (4) In the
_Species piscium_ descriptions of seventy-two species, examined by
himself, are given--descriptions which even now are models of
exactitude and method. (5) Finally, in the _Synonymia piscium_
references to all previous authors are arranged for every species,
very much in the manner which is adopted in the systematic works of
the present day.
Linnaeus.
Artedi has been justly called the father of ichthyology. So admirable
was his treatment of the subject, that even Linnaeus could only modify
and add to it. Indeed, so far as ichthyology is concerned, Linnaeus has
scarcely done anything beyond applying binominal terms to the species
properly described and classified by Artedi. His classification of the
genera appears in the 12th edition of the _Systema_ thus:--
A. _Amphibia nantia._--_Spiraculis compositis._--Petromyzon, Raía,
Squalus, Chimaera. _Spiraculis solitariis._--Lophius, Acipenser,
Cyclopterus, Balistes, Ostracion, Tetrodon, Diodon, Centriscus,
Syngnathus, Pegasus.
B. _Pisces apodes._--Muraena, Gymnotus, Trichiurus, Anarrhichas,
Ammodytes, Ophidium, Stromateus, Xiphias.
C. _Pisces jugulares._--Callionymus, Uranoscopus, Trachinus, Gadus,
Blennius.
D. _Pisces thoracici._--Cepola, Echeneis, Coryphaena, Gobius, Cottus,
Scorpaena, Zeus, Pleuronectes, Chaetodon, Sparus, Labrus, Sciaena,
Perca, Gasterosteus, Scomber, Mullus, Trigla.
E. _Pisces abdominales._--Cobitis, Amia, Silurus, Teuthis, Loricaria,
Salmo, Fistularia, Esox, Elops, Argentina, Atherina, Mugil, Mormyrus,
Exocoetus, Polynemus, Clupea, Cyprinus.
Two contemporaries of Linnaeus, L. T. Gronow and J. T. Klein, attempted
a systematic arrangement of fishes.
The works of Artedi and Linnaeus led to an activity of research,
especially in Scandinavia, Holland, Germany and England, such as has
never been equalled in the history of biological science. Whilst some of
the pupils and followers of Linnaeus devoted themselves to the
examination and study of the fauna of their native countries, others
proceeded on voyages of discovery to foreign and distant lands. Of these
latter the following may be especially mentioned: O. Fabricius worked
out the fauna of Greenland; Peter Kalm collected in North America, F.
Hasselquist in Egypt and Palestine, M. T. Brünnich in the Mediterranean,
Osbeck in Java and China, K. P. Thunberg in Japan; Forskål examined and
described the fishes of the Red Sea; G. W. Steller, P. S. Pallas, S. G.
Gmelin, and A. J. Güldenstädt traversed nearly the whole of the Russian
empire in Europe and Asia. Others attached themselves as naturalists to
celebrated navigators, such as the two Forsters (father and son) and
Solander, who accompanied Cook; P. Commerson, who travelled with
Bougainville; and Pierre Sonnerat. Of those who studied the fishes of
their native countries, the most celebrated were Pennant (Great
Britain), O. F. Müller (Denmark), Duhamel du Monceau (France), C. von
Meidinger (Austria), J. Cornide (Spain), and A. Parra (Cuba).
The mass of materials brought together was so great that, not long after
the death of Linnaeus, the necessity made itself felt for collecting
them in a compendious form. Several compilers undertook this task; they
embodied the recent discoveries in new editions of the classical works
of Artedi and Linnaeus, but, they only succeeded in burying those noble
monuments under a chaotic mass of rubbish. For ichthyology it was
fortunate that two men at least, Bloch and Lacepède, made it a subject
of prolonged original research.
Bloch.
Mark Eliezer Bloch (1723-1799), a physician of Berlin, had reached the
age of fifty-six when he began to write on ichthyological subjects. His
work consists of two divisions:-- (1) _Öconomische Naturgeschichte der
Fische Deutschlands_ (Berl., 1782-1784); (2) _Naturgeschichte der
ausländischen Fische_ (Berl., 1785-1795). The first division, which is
devoted to a description of the fishes of Germany, is entirely original.
His descriptions as well as figures were made from nature, and are, with
few exceptions, still serviceable; indeed many continue to be the best
existing in literature. Bloch was less fortunate, and is much less
trustworthy, in his natural history of foreign fishes. For many of the
species he had to trust to more or less incorrect drawings and
descriptions by travellers; frequently, also, he was deceived as to the
origin of specimens which he purchased. Hence his accounts contain
numerous errors, which it would have been difficult to correct had not
nearly the whole of the materials on which his work is based been
preserved in the collections at Berlin.
After the completion of his great work Bloch prepared a general system
of fishes, in which he arranged not only those previously described, but
also those with which he had afterwards become acquainted. The work was
ably edited and published after Bloch's death by a philologist, J. G.
Schneider, under the title _M. E. Blochii Systema ichthyologiae iconibus
cx. illustratum_ (Berl., 1801). The number of species enumerated amounts
to 1519. The system is based upon the number of the fins, the various
orders being termed _Hendecapterygii_, _Decapterygii_, &c. An artificial
method like this led to the most unnatural combinations and
distinctions.
Bloch's _Naturgeschichte_ remained for many years the standard work. But
as regards originality of thought Bloch was far surpassed by his
contemporary, B. G. E. de Lacepède, born at Agen, in France, in 1756,
who became professor at the museum of natural history in Paris, where he
died in 1825.
Lacepède.
Lacepède had to contend with great difficulties in the preparations of
his _Histoire des poissons_ (Paris, 1798-1803, 5 vols.), which was
written during the most disturbed period of the French Revolution. A
great part of it was composed whilst the author was separated from
collections and books, and had to rely on his notes and manuscripts
only. Even the works of Bloch and other contemporaneous authors remained
unknown or inaccessible to him for a long time. His work, therefore,
abounds in the kind of errors into which a compiler is liable to fall.
Thus the influence of Lacepède on the progress of ichthyology was vastly
less than that of his fellow-labourer; and the labour laid on his
successors in correcting numerous errors probably outweighed the
assistance which they derived from his work.
The work of the principal students of ichthyology in the period between
Ray and Lacepède was chiefly systematizing and describing; but the
internal organization of fishes also received attention from more than
one great anatomist. Albrecht von Haller, Peter Camper and John Hunter
examined the nervous system and the organs of sense; and Alexander
Monro, _secundus_, published a classical work, _The Structure and
Physiology of Fishes Explained and Compared with those of Man and other
Animals_ (Edin., 1785). The electric organs of fishes (_Torpedo_ and
_Gymnotus_) were examined by Réaumur, J. N. S. Allamand, E. Bancroft,
John Walsh, and still more exactly by J. Hunter. The mystery of the
propagation of the eel called forth a large number of essays, and even
the artificial propagation of _Salmonidae_ was known and practised by J.
G. Gleditsch (1764).
Bloch and Lacepède's works were almost immediately succeeded by the
labours of Cuvier, but his early publications were tentative,
preliminary and fragmentary, so that some little time elapsed before the
spirit infused into ichthyology by this great anatomist could exercise
its influence on all the workers in this field.
The _Descriptions and Figures of Two Hundred Fishes collected at
Vizagapatam on the Coast of Coromandel_ (Lond., 1803, 2 vols.) by
Patrick Russel, and _An Account of the Fishes found in the River
Ganges and its Branches_ (Edin., 1822, 2 vols.) by F. Hamilton
(formerly Buchanan), were works distinguished by greater accuracy of
the drawings (especially the latter) than was ever attained before. A
_Natural History of British Fishes_ was published by E. Donovan
(Lond., 1802-1808); and the Mediterranean fauna formed the study of
the lifetime of A. Risso, _Ichthyologie de Nice_ (Paris, 1810); and
_Histoire naturelle de l'Europe méridionale_ (Paris, 1827). A slight
beginning in the description of the fishes of the United States was
made by Samuel Latham Mitchell (1764-1831), who published, besides
various papers, a _Memoir on the Ichthyology of New York_, in 1815.
Cuvier.
Valenciennes.
G. Cuvier (1769-1832) devoted himself to the study of fishes with
particular predilection. The investigation of their anatomy, and
especially their skeleton, was continued until he had succeeded in
completing so perfect a framework of the system of the whole class that
his immediate successors required only to fill up those details for
which their master had had no leisure. He ascertained the natural
affinities of the infinite variety of forms, and accurately defined the
divisions, orders, families and genera of the class, as they appear in
the various editions of the _Règne Animal_. His industry equalled his
genius; he formed connections with almost every accessible part of the
globe; and for many years the museum of the Jardin des Plantes was the
centre where all ichthyological treasures were deposited. Thus Cuvier
brought together a collection which, as it contains all the materials on
which his labours were based, must still be considered as the most
important. Soon after the year 1820, Cuvier, assisted by one of his
pupils, A. Valenciennes, commenced his great work on fishes, _Historie
naturelle des Poissons_, of which the first volume appeared in 1828.
After Cuvier's death in 1832 the work was left entirely in the hands of
Valenciennes, whose energy and interest gradually slackened, rising to
their former pitch in some parts only, as, for instance, in the
treatise, on the herring. He left the work unfinished with the
twenty-second volume (1848), which treats of the Salmonoids. Yet,
incomplete as it is, it is indispensable to the student.
The system finally adopted by Cuvier is the following:--
A. POISSONS OSSEUX.
I. A BRANCHIES EN PEIGNES OU EN LAMES.
1. _A Mâchoire Supérieure Libre._
a. Acanthoptérygiens.
Percoïdes. Sparoïdes. Branchies labyrinthiques.
Polynèmes. Chétodonoïdes. Lophioïdes.
Mulles. Scombéroïdes. Gobioïdes.
Joues cuirassées. Muges. Labroïdes.
Sciénoïdes.
b. Malacoptérygiens.
_Abdominaux._ _Subbrachiens._ _Apodes._
--- --- ---
Cyprinoïdes. Gadoïdes. Murénoïdes.
Siluroïdes. Pleuronectes.
Salmonoïdes. Discoboles.
Clupéoïdes.
Lucioïdes.
2. _A Mâchoire Supérieure Fixée._
Selérodermes. Gymnodontes.
II. A BRANCHIES EN FORME DE HOUPPES.
Lophobranches.
B. CARTILAGINEUX OU CHONDROPTÉRYGIENS.
Sturioniens. Plagiostomes. Cyclostomes.
We have only to compare this system with that of Linnaeus if we wish to
measure the gigantic stride made by ichthyology during the intervening
period of seventy years. The various characters employed for
classification have been examined throughout the whole class, and their
relative importance has been duly weighed and understood. The important
category of "family" appears now in Cuvier's system fully established as
intermediate between genus and order. Important changes in Cuvier's
system have been made and proposed by his successors, but in the main it
is still that of the present day.
Cuvier had extended his researches beyond the living forms, into the
field of palaeontology; he was the first to observe the close
resemblance of the scales of the fossil _Palaeoniscus_ to those of the
living _Polypterus_ and _Lepidosteus_, the prolongation and identity of
structure of the upper caudal lobe in _Palaeoniscus_ and the sturgeons,
the presence of peculiar "fulcra" on the anterior margin of the dorsal
fin in _Palaeoniscus_ and _Lepidosteus_, and inferred from these facts
that the fossil genus was allied either to the sturgeons or to
_Lepidosteus_. But it did not occur to him that there was a close
relationship between those recent fishes. _Lepidosteus_ and, with it,
the fossil genus remained in his system a member of the order of
_Malacopterygii abdominales_.
Agassiz.
It was left to L. Agassiz (1807-1873) to point out the importance of the
structure of the scales as a characteristic, and to open a path towards
the knowledge of a whole new subclass of fishes, the _Ganoidei_.
Impressed with the fact that the peculiar scales of _Polypterus_ and
_Lepidosteus_ are common to all fossil osseous fishes down to the Chalk,
he takes the structure of the scales generally as the base for an
ichthyological system, and distinguishes four orders:--
1. _Placoids._--Without scales proper, but with scales of enamel,
sometimes large, sometimes small, and reduced to mere points (Rays,
Sharks and Cyclostomi, with the fossil Hybodontes). 2.
_Ganoids._--With angular bony scales, covered with a thick stratum of
enamel: to this order belong the fossil Lepidoides, Sauroides,
Pycnodontes and Coelacanthi; the recent Polypterus, Lepidosteus,
Sclerodermi, Gymnodontes, Lophobranches and Siluroides; also the
Sturgeons. 3. _Ctenoids._--With rough scales, which have their free
margins denticulated: Chaetodontidae, Pleuronectidae, Percidae,
Polyacanthi, Sciaenidae, Sparidae, Scorpaenidae, Aulostomi. 4.
_Cycloids._--With smooth scales, the hind margin of which lacks
denticulation: Labridae, Mugilidae, Scombridae, Gadoidei, Gobiidae,
Muraenidae, Lucioidei, Salmonidae, Clupeidae, Cyprinidae.
If Agassiz had had an opportunity of acquiring a more extensive and
intimate knowledge of existing fishes before his energies were absorbed
in the study of fossil remains, he would doubtless have recognized the
artificial character of his classification. The distinctions between
cycloid and ctenoid scales, between placoid and ganoid fishes, are
vague, and can hardly be maintained. So far as the living and
post-Cretacean forms are concerned, he abandoned the vantage-ground
gained by Cuvier; and therefore his system could never supersede that of
his predecessor, and finally shared the fate of every classification
based on the modifications of one organ only. But Agassiz opened an
immense new field of research by his study of the infinite variety of
fossil forms. In his principal work, _Recherches sur les poissons
fossiles_, Neuchâtel, 1833-1843, 4to, atlas in fol., he placed them
before the world arranged in a methodical manner, with excellent
descriptions and illustrations. His power of discernment and penetration
in determining even the most fragmentary remains is astonishing; and, if
his order of Ganoids is an assemblage of forms very different from what
is now understood by that term, he was the first who recognized that
such an order of fishes exists.
The discoverer of the _Ganoidei_ was succeeded by their explorer
Johannes Müller (1801-1858). In his classical memoir _Über den Bau und
die Grenzen der Ganoiden_ (Berl., 1846) he showed that the Ganoids
differ from all the other osseous fishes, and agree with the
Plagiostomes, in the structure of the heart. By this primary character,
all heterogeneous elements, as Siluroids, _Osteoglossidae_, &c., were
eliminated from the order as understood by Agassiz. On the other hand,
he did not recognize the affinity of _Lepidosiren_ to the Ganoids, but
established for it a distinct subclass, _Dipnoi_, which he placed at the
opposite end of the system. By his researches into the anatomy of the
lampreys and _Amphioxus_, their typical distinctness from other
cartilaginous fishes was proved; they became the types of two other
subclasses, _Cyclostomi_ and _Leptocardii_.
Müller proposed several other modifications of the Cuvierian system;
and, although all cannot be maintained as the most natural arrangements,
yet his researches have given us a much more complete knowledge of the
organization of the Teleostean fishes, and later inquiries have shown
that, on the whole, the combinations proposed by him require only some
further modification and another definition to render them perfectly
natural.
The discovery (in the year 1871) of a living representative of a genus
hitherto believed to be long extinct, _Ceratodus_, threw a new light on
the affinities of fishes. The writer of the present article, who had the
good fortune to examine this fish, was enabled to show that, on the one
hand, it was a form most closely allied to _Lepidosiren_, and, on the
other, that it could not be separated from the Ganoid fishes, and
therefore that _Lepidosiren_ also was a Ganoid,--a relation already
indicated by Huxley in a previous paper on "Devonian Fishes."
Having followed the development of the ichthyological system down to
this period, we now enumerate the most important contributions to
ichthyology which appeared contemporaneously with or subsequently to the
publication of the great work of Cuvier and Valenciennes. For the sake
of convenience we may arrange these works under two heads.
I. VOYAGES, CONTAINING GENERAL ACCOUNTS OF ZOOLOGICAL COLLECTIONS
A. _French._--1. _Voyage autour du monde sur les corvettes de S. M.
l'Uranie et la Physicienne, sous le commandement de M. Freycinet_,
"Zoologie--Poissons," par Quoy et Gaimard (Paris, 1824). 2. _Voyage de
la Coquille_, "Zoologie," par Lesson (Paris, 1826-1830). 3. _Voyage de
l'Astrolabe, sous le commandement de M. J. Dumont d'Urville_,
"Poissons," par Quoy et Gaimard (Paris, 1834). 4. _Voyage au Pôle Sud
par M. J. Dumont d'Urville_, "Poissons," par Hombron et Jacquinot
(Paris, 1853-1854).
B. _English._--1. _Voyage of H.M.S. Sulphur_, "Fishes," by J.
Richardson (Lond., 1844-1845). 2. _Voyage of H.M.SS. Erebus and
Terror_, "Fishes," by J. Richardson (Lond., 1846). 3. _Voyage of
H.M.S. Beagle_, "Fishes," by L. Jenyns (Lond., 1842).
_C. German._--1. _Reise der österreichischen Fregatte Novara_,
"Fische," von R. Kner (Vienna, 1865).
II. FAUNAE
A. _Great Britain._--1. R. Parnell, _The Natural History of the Fishes
of the Firth of Forth_ (Edin., 1838). 2. W. Yarrell, _A History of
British Fishes_ (3rd ed., Lond., 1859). 3. J. Couch, _History of the
Fishes of the British Islands_ (Lond., 1862-1865).
B. _Denmark and Scandinavia._--1. H. Kröyer, _Danmark's Fiske_
(Copenhagen, 1838-1853). 2. S. Nilsson, _Skandinavisk Fauna_, vol. iv.
"Fiskarna" (Lund, 1855). 3. Fries och Ekström, _Skandinaviens Fiskar_
(Stockh., 1836).
C. _Russia._--1. Nordmann, "Ichthyologie pontique," in Demidoff's
_Voyage dans la Russie méridionale_, tome iii. (Paris, 1840).
D. _Germany._--1. Heckel und Kner, _Die Süsswasserfische der
österreichischen Monarchie_ (Leipz., 1858). 2. C. T. E. Siebold, _Die
Süsswasserfische von Mitteleuropa_ (Leipz., 1863).
E. _Italy and Mediterranean._--1. Bonaparte, _Iconografia della fauna
italica_, tom iii., "Pesci" (Rome, 1832-1841). 2. Costa, _Fauna del
regno di Napoli_, "Pesci" (Naples, about 1850).
F. _France._--1. E. Blanchard, _Les Poissons des eaux douces de la
France_ (Paris, 1866).
G. _Spanish Peninsula._--The fresh-water fish fauna of Spain and
Portugal was almost unknown, until F. Steindachner paid some visits to
those countries for the purpose of exploring the principal rivers. His
discoveries are described in several papers in the _Sitzungsberichte
der Akademie zu Wien_. B. du Bocage and F. de B. Capello made
contributions to our knowledge of the marine fishes on the coast of
Portugal (_Jorn. Scienc. Acad. Lisb._).
H. _North America._--1. J. Richardson, _Fauna Bareali-Americana_, part
iii., "Fishes" (Lond., 1836). The species described in this work are
nearly all from the British possessions in the north. 2. Dekay,
_Zoology of New York_, part iv., "Fishes" (New York, 1842). 3.
_Reports of the U.S. Comm. of Fish and Fisheries_ (5 vols.,
Washington, 1873-1879) and _Reports_ and special publications of the
U.S. Bureau of Fisheries contain valuable information. Numerous
descriptions of North American fresh-water fishes have been published
in the reports of the various U.S. Government expeditions, and in
North American scientific journals, by D. H. Storer, S. F. Baird, C.
Girard, W. O. Ayres, E. D. Cope, D. S. Jordan, G. Brown Goode, &c.
I. _Japan._--1. _Fauna Japonica_, "Poissons," par H. Schlegel,
(Leiden, 1850).
J. _East Indies; Tropical parts of the Indian and Pacific Oceans._--1.
E. Rüppell, _Atlas zu der Reise im nördlichen Afrika_ (Frankf., 1828).
2. E. Rüppell, _Neue Wirbelthiere_, "Fische" (Frankf., 1837). 3. R. L.
Playfair and A. Günther, _The Fishes of Zanzibar_ (Lond., 1876). 4. C.
B. Klunzinger, _Synopsis der Fische des Rothen Meers_ (Vienna,
1870-1871). 5. F. Day, _The Fishes of India_ (Lond., 1865, 4to)
contains an account of the fresh-water and marine species. 6. A.
Günther, _Die Fische der Südsee_ (Hamburg, 4to), from 1873 (in
progress). 7. Unsurpassed in activity, as regards the exploration of
the fish fauna of the East Indian archipelago, is P. Bleeker
(1819-1878), a surgeon in the service of the Dutch East Indian
Government, who, from the year 1840, for nearly thirty years, amassed
immense collections of the fishes of the various islands, and
described them in extremely numerous papers, published chiefly in the
journals of the Batavian Society. Soon after his return to Europe
(1860) Bleeker commenced to collect the final results of his labours
in a grand work, illustrated by coloured plates, _Atlas ichthyologique
des Indes Orientales Néerlandaises_ (Amsterd., fol., 1862), the
publication of which was interrupted by the author's death in 1878.
K. _Africa._--1. A. Günther, "The Fishes of the Nile," in Petherick's
_Travels in Central Africa_ (Lond., 1869). 2. W. Peters,
_Naturwissenschaftliche Reise nach Mossambique_, iv., "Flussfische"
(Berl., 1868, 4to).
L. _West Indies and South America._--1. L. Agassiz, _Selecta genera et
species piscium, quae in itinere per Brasiliam, collegit J. B. de
Spix_ (Munich, 1829, fol.). 2. F. de Castelnau, _Animaux nouveaux ou
rares, recueillis pendant l'expédition dans les parties centrales de
l'Amérique du Sud_, "Poissons" (Paris, 1855). 3. L. Vaillant and F.
Bocourt, _Mission scientifique au Mexique et dans l'Amérique
centrale_, "Poissons" (Paris, 1874). 4. F. Poey, the celebrated
naturalist of Havana, devoted many years of study to the fishes of
Cuba. His papers and memoirs are published partly in two periodicals,
issued by himself, under the title of _Memorias sobre la historia
natural de la isla de Cuba_ (from 1851), and _Repertorio
fisico-natural de la isla de Cuba_ (from 1865), partly in North
American scientific journals. And, finally, F. Steindachner and A.
Günther have published many contributions, accompanied by excellent
figures, to our knowledge of the fishes of Central and South America.
M. _New Zealand._--1. F. W. Hutton and J. Hector, _Fishes of New
Zealand_ (Wellington, 1872).
N. _Arctic Regions._--1. C. Lütken, "A Revised Catalogue of the Fishes
of Greenland," in _Manual of the Natural History, Geology and Physics
of Greenland_ (Lond., 1875, 8vo). 2. The fishes of Spitzbergen were
examined by A. J. Malmgren (1865). (A. C. G.)
II. HISTORY AND LITERATURE FROM 1880
In the systematic account which followed the above chapter in the 9th
edition of the _Encyclopaedia Britannica_, the following classification,
which is the same as that given in the author's _Introduction to the
Study of Fishes_ (London, 1880) was adopted by Albert Günther:--
Subclass I. : PALAEICHTHYES.
Order I. : _Chondropterygii._
With two suborders : Plagiostomata and Holocephala.
Order II. : _Ganoidei._
With eight suborders : Placodermi, Acanthodini, Dipnoi,
Chondrostei, Polypteroidei, Pycnodontoidei, Lepidosteoidei,
Amioidei.
Subclass II. : TELEOSTEI.
Order I. : _Acanthopterygii._
With the divisions Perciformes, Beryciformes, Kurtiformes,
Polynemiformes, Sciaeniformes, Xiphiiformes, Trichiuriformes,
Cotto-Scombriformes, Gobiiformes, Blenniformes, Mugiliformes,
Gastrosteiformes, Centrisciformes, Gobiesociformes, Channiformes,
Labyrinthibranchii, Lophotiformes, Taeniiformes and
Notacanthiformes.
Order II. : _Acanthopterygii Pharyngognathi._
Order III. : _Anacanthini._
With two divisions : Gadoidei and Pleuronectoidei.
Order IV. : _Physostomi._
Order V. : _Lophobranchii._
Order VI. : _Plectognathi._
Subclass III. : CYCLOSTOMATA.
Subclass IV. : LEPTOGARDII.
It was an artificial system, in which the most obvious relationships of
the higher groups were lost sight of, and the results of the already
fairly advanced study of the fossil forms to a great extent discarded.
This system gave rise to much adverse criticism; as T. H. Huxley
forcibly put it in a paper published soon after (1883), opposing the
division of the main groups into Palaeichthyes and Teleostei:
"Assuredly, if there is any such distinction to be drawn on the basis of
our present knowledge among the higher fishes, it is between the Ganoids
and the Plagiostomes, and not between the Ganoids and the Teleosteans";
at the same time expressing his conviction, "first, that there are no
two large groups of animals for which the evidence of a direct genetic
connexion is better than in the case of the Ganoids and the Teleosteans;
and secondly, that the proposal to separate the Elasmobranchii
(Chondropterygii of Günther), Ganoidei and Dipnoi of Müller into a group
apart from, and equivalent to, the Teleostei appears to be inconsistent
with the plainest relations of these fishes." This verdict has been
endorsed by all subsequent workers at the classification of fishes.
Günther's classification would have been vastly improved had he made
use of a contribution published as early as 1871, but not referred to by
him. As not even a passing allusion is made to it in the previous
chapter, we must retrace our steps to make good this striking omission.
Edward Drinker Cope (1840-1897) was a worker of great originality and
relentless energy, who, in the sixties of the last century, inspired by
the doctrine of evolution, was one of the first to apply its principles
to the classification of vertebrates. Equally versed in recent and
fossil zoology, and endowed with a marvellous gift, or "instinct" for
perceiving the relationship of animals, he has done a great deal for the
advance of our knowledge of mammals, reptiles and fishes. Although often
careless in the working out of details and occasionally a little too
bold in his deductions, Cope occupies a high rank among the zoologists
of the 19th century, and much of his work has stood the test of time.
The following was Cope's classification, 1871 (_Tr. Amer. Philos. Soc._
xiv. 449).
Subclass I. Holocephali.
" II. Selachii.
" III. Dipnoi.
" IV. Crossopterygia, with two orders: Haplistia and Cladistia.
" V. Actinopteri.
The latter is subdivided in the following manner:--
Tribe I. : Chondrostei.
Two orders : Selachostomi and Glaniostomi.
Tribe II. : Physostomi.
Twelve orders: Ginglymodi, Halecomorphi, Nematognathi, Scyphophori,
Plectospondyli, Isospondyli, Haplomi, Glanencheli, Ichthyocephali,
Holostomi, Enchelycephali, Colocephali.
Tribe III. : Physoclysti.
Ten orders : Opisthomi, Percesoces, Synentognathi, Hemibranchii,
Lophobranchii, Pediculati, Heterosomata, Plectognathi, Percomorphi,
Pharyngognathi.
Alongside with so much that is good in this classification, there are
many suggestions which cannot be regarded as improvements on the views
of previous workers. Attaching too great an importance to the mode of
suspension of the mandible, Cope separated the Holocephali from the
Selachii and the Dipnoi from the Crossopterygii, thus obscuring the
general agreement which binds these groups to each other, whilst there
is an evident want of proportion in the five subclasses. The exclusion
from the class Pisces of the Leptocardii, or lancelets, as first
advocated by E. Haeckel, was a step in the right direction, whilst that
of the Cyclostomes does not seem called for to such an authority as R.
H. Traquair, with whom the writer of this review entirely concurs.
The group of Crossopterygians, first separated as a family from the
other Ganoids by Huxley, constituted a fortunate innovation, and so was
its division into two minor groups, by which the existing forms
(_Polypteroidei_) were separated as Cladistia. The divisions of the
Actinopteri, which includes all Teleostomes other than the Dipneusti and
Crossopterygii also showed, on the whole, a correct appreciation of
their relationships, the Chondrostei being well separated from the other
Ganoids with which they were generally associated. In the groupings of
the minor divisions, which Cope termed orders, we had a decided
improvement on the Cuvierian-Müllerian classification, the author having
utilized many suggestions of his fellow countrymen Theodore Gill, who
has done much towards a better understanding of their relationships. In
the association of the Characinids with the Cyprinids (Plectospondyli)
in the separation of the flat-fishes from the Ganoids, in the
approximation of the Lophobranchs to the sticklebacks and of the
Plectognaths to the Acanthopterygians, and in many other points, Cope
was in advance of his time, and it is to be regretted that his
contemporaries did not more readily take up many of his excellent
suggestions for the improvement of their systems.
In the subsequent period of his very active scientific life, Cope made
many alterations to his system, the latest scheme published by him being
the following ("Synopsis of the families of Vertebrata," _Amer. Natur._,
1889, p. 849):--
Class : Agnatha.
I. Subclass : OSTRACODERMI.
Orders : Arrhina, Diplorrhina.
II. Subclass : MARSIPOBRANCHII.
Orders : Hyperotreti, Hyperoarti.
Class : Pisces.
I. Subclass : HOLOCEPHALI.
II. Subclass : DIPNOI.
III. Subclass : ELASMOBRANCHII.
Orders : Ichthyotomi, Selachii.
IV. Subclass : TELEOSTOMI.
(i.) Superorder : _Rhipidopterygia._
Orders : Rhipidistia, Actinistia.
(ii.) Superorder : _Crossopterygia._
Orders : Placodermi, Haplistia, Taxistia, Cladistia.
(iii.) Superorder : _Podopterygia_ (Chondrostei).
(iv.) Superorder : _Actinopterygia._
Orders : Physostomi, Physoclysti.
This classification is that followed, with many emendations, by A. S.
Woodward in his epoch-making _Catalogue of Fossil Fishes_ (4 vols.,
London, 1889-1901), and in his most useful _Outlines of Vertebrate
Paleontology_ (Cambridge, 1898), and was adopted by Günther in the 10th
edition of the _Encyclopaedia Britannica_:--
Class : Agnatha.
I. Subclass : CYCLOSTOMI.
With three orders : (a) _Hyperoartia_ (Lampreys); (b)
_Hyperotreti_ (Myxinoids); (c) _Cycliae_ (Palaeospondylus).
II. Subclass : OSTRACODERMI.
With four orders : (a) _Heterostraci_ (Coelolepidae,
Psammosteidae, Drepanaspidae, Pteraspidae); (b) _Osteostraci_
(Cephalaspidae, Ateleaspidae, &c.); (c) Antiarchi
(Asterolepidae, Pterichthys, Bothrolepis, &c.); (d) Anaspida
(Birkeniidae).
Class : Pisces.
I. Subclass : ELASMOBRANCHII.
With four orders : (a) _Pleuropterygii_ (Cladoselache); (b)
_Ichthyotomi_ (Pleuracanthidae); (c) _Acanthodii_
(Diplacanthidae, and Acanthodidae); (d) _Selachii_ (divided
from the structure of the vertebral centres into
Asterospondyli and Tectospondyli).
II. Subclass : HOLOCEPHALI.
With one order : _Chimaeroidei._
III. Subclass : DIPNOI.
With two orders : (a) _Sirenoidei_ (Lepidosiren, Ceratodus,
Uronemidae, Ctenodontidae); (b) _Arthrodira_ (Homosteus,
Coccosteus, Dinichthys).
IV. Subclass : TELEOSTOMI.
A. Order : _Crossopterygii._
With four suborders: (1) _Haplistia_ (Tarassius); (2)
_Rhipidistia_ (Holoptychidae, Rhizodontidae, Osteolepidae);
(3) _Actinistia_ (Coelacanthidae); (4) _Cladistia_
(Polypterus).
B. Order : _Actinopterygii._
With about twenty suborders: (1) _Chondrostei_
(Palaeoniscidae, Platysomidae, Chondrosteidae, Sturgeons);
(2) _Protospondyli_ (Semionotidae, Macrosemiidae,
Pycnodontidae, Eugnathidae, Amiidae, Pachycormidae); (3)
_Aetheospondyli_ (Aspidorhynchidae, Lepidosteidae); (4)
_Isospondyli_ (Pholidophoridae, Osteoglossidae, Clupeidae,
Leptolepidae, &c.); (5) _Plectospondyli_ (Cyprinidae,
Characinidae); (6) _Nematognathi_; (7) _Apodes_; and the
other Teleosteans.
There are, however, grave objections to this system, which cannot be
said to reflect the present state of our knowledge. In his masterly
paper on the evolution of the Dipneusti, L. Dollo has conclusively shown
that the importance of the autostyly on which the definition of the
Holocephali from the Elasmobranchii or Selachii and of the Dipneusti
from the Teleostomi rested, had been exaggerated, and that therefore the
position assigned to these two groups in Günther's classification of
1880 still commended itself. Recent work on _Palaeospondylus_, on the
Ostracoderms, and on the Arthrodira, throws great doubt on the propriety
of the positions given to them in the above classification, and the rank
assigned to the main divisions of the Teleostomi do not commend
themselves to the writer of the present article, who would divide the
fishes into three subclasses:--
I. Cyclostomi
II. Selachii
III. Teleostomi,
the characters and contents of which will be found in separate
articles; in the present state of uncertainty as to their position,
_Palaeospondylus_ and the _Ostracodermi_ are best placed _hors cadre_
and will be dealt with under these names.
The three subclasses here adopted correspond exactly with those proposed
in Theo. Gill's classification of the recent fishes ("Families and
Subfamilies of Fishes," _Mem. Nat. Ac. Sci._ vi. 1893), except that they
are regarded by that authority as classes.
The period dealt with in this chapter, ushered in by the publication of
Günther's _Introduction to the Study of Fishes_, has been one of
extraordinary activity in every branch of ichthyology, recent and
fossil. A glance at the _Zoological Record_, published by the Zoological
Society of London, will show the ever-increasing number of monographs,
morphological papers and systematic contributions, which appear year
after year. The number of new genera and species which are being
proposed is amazing, but it is difficult to tell how many of them will
simply go to swell the already overburdened synonymy. Perhaps a
reasonable estimate of the living species known at the present day would
assess their number at about 13,000.
It is much to be regretted that there is not a single general modern
systematic work on fishes. The most important treatises, the 7th volume
of the _Cambridge Natural History_, by T. W. Bridge and G. A. Boulenger,
and D. S. Jordan's _Guide to the Study of Fishes_, only profess to give
definitions of the families with enumerations of the principal genera.
Günther's _Catalogue of the Fishes in the British Museum_ therefore
remains the only general descriptive treatise, but its last volume dates
from 1870, and the work is practically obsolete. A second edition of it
was begun in 1894, but only one volume, by Boulenger, has appeared, and
the subject is so vast that it seems doubtful now whether any one will
ever have the time and energy to repeat Günther's achievement. The fish
fauna of the different parts of the world will have to be dealt with
separately, and it is in this direction that descriptive ichthyology is
most likely to progress.
North America, the fishes of which were imperfectly known in 1880, now
possesses a _Descriptive Catalogue_ in 4 stout volumes, by D. S. Jordan
and B. W. Evermann, replacing the synopsis brought out in 1882 by D. S.
Jordan and C. H. Gilbert. A similar treatise should embrace all the
fresh-water species of Africa, the fishes of the two principal river
systems, the Nile and the Congo, having recently been worked out by G.
A. Boulenger. Japanese ichthyology has been taken in hand by D. S.
Jordan and his pupils.
The fishes of the deep sea have been the subject of extensive monographs
by L. Vaillant (_Travailleur_ and _Talisman_), A. Günther
(_Challenger_), A. Alcock (_Investigator_), R. Collett (_Hirondelle_),
S. Garman (_Albatross_) and a general résumé up to 1895 was provided in
G. B. Goode's and T. H. Bean's _Oceanic Ichthyology_. More than 600 true
bathybial fishes are known from depths of 1000 fathoms and more, and a
great deal of evidence has been accumulated to show the general
transition of the surface fauna into the bathybial.
A recent departure has been the exploration of the Antarctic fauna.
Three general reports, on the results of the _Southern Cross_, the
_Belgica_ and the Swedish _South Polar_ expeditions, had already been
published in 1907, and others on the _Scotia_ and _Discovery_ were in
preparation. No very striking new types of fishes have been discovered,
but the results obtained are sufficient to entirely disprove the theory
of bipolarity which some naturalists had advocated. Much has been done
towards ascertaining the life-histories of the fishes of economic
importance, both in Europe and in North America, and our knowledge of
the larval and post-larval forms has made great progress.
Wonderful activity has been displayed in the field of palaeontology, and
the careful working out of the morphology of the archaic types has led
to a better understanding of the general lines of evolution; but it is
to be regretted that very little light on the relationships of the
living groups of Teleosteans has been thrown by the discoveries of
palaeontologists.
Among the most remarkable additions made in recent years, the work of R.
H. Traquair on the problematic fishes _Palaeospondylus_, _Thelodus_,
_Drepanaspis_, _Lanarkia_, _Ateleaspis_, _Birkenia_ and _Lanasius_,
ranks foremost; next to it must be placed the researches of A. S.
Woodward and Bashford Dean on the primitive shark _Cladoselache_, and of
the same authors, J. S. Newberry, C. R. Eastman, E. W. Claypole and L.
Hussakof, on the Arthrodira, a group the affinities of which have been
much discussed.
AUTHORITIES.--The following selection from the extremely extensive
ichthyological literature which has appeared during the period
1880-1906 will supplement the bibliographical notice appended to
section I. I. The General Subject: A. Günther, _Introduction to the
Study of Fishes_ (Edinburgh, 1880); B. Dean, _Fishes Living and
Fossil_ (New York, 1895); T. W. Bridge and G. A. Boulenger, "Fishes,"
_Cambridge Natural History_, vii. (1904); D. S. Jordan, _Guide to the
Study of Fishes_ (2 vols., New York, 1905). II. Palaeontological: A.
Fritsch, _Fauna der Gaskohle und der Kalksteine der Permformation
Böhmens_ (vols, i.-iii., Prague, 1879-1894); K. A. von Zittel,
_Handbuch der Paläontologie_, vol. iii. (Munich, 1887); A. Smith
Woodward, _Catalogue of Fossil Fishes in the British Museum_, vols.
i.-iii. (London, 1889-1895); A. Smith Woodward, _Outlines of
Vertebrate Palaeontology for Students of Zoology_ (Cambridge, 1898);
J. S. Newberry, "The Palaeozoic Fishes of North America," _Mon. U.S.
Geol. Surv._ vol. xvi. (1889); J. V. Rohon, "Die obersilurischen
Fische von Ösel, Thyestidae und Tremataspidae," _Mém. Ac. Imp. Sc.
St-Pétersb._ xxxviii. (1892); O. Jaekel, _Die Selachier von Bolca, ein
Beitrag zur Morphogenie der Wirbeltiere_ (Berlin, 1894); B. Dean,
"Contributions to the Morphology of Cladoselache," _Journ. Morphol._
ix. (1894); R. H. Traquair, "The Asterolepidae," _Mon. Palaeont. Soc._
(1894-1904, in progress); "Report on Fossil Fishes collected by the
Geological Survey of Scotland in the Silurian Rocks of the South of
Scotland," _Trans. Roy. Soc. Edin._ xxxix. (1899); L. Dollo, "Sur la
phylogénie des Dipneustes," _Bull. Soc. Belge Géol._ vol. ix. (1895);
E. W. Claypole, "The Ancestry of the Upper Devonian Placoderms of
Ohio," _Amer. Geol._ xvii. (1896); B. Dean, "Palaeontlogical Notes,"
_Mem. N.Y. Ac._ ii. (1901); A. Stewart and S. W. Williston,
"Cretaceous Fishes of Kansas," _Univ. Geol. Surv. Kansas_, vi.
(Topeka, 1901); A. S. Woodward, "Fossil Fishes of the English Chalk,"
_Palaeontogr. Soc._ (1902-1903, etc.); R. H. Traquair, "The Lower
Devonian Fishes of Gemünden.," _Roy. Soc. Edin. Trans._ 40 (1903); W.
J. and I. B. J. Sollas, "Account of the Devonian Fish
Palaeospondylus," _Phil. Trans._ 196 (1903); C. T. Regan, "Phylogeny
of the Teleostomi," _Ann. & Mag. N.H._ (7) 13 (1904); C. R. Eastman,
"Fishes of Monte Bolca," _Bull. Mus. C.Z._ 46 (1904); "Structure and
Relations of Mylostoma," _Op. cit._ 2 (1906); O. Abel, "Fossile
Flugfische," _Jahrb. Geol. Reichsanst._ 56 (Wien, 1906); L. Hussakof.
"Studies on the Arthrodira," _Mem. Amer. Mus. N.H._ ix. (1906). III.
Faunistic (recent fishes): (A) EUROPE: E. Bade, _Die
mitteleuropäischen Süsswasserfische_ (2 vols., Berlin, 1901-1902).
GREAT BRITAIN: F. Day, _The Fishes of Great Britain and Ireland_ (2
vols., London, 1880-1884); J. T. Cunningham, _The Natural History of
the Marketable Marine Fishes of the British Islands_ (London, 1896);
W. C. M'Intosh and A. T. Masterman, _The Life-Histories of the British
Marine Food-Fishes_ (London, 1897); Sir H. Maxwell, _British
Fresh-water Fish_ (London, 1904); F. G. Aflalo, _British Salt-water
Fish_ (London, 1904). Numerous important researches into the
development, life-conditions and distributions, carried out at the
Biological Laboratories at Plymouth and St Andrews and during the
survey of the fishing grounds of Ireland, have been published by W. L.
Calderwood, J. T. Cunningham, E. W. L. Holt, W. C. M'Intosh, J. W.
Fulton, W. Garstang and Prince in the _Journ. Mar. Biolog. Assoc._,
_The Reports of the Fishery Board of Scotland_, _Scient. Trans. R.
Dublin Soc._ and other periodicals. (B) DENMARK AND SCANDINAVIA: W.
Lilljeborg, _Sveriges och Norges Fiskar_ (3 vols., Upsala, 1881-1891);
F. A. Smith, _A History of Scandinavian Fishes by B. Fries, C. U.
Ekström and C. Sundevall, with Plates by W. von Wright_ (second
edition, revised and completed by F. A. S., Stockholm, 1892); A.
Stuxberg, _Sveriges och Norges Fiskar_ (Göteborg, 1895); C. G. J.
Petersen, _Report of the Danish Biological Station_ (Copenhagen,
1802-1900) (annual reports containing much information on fishes of
and fishing in the Danish seas). (C) FINLAND: G. Sundman and A. J.
Mela, _Finland's Fiskar_ (Helsingfors, 1883-1891). (D) GERMANY: K.
Möbius and F. Heincke, "Die Fische der Ostsee," _Bericht Commiss.
Untersuch. deutsch. Meere_ (Kiel, 1883); F. Heincke, E. Ehrenbaum and
G. Duncker have published their investigations into the life-history
and development of the fishes of Heligoland in _Wissenschaftl.
Meeresuntersuchungen_ (Kiel and Leipzig, 1894-1899); (E) SWITZERLAND:
V. Fatio, _Faune des vertébrés de la Suisse: Poissons_ (2 vols.,
Geneva and Basel, 1882-1890). (F) FRANCE: E. Moreau, _Histoire
naturelle des poissons de la France_ (3 vols., Paris, 1881);
_Supplément_ (Paris, 1891). (G) PYRENEAN PENINSULA: D. Carlos de
Bragança, _Resultados das investigações scientificas feitas a bordo do
yacht "Amelia." Pescas maritimas_, i. and ii. (Lisbon, 1899-1904). (H)
ITALY AND MEDITERRANEAN: P. Döderlein, _Manuale ittiologico del
Mediterraneo_ (Palermo, 1881-1891, not completed; interrupted by the
death of the author); E. W. L. Holt, "Recherches sur la reproduction
des poissons osseux, principalement dans le golfe de Marseille," _Ann.
Mus. Mars._ v. (Marseilles, 1899); (I) WESTERN AND CENTRAL ASIA: L.
Lortet, "Poissons et reptiles du lac de Tibériade," _Arch. Mus.
d'Hist. Nat. Lyon_, iii. (1883); S. Herzenstein, _Wissenschaftliche
Resultate der von N. M. Przewalski nach Central Asien unternommenen
Reisen: Fische_ (St Petersburg, 1888-1891); L. Berg, _Fishes of
Turkestan_ (Russian text, St Petersburg, 1905); G. Radde, S. Kamensky
and F. F. Kawraisky have worked out the Cyprinids and Salmonids of the
Caucasus (Tiflis, 1896-1899). (J) JAPAN: F. Steindachner and L.
Döderlein, "Beiträge zur Kenntniss der Fische Japans," _Denkschr. Ak.
Wien_, (vols. 67 and 68, 1883); K. Otaki, T. Fujita and T. Higurashi,
_Fishes of Japan_ (in Japanese) (Tokyo, 1903, in progress). Numerous
papers by D. S. Jordan, in collaboration with J. O. Snyder, E. C.
Starks, H. W. Fowler and N. Sindo. (K) EAST INDIES: F. Day, _The Fauna
of British India: Fishes_ (2 vols., London, 1889) (chiefly an
abridgment of the author's _Fishes of India_); M. Weber, "Die
Süsswasserfische des Indischen Archipels," _Zool. Ergebnisse e. Reise
in Niederl. Ostind._ iii. (Leiden, 1894). Numerous contributions to
the fauna of the Malay Peninsula and Archipelago by G. A. Boulenger,
L. Vaillant, F. Steindachner, G. Duncker, W. Volz and C. L. Popta. (L)
AFRICA: G. A. Boulenger, _Matériaux pour la faune du Congo: poissons
nouveaux_ (Brussels, 1898-1902, in progress); and _Poissons du bassin
du Congo_ (Brussels, 1901); G. Pfeffer, _Die Thierwelt Ostafrikas:
Fische_ (Berlin, 1896); A. Günther, G. A. Boulenger, G. Pfeffer, F.
Steindachner, D. Vinciguerra, J. Pellegrin and E. Lönnberg have
published numerous contributions to the fish-fauna of tropical Africa
in various periodicals. The marine fishes of South Africa have
received special attention on the part of J. D. F. Gilchrist, _Marine
Investigations in South Africa_, i. and ii. (1898-1904), and new
species have been described by G. A. Boulenger and C. T. Regan. (M)
NORTH AMERICA: D. S. Jordan and B. W. Evermann, _The Fishes of North
and Middle America_ (4 vols., Washington, 1896-1900); D. S. Jordan and
B. W. Evermann, _American Food and Game Fishes_ (New York, 1902); D.
S. Jordan and C. H. Gilbert "The Fishes of Bering Sea," in _Fur-Seals
and Fur-Seal Islands_ (Washington, 1899); The U.S. Bureau of Fisheries
(since 1903) has published annually a _Report_ and a _Bulletin_,
containing a vast amount of information on North American fishes and
every subject having a bearing on the fisheries of the United States;
S. E. Meek, "Fresh-water Fishes of Mexico," _Field Columb. Mus. Zool._
v. (1904). (N) SOUTH AMERICA: C. H. and R. S. Eigenmann, "A Catalogue
of the Fresh-water Fishes of South America," _Proc. U.S. Nat. Mus._ 14
(Washington, 1891); the same authors, F. Steindachner, G. A.
Boulenger, C. Berg and C. T. Regan have published contributions in
periodicals on this fauna. (O) AUSTRALIA: J. E. Tenison-Woods, _Fish
and Fisheries of New South Wales_ (Sydney, 1882); J. Douglas Ogilby,
Edible Fishes and Crustaceans of New South Wales (Sydney, 1893); J.
Douglas Ogilby and E. R. Waite are authors of numerous papers on
Australian fishes in _Proc. Linn. Soc. N.S. Wales_ and _Rec. Austral.
Mus._ (P) SOUTH PACIFIC: D S. Jordan and B. W. Evermann, "Shore Fishes
of the Hawaiian Islands," _Bull. U.S. Fish. Comm._ 23 (1905). (Q)
MADAGASCAR: H. E. Sauvage, _Histoire physique, naturelle et politique
de Madagascar_, par A. Grandidier. xvi.; _Poissons_ (Paris, 1891). (R)
OCEANIC FISHES: G. B. Goode and T. H. Bean, _Oceanic Ichthyology_
(Washington, 1895); A. Günther, _Deep-sea Fishes of the "Challenger"
Expedition_ (London, 1887); C. H. Gilbert, "Deep-sea Fishes of the
Hawaiian Islands," _Bull. U.S. Fish. Comm._ 23 (1905); R. Collett,
_Norske Nordhavs Expedition: Fiske_ (Christiania, 1880); C. F. Lütken,
_Dijmphna-Togtets Zoologisk-botaniske Udbytte: Kara-Havets Fiske_
(Copenhagen, 1886); L. Vaillant, _Expéditions scientifiques du
"Travailleur" et du "Talisman": Poissons_ (Paris, 1888); A. Agassiz,
_Three Cruises of the U.S. Coast and Geodetic Survey Steamer "Blake"_
(Boston and New York, 1888); A. Alcock, _Illustrations of the Zoology
of H.M.S. "Investigator": Fishes_ (Calcutta, 1892-1899, in progress);
A. Alcock, _Descriptive Catalogue of the Indian Deep-sea Fishes in the
Indian Museum_ (Calcutta, 1899, contains references to all the
previous papers of the author on the subject); R. Collett, _Résultats
des campagnes scientifiques accomplies par Albert I^er prince de
Monaco: poissons provenant des campagnes du yacht "l'Hirondelle,"_
(Monaco, 1896); R. Koehler, _Résultats scientifiques de la campagne du
"Caudan,"_ (Paris, 1896); C. H. Gilbert and F. Cramer, "Report on the
Fishes dredged in Deep Water near the Hawaiian Islands," _Proc. U.S.
Nat. Mus._ xix. (Washington, 1896); C. Lütken, "Spolia Atlantica,"
_Vidensk. Selsk. Skr._ vii. and ix. (Copenhagen, 1892-1898); C.
Lütken, _Danish Ingolf Expedition_, ii.: _Ichthyological Results_
(Copenhagen, 1898); S. Garman, "Reports on an Exploration off the West
Coast of Mexico, Central and South America, and off the Galapagos
Islands in charge of Alexander Agassiz, by the U.S. Fish Commission
Steamer "Albatross," during 1891," _Mem. Mus. Comp. Zool._ vol. xxiv.
(Cambridge, U.S.A., 1899). (S) ANTARCTIC FISHES: G. A. Boulenger,
_Report on the Collections made during the voyage of the "Southern
Cross": Fishes_ (London, 1902); L. Dollo, _Expédition Antarctique
Belge_ (S.Y. "Belgica"). _Poissons_ (Antwerp, 1904); E. Lönnberg,
_Swedish South Polar Expedition: Fishes_ (Stockholm, 1905); G. A.
Boulenger, _Fishes of the "Discovery" Antarctic Expedition_ (London,
1906). (G. A. B.)
III. DEFINITION OF THE CLASS _Pisces_. ITS PRINCIPAL DIVISIONS
Fishes, constituting the class _Pisces_, may be defined as Craniate
Vertebrata, or Chordata, in which the anterior portion of the central
nervous system is expanded into a brain surrounded by an unsegmented
portion of the axial skeleton; which are provided with a heart,
breathing through gills; and in which the limbs, if present, are in the
form of fins, as opposed to the pentadactyle, structure common to the
other Vertebrata. With the exception of a few forms in which lungs are
present in addition to the gills, thus enabling the animal to breathe
atmospheric air for more or less considerable periods (Dipneusti), all
fishes are aquatic throughout their existence.
In addition to the paired limbs, median fins are usually present,
consisting of dermal rays borne by endoskeletal supports, which in the
more primitive forms are strikingly similar in structure to the paired
fins that are assumed to have arisen from the breaking up of a lateral
fold similar to the vertical folds out of which the dorsal, anal and
caudal fins have been evolved. The body is naked, or scaly, or covered
with bony shields or hard spines.
Leaving aside the Ostracophori, which are dealt with in a separate
article, the fishes may be divided into three subclasses--
I. Cyclostomi or Marsipobranchii, with the skull imperfectly developed,
without jaws, with a single nasal aperture, without paired fins, and
with an unpaired fin without dermal rays. Lampreys and hag-fishes.
II. Selachii or Chondropterygii, with the skull well developed but
without membrane bones, with paired nasal apertures, with median and
paired fins, the ventrals bearing prehensile organs (claspers) in the
males. Sharks, skates and chimaeras.
III. Teleostomi, with the skull well developed and with membrane bones,
with paired nasal apertures, primarily with median and paired fins,
including all other fishes. (G. A. B.)
IV. ANATOMY[1]
The special importance of a study of the anatomy of fishes lies in the
fact that fishes are on the whole undoubtedly the most archaic of
existing craniates, and it is therefore to them especially that we must
look for evidence as to the evolutionary history of morphological
features occurring in the higher groups of vertebrates.
In making a general survey of the morphology of fishes it is essential
to take into consideration the structure of the young developing
individual (embryology) as well as that of the adult (comparative
anatomy in the narrow sense). Palaeontology is practically dumb
excepting as regards external form and skeletal features, and even of
these our knowledge must for long be in a hopelessly imperfect state.
While it is of the utmost importance to pay due attention to
embryological data it is equally important to consider them critically
and in conjunction with broad morphological considerations. Taken by
themselves they are apt to be extremely misleading.
_External Features._--The external features of a typical fish are
intimately associated with its mode of life. Its shape is more or less
that of a spindle; its surface is covered with a highly glandular
epidermis, which is constantly producing lubricating mucus through the
agency of which skin-friction is reduced to an extraordinary degree; and
finally it possesses a set of remarkable propelling organs or fins.
The exact shape varies greatly from the typical spindle shape with
variations in the mode of life; e.g. bottom-living fishes may be much
flattened from above downwards as in the rays, or from side to side in
the Pleuronectids such as flounder, plaice or sole, or the shape may
be much elongated as in the eels.
_Head, Trunk and Tail._--In the body of the fish we may recognize the
three main sub-divisions of the body--head, trunk and tail--as in the
higher vertebrates, but there is no definite narrowing of the anterior
region to form a neck such as occurs in the higher groups, though a
suspicion of such a narrowing occurs in the young _Lepidosiren_.
The tail, or postanal region, is probably a secondary development--a
prolongation of the hinder end of the body for motor purposes. This is
indicated by the fact that it frequently develops late in ontogeny.
The vertebrate, in correlation perhaps with its extreme cephalization,
develops from before backwards (except the alimentary canal, which
develops more _en bloc_), there remaining at the hind end for a
prolonged period a mass of undifferentiated embryonic tissue from the
anterior side of which the definitive tissues are constantly being
developed. After development has reached the level of the anus it
still continues backwards and the tail region is formed, showing a
continuation of the same tissues as in front, notochord, nerve cord,
gut, myotomes. Of these the (postanal) gut soon undergoes atrophy.
_Fins._--The fins are extensions of the body surface which serve for
propulsion. To give the necessary rigidity they are provided with
special skeletal elements, while to give mobility they are provided with
special muscles. These muscles, like the other voluntary muscles of the
body, are derived from the primitive myotomes and are therefore
segmental in origin. The fins are divisible into two main
categories--the median or unpaired fins and the paired fins.
[Illustration: FIG. 1.--Heterocercal Tail of _Acipenser_. a, Modified
median scales ("fulcra"); b, bony plates.]
[Illustration: From _Cambridge Natural History_, vol. vii., "Fishes,
&c.," by permission of Messrs. Macmillan & Co., Ltd.
FIG. 2.--_Cladoselache._ (After Dean.)]
The median fins are to be regarded as the more primitive. The
fundamental structure of the vertebrate, with its median skeletal axis
and its great muscular mass divided into segments along each side of the
body, indicates that its primitive method of movement was by waves of
lateral flexure, as seen in an Amphioxus, a cyclostome or an eel. The
system of median fins consists in the first instance of a continuous
fin-fold extending round the posterior end of the body--as persists even
in the adult in the existing Dipneusti. A continuous median fin-fold
occurs also in various Teleosts (many deep-sea Teleosts, eels, &c.),
though the highly specialized features in other respects make it
probable that we have here to do with a secondary return to a condition
like the primitive one. In the process of segmentation of the originally
continuous fin-fold we notice first of all a separation of and an
increase in size of that portion of the fin which from its position at
the tip of the tail region is in the most advantageous position for
producing movements of the body. There is thus formed the _caudal_ fin.
In this region there is a greatly increased size of the fin-fold--both
dorsally and ventrally. There is further developed a highly
characteristic asymmetry. In the original symmetrical or _protocercal_ (
= _diphycercal_) type of tail (as seen in a cyclostome, a Dipnoan and in
most fish embryos) the skeletal axis of the body runs straight out to
its tip--the tail fold being equally developed above and below the axis.
In the highly developed caudal fin of the majority of fishes, however,
the fin-fold is developed to a much greater extent on the ventral side,
and correlated with this the skeletal axis is turned upwards as in the
_heterocercal_ tail of sharks and sturgeons. The highest stage in this
evolution of the caudal fin is seen in the Teleostean fishes, where the
ventral tail-fold becomes developed to such an extent as to produce a
secondarily symmetrical appearance (_homocercal_ tail, fig. 4).
[Illustration: From _"Challenger" Reports Zool._, published by H.M.
Stationery Office.
FIG. 3.--_Chlamydoselachus_. (After Günther.)]
The sharks have been referred to as possessing heterocercal tails,
but, though this is true of the majority, within the limits of the
group all three types of tail-fin occur, from the protocercal tail of
the fossil Pleuracanthids and the living _Chlamydoselachus_ to the
highly developed, practically homocercal tail of the ancient
_Cladoselache_(fig. 2).
The praecaudal portion of the fin-fold on the dorsal side of the body
becomes broken into numerous finlets in living Crossopterygians, while
in other fishes it disappears throughout part of its length, leaving
only one, two or three enlarged portions--the _dorsal_ fins (fig. 4,
d.f.). Similarly the praecaudal part of the fin-fold ventrally becomes
reduced to a single _anal_ fin (a.f.), occasionally continued backwards
by a series of finlets (_Scombridae_). In the sucker-fishes (_Remora_,
_Eckeneis_) the anterior dorsal fin is metamorphosed into a sucker by
which the creature attaches itself to larger fishes, turtles, &c.
[Illustration: From _Cambridge Natural History_, vol. vii., "Fishes,
&c.," by permission of Messrs. Macmillan & Co., Ltd.
FIG. 4.--_Tilapia dolloi_, a teleostean fish, to illustrate external
features. (After Boulenger.)
A, Side view. g.r, Gill rakers.
B, First branchial arch. l.l, Lateral line organs.
a.f, Anal fin. n, Nasal opening.
c.f, Caudal fin. p.f, Pelvic fin.
d.f, Dorsal fin. p.op, Preoperculum.
g.f, Gill lamellae. pt.f, Pectoral fin.]
The paired fins--though more recent developments than the median--are
yet of very great morphological interest, as in them we are compelled to
recognize the homologues of the paired limbs of the higher vertebrates.
We accordingly distinguish the two pairs of fins as pectoral or anterior
and pelvic ( = "ventral") or posterior. There are two main types of
paired fin--the _archipterygial_ type, a paddle-like structure supported
by a jointed axis which bears lateral rays and exists in an unmodified
form in _Neoceratodus_ alone amongst living fishes, and the
_actinopterygial_ type, supported by fine raylike structures as seen in
the fins of any ordinary fish. The relatively less efficiency of the
archipterygium and its predominance amongst the more ancient forms of
fishes point to its being the more archaic of these two types.
In the less highly specialized groups of fishes the pectoral fins are
close behind the head, the pelvic fins in the region of the cloacal
opening. In the more specialized forms the pelvic fins frequently show a
more or less extensive shifting towards the head, so that their position
is described as thoracic (fig. 4) or jugular (_Gadus_--cod, haddock,
&c., fig. 5).
[Illustration: FIG. 5.--Burbot (_Lota vulgaris_), with jugular ventral
fins.]
The median fin, especially in its caudal section, is the main
propelling organ: the paired fins in the majority of fishes serve for
balancing. In the Dipneusti the paired fins are used for clambering
about amidst vegetation, much in the same fashion as the limbs of
Urodeles. In _Ceratodus_ they also function as paddles. In various
Teleosts the pectoral fins have acquired secondarily a leg-like
function, being used for creeping or skipping over the mud
(_Periophthalmus_; cf. also Trigloids, Scorpaenids and Pediculati). In
the "flying" fishes the pectoral fins are greatly enlarged and are
used as aeroplanes, their quivering movements frequently giving a
(probably erroneous) impression of voluntary flapping movements. In
the gobies and lumpsuckers (_Cyclopteridae_) the pelvic fins are fused
to form an adhesive sucker; in the _Gobiesocidae_ they take part in
the formation of a somewhat similar sucker.
The evolutionary history of the paired limbs forms a fascinating
chapter in vertebrate morphology. As regards their origin two
hypotheses have attracted special attention: (1) that enunciated by
Gegenbaur, according to which the limb is a modified gill septum, and
(2) that supported by James K. Thacher, F. M. Balfour, St George
Mivart and others, that the paired fins are persisting and modified
portions of a once continuous fin-fold on each side of the body. The
majority of morphologists are now inclined to accept the second of
these views. Each has been supported by plausible arguments, for which
reference must be made to the literature of the subject.[2] Both views
rest upon the assumed occurrence of stages for the existence of which
there is no direct evidence, viz. in the case of (1) transitional
stages between gill septum and limb, and in the case of (2) a
continuous lateral fin-fold. (There is no evidence that the lateral
row of spines in the acanthodian _Climatius_ has any other than a
defensive significance.) In the opinion of the writer of this article,
such assumptions are without justification, now that our knowledge of
Dipnoan and Crossopterygian and Urodele embryology points towards the
former possession by the primitive vertebrate of a series of
projecting, voluntarily movable, and hence potentially motor structure
on each side of the body. It must be emphasized that these--the true
external gills--are the _only_ organs known actually to exist in
vertebrates which might readily be transformed into limbs. When
insuperable objections are adduced to this having actually taken place
in the course of evolution, it will be time enough to fall back upon
purely hypothetical ancestral structures on which to base the
evolutionary history of the limbs.
The ectoderm covering the general surface is highly glandular. In the
case of the Dipneusti, flask-shaped multicellular glands like those of
Amphibians occur in addition to the scattered gland cells.
A characteristic feature of glandular activity is the production of a
slight electrical disturbance. In the case of _Malopterurus_ this
elsewhere subsidiary function of the skin has become so exaggerated as
to lead to the conversion of the skin of each side of the body into a
powerful electrical organ.[3] Each of these consists of some two
million small chambers, each containing an electric disk and all
deriving their nerve supply from the branches of a single enormous
axis cylinder. This takes its origin from a gigantic ganglion cell
situated latero-dorsally in the spinal cord between the levels of the
first and second spinal nerves.
_Cement Organs._--The larvae of certain Teleostomes and Dipnoans possess
special glandular organs in the head region for the secretion of a
sticky cement by which the young fish is able to attach itself to
water-plants or other objects. As a rule these are ectodermal in origin;
e.g. in _Lepidosiren_ and _Protopterus_[4] the crescentic cement organ
lying ventrally behind the mouth consists of a glandular thickening of
the deep layer of the ectoderm. In young ganoid fishes preoral cement
organs occur. In Crossopterygians there is one cup-shaped structure on
each side immediately in front of the mouth. Here the glandular
epithelium is endodermal, developed[5] as an outgrowth from the wall of
the alimentary canal, closely resembling a gill pouch. In _Amia_[6] the
same appears to be the case. In a few Teleosts similar organs occur,
e.g. _Sarcodaces_, _Hyperopisus_,[7] where so far as is known they are
ectodermal.
_Photogenic Organs._--The slimy secretion produced by the epidermal
glands of fishes contains in some cases substances which apparently
readily undergo a slow process of oxidation, giving out light of low
wave-length in the process and so giving rise to a phosphorescent
appearance. In many deep-sea fishes this property of producing
light-emitting secretion has undergone great development, leading to the
existence of definite photogenic organs. These vary much in character,
and much remains to be done in working out their minute structure. Good
examples are seen in the Teleostean family _Scopelidae_, where they form
brightly shining eye-like spots scattered about the surface of the body,
especially towards the ventral side.
[Illustration: From _Trans. Zool. Soc. of London_.
FIG. 6.--Larva of Polypterus. (After Budgett.)]
[Illustration: From _Phil. Transactions, Royal Society of London_.
FIG. 7.--Thirty Days' Larval Lepidosiren. (After Graham Kerr.)]
_External Gills._--In young Crossopterygians and in the young
_Protopterus_ and _Lepidosiren_ true external gills occur of the same
morphological nature as those of Urodele amphibians. In Crossopterygians
a single one is present on each side on the hyoid arch; in the two
Dipnoans mentioned four are present on each side--on visceral arches
III., IV., V. and VI. (It may be recalled that in Urodeles they occur on
arches III., IV. and V., with vestiges[8] on arches I. and II.). Each
external gill develops as a projection of ectoderm with mesodermal core
near the upper end of its visceral arch; the main aortic arch is
prolonged into it as a loop. When fully developed it is pinnate, and is
provided with voluntary muscles by which it can be moved freely to renew
the water in contact with its respiratory surface. In the case of
_Polypterus_ a short rod of cartilage projects from the hyoid arch into
the base of the external gill. Their occurrence with identical main
features in the three groups mentioned indicates that the external gills
are important and archaic organs of the vertebrata. Their non-occurrence
in at least some of the groups where they are absent is to be explained
by the presence of a large vascular yolk sac, which necessarily fulfils
in a very efficient way the respiratory function.
_Alimentary Canal._--The alimentary canal forms a tube traversing the
body from mouth to cloacal opening. Corresponding with structural and
functional differences it is for descriptive purposes divided into the
following regions--(1) Buccal cavity or mouth cavity, (2) Pharynx, (3)
Oesophagus or gullet, (4) Stomach, (5) Intestine, and (6) Cloaca. The
buccal cavity or mouth cavity is morphologically a stomodaeum, i.e. it
represents an inpushing of the external surface. Its opening to the
exterior is wide and gaping in the embryo in certain groups (Selachians
and Crossopterygians), and even in the adult among the Cyclostomata, but
in the adult Gnathostome it can be voluntarily opened and shut in
correlation with the presence of a hinged jaw apparatus. The mouth
opening is less or more ventral in position in Cyclostomes and
Selachians, while in Dipnoans and Teleostomes it is usually terminal.
[Illustration: From Bridge, _Cambridge Natural History_, vol. vii.,
"Fishes, &c." (by permisson of Macmillan & Co., Ltd.). After Boas,
_Lehrbuch der Zoologie_ (by permission of Gustav Fischer).
FIG. 8.--Diagrams to illustrate the relations of branchial clefts and
pharynx in an Elasmobranch (A) and a Teleost (B); 1, 2, &c., Branchial
septa.
b.c, Opercular cavity.
b.l, Respiratory lamellae.
c, Coelom.
e.b.a, Opercular opening.
hy.a, Hyoid arch.
hy.c, Hyobranchial cleft.
l.s, Valvular outer edge of gill septum.
n, Nasal aperture.
oes, Oesophagus.
op, Operculum.
p.q, Palato quadrate cartilage.
Ph, Pharynx.
sp, Spiracle.]
In certain cases (e.g. _Lepidosiren_)[9] the buccal cavity arises by
secondary excavation without any actual pushing in of ectoderm.
It is highly characteristic of the vertebrata that the pharynx--the
portion of the alimentary canal immediately behind the buccal
cavity--communicates with the exterior by a series of paired clefts
associated with the function of respiration and known as the visceral
clefts. It is especially characteristic of fishes that a number of these
clefts remain open as functional breathing organs in the adult.
The visceral clefts arise as hollow pouches (or at first solid
projections) of the endoderm. Each pouch fuses with the ectoderm at its
outer end and then becomes perforated so as to form a free communication
between pharynx and exterior.
The mesenchymatous packing tissue between consecutive clefts forms the
visceral arches, and local condensation within each gives rise to
important skeletal elements--to which the name visceral arches is often
restricted. From the particular skeletal structures which develop in the
visceral arches bounding it the anterior cleft is known as the
hyomandibular cleft, the next one as hyobranchial. In common usage the
hyomandibular cleft is called the spiracle, and the series of clefts
behind it the branchial clefts.
The typical functional gill cleft forms a vertical slit, having on each
side a gill septum which separates it from its neighbours in the series.
The lining of the gill cleft possesses over a less or greater extent of
its area a richly developed network of capillary blood-vessels, through
the thin covering of which the respiratory exchange takes place between
the blood and the water which washes through the gill cleft. The area of
respiratory surface tends to become increased by the development of
outgrowths. Frequently these take the form of regular plate-like
structures known as gill lamellae. In the Selachians these lamellae are
strap-like structures (_Elasmobranch_) attached along nearly their
whole length to the gill septum as shown in fig. 8, A. In the
Holocephali and in the sturgeon the outer portions of the gill septa
have disappeared and this leads to the condition seen in the higher
Teleostomes (fig. 8, B), where the whole of the septum has disappeared
except its thick inner edge containing the skeletal arch. It follows
that in these higher Teleostomes--including the ordinary Teleosts--the
gill lamellae are attached only at their extreme inner end.
In the young of Selachians and certain Teleosts (e.g. _Gymnarchus_ and
_Heterotis_)[10] the gill lamellae are prolonged as filaments which
project freely to the exterior. These must not be confused with true
external gills.
The partial atrophy of the gill septa in the Teleostomes produces an
important change in their appearance. Whereas in the Selachian a series
of separate gill clefts is seen in external view each covered by a soft
valvular backgrowth of its anterior lip, in the Teleostean fish, on the
other hand, a single large opening is seen on each side (opercular
opening) covered over by the enormously enlarged valvular flap belonging
to the anterior lip of the hyobranchial cleft. This flap, an outgrowth
of the hyoid arch, is known as the operculum.
In the Teleostomi there are usually five functional clefts, but these
are the survivors of a formerly greater number. Evidence of reduction is
seen at both ends of the series. In front of the first functional cleft
(the hyobranchial) there is laid down in the embryo the rudiment of a
spiracular cleft. In the less highly organized fishes this survives in
many cases as an open cleft.
In many sharks and in sturgeons the spiracle forms a conspicuous
opening just behind the eye. In rays and skates, which are modified in
correlation with their ground feeding habit, the spiracle is a large
opening which during the great widening out of the body during
development comes to be situated on the dorsal side, while the
branchial clefts come to be ventral in position. In existing
Crossopterygians the spiracle is a slit-like opening on the dorsal
side of the head which can be opened or closed at will. In Dipneusti,
as in the higher Teleostomes, the spiracle is found as an embryonic
rudiment, but in this case it gives rise in the adult to a remarkable
sense organ of problematical function.[11]
Traces of what appear to be pre-spiracular clefts exist in the embryos
of various forms. Perhaps the most remarkable of these is to be found in
the larval Crossopterygian,[12] and apparently also in _Amia_[13] at
least, amongst the other ganoids, where a pair of entodermal pouches
become cut off from the main entoderm and, establishing an opening to
the exterior, give rise to the lining of the cement organs of the larva.
Posteriorily there is evidence that the extension backwards of the
series of gill clefts was much greater in the primitive fishes. In the
surviving sharks (_Chlamydoselachus_ and _Notidanus cinereus_), there
still exist in the adult respectively six and seven branchial clefts,
while in embryonic Selachians there are frequently to be seen pouch-like
outgrowths of entoderm apparently representing rudimentary gill pouches
but which never develop. Further evidence of the progressive reduction
in the series of clefts is seen in the reduction of their functional
activity at the two ends of the series. The spiracle, even where
persisting in the adult, has lost its gill lamellae either entirely or
excepting a few vestigial lamellae forming a "pseudobranch" on its
anterior wall (Selachians, sturgeons). A similar reduction affects the
lamellae on the anterior wall of the hyobranchial cleft (except in
Selachians) and on the posterior wall of the last branchial cleft.
A pseudobranch is frequently present in Teleostomes on the anterior
wall of the hyobranchial cleft, i.e. on the inner or posterior face of
the operculum. It is believed by some morphologists to belong really
to the cleft in front.[14]
_Phylogeny._--The phylogeny of the gill clefts or pouches is
uncertain. The only organs of vertebrates comparable with them
morphologically are the enterocoelic pouches of the entoderm which
give rise to the mesoderm. It is possible that the respiratory
significance of the wall of the gill cleft has been secondarily
acquired. This is indicated by the fact that they appear in some cases
to be lined by an ingrowth of ectoderm. This suggests that there may
have been a spreading inwards of respiratory surface from the external
gills. It is conceivable that before their walls became directly
respiratory the gill clefts served for the pumping of fresh water over
the external gills at the bases of which they lie.
_Lung._--As in the higher vertebrates, there develops in all the main
groups of gnathostomatous fishes, except the Selachians, an outgrowth of
the pharyngeal wall intimately associated with gaseous interchange. In
the Crossopterygians and Dipnoans this pharyngeal outgrowth agrees
exactly in its mid-ventral origin and in its blood-supply with the lungs
of the higher vertebrates, and there can be no question about its being
morphologically the same structure as it is also in function.
[Illustration: FIG. 9.--Lung of _Neoceratodus_, opened in its lower half
to show its cellular pouches. a, Right half; b, Left half; c, Cellular
pouches; e, Pulmonary vein; f, Arterial blood-vessel; oe, Oesophagus,
opened to show glottis (gl.)]
In the Crossopterygian the ventrally placed slit-like glottis leads
into a common chamber produced anteriorly into two horns and continued
backwards into two "lungs." These are smooth, thin-walled, saccular
structures, the right one small, the left very large and extending to
the hind end of the splanchnocoele. In the Dipnoans the lung has taken
a dorsal position close under the vertebral column and above the
splanchnocoele. Its walls are sacculated, almost spongy in
_Lepidosiren_ and _Protopterus_, so as to give increase to the
respiratory surface. In _Nexeratodus_ (fig. 9) an indication of
division into two halves is seen in the presence of two prominent
longitudinal ridges, one dorsal and one ventral. In _Lepidosiren_ and
_Protopterus_ the organ is completely divided except at its anterior
end into a right and a left lung. The anterior portion of the lung or
lungs is connected with the median ventral glottis by a short wide
vestibule which lies on the right side of the oesophagus.
In the Teleostei the representative of the lung, here termed the
swimbladder, has for its predominant function a hydrostatic one; it acts
as a float. It arises as a diverticulum of the gut-wall which may retain
a tubular connexion with the gut (_physostomatous_ condition) or may in
the adult completely lose such connexion (_physoclistic_). It shows two
conspicuous differences from the lung of other forms: (1) it arises in
the young fish as a dorsal instead of as a ventral diverticulum, and (2)
it derives its blood-supply not from the sixth aortic arch but from
branches of the dorsal aorta.
These differences are held by many to be sufficient to invalidate the
homologizing of the swimbladder with the lung. The following facts,
however, appear to do away with the force of such a contention. (1) In
the Dipneusti (e.g. _Neoceratodus_) the lung apparatus has acquired a
dorsal position, but its connexion with the mid-ventral glottis is
asymmetrical, passing round the right side of the gut. Were the
predominant function of the lung in such a form to become hydrostatic
we might expect the course of evolution to lead to a shifting of the
glottis dorsalwards so as to bring it nearer to the definitive
situation of the lung. (2) In _Erythrinus_ and other Characinids the
glottis is not mid-ventral but decidedly lateral in position,
suggesting either a retention of, or a return to, ancestral stages in
the dorsalward migration of the glottis. (3) The blood-supply of the
Teleostean swimbladder is from branches of the dorsal aorta, which may
be distributed over a long anteroposterior extent of that vessel.
Embryology, however, shows that the swimbladder arises as a localized
diverticulum. It follows that the blood-supply from a long stretch of
the aorta can hardly be primitive. We should rather expect the
primitive blood-supply to be from the main arteries of the pharyngeal
wall, i.e. from the hinder aortic arch as is the case with the lungs
of other forms. Now in _Amia_ at least we actually find such a
blood-supply, there being here a pulmonary artery corresponding with
that in lung-possessing forms. Taking these points into consideration
there seems no valid reason for doubting that in lung and swimbladder
we are dealing with the same morphological structure.
_Function._--In the Crossopterygians and Dipnoans the lung is used for
respiration, while at the same time fulfilling a hydrostatic function.
Amongst the Actinopterygians a few forms still use it for respiration,
but its main function is that of a float. In connexion with this
function there exists an interesting compensatory mechanism whereby the
amount of gas in the swimbladder may be diminished (by absorption), or,
on the other hand, increased, so as to counteract alterations in
specific gravity produced, e.g. by change of pressure with change of
depth. This mechanism is specially developed in physoclistic forms,
where there occur certain glandular patches ("red glands") in the lining
epithelium of the swimbladder richly stuffed with capillary
blood-vessels and serving apparently to secrete gas into the
swimbladder. That the gas in the swimbladder is produced by some vital
process, such as secretion, is already indicated by its composition, as
it may contain nearly 90% of oxygen in deep-sea forms or a similar
proportion of nitrogen in fishes from deep lakes, i.e. its composition
is quite different from what it would be were it accumulated within the
swimbladder by mere ordinary diffusion processes. Further, the formation
of gas is shown by experiment to be controlled by branches of the vagus
and sympathetic nerves in an exactly similar fashion to the secretion of
saliva in a salivary gland. (See below for relations of swimbladder to
ear).
Of the important non-respiratory derivatives of the pharyngeal wall
(thyroid, thymus, postbranchial bodies, &c.), only the thyroid calls for
special mention, as important clues to its evolutionary history are
afforded by the lampreys. In the larval lamprey the thyroid develops as
a longitudinal groove on the pharyngeal floor. From the anterior end of
this groove there pass a pair of peripharyngeal ciliated tracts to the
dorsal side of the pharynx where they pass backwards to the hind end of
the pharynx. Morphologically the whole apparatus corresponds closely
with the endostyle and peripharyngeal and dorsal ciliated tracts of the
pharynx of _Amphioxus_. The correspondence extends to function, as the
open thyroid groove secretes a sticky mucus which passes into the
pharyngeal cavity for the entanglement of food particles exactly as in
_Amphioxus_. Later on the thyroid groove becomes shut off from the
pharynx; its secretion now accumulates in the lumina of its interior and
it functions as a ductless gland as in the Gnathostomata. The only
conceivable explanation of this developmental history of the thyroid in
the lamprey is that it is a repetition of phylogenetic history.
Behind the pharynx comes the main portion of the alimentary canal
concerned with the digestion and absorption of the food. This forms a
tube varying greatly in length, more elongated and coiled in the higher
Teleostomes, shorter and straighter in the Selachians, Dipnoans and
lower Teleostomes. The oesophagus or gullet, usually forming a short,
wide tube, leads into the glandular, more or less dilated stomach. This
is frequently in the form of a letter J, the longer limb being
continuous with the gullet, the shorter with the intestine. The curve of
the J may be as in _Polypterus_ and the perch produced backwards into a
large pocket. The intestine is usually marked off from the stomach by a
ring-like sphincter muscle forming the pyloric valve. In the lower
gnathostomatous fishes (Selachians, Crossopterygians, Dipnoans,
sturgeons) the intestine possesses the highly characteristic spiral
valve, a shelf-like projection into its lumen which pursues a spiral
course, and along the turns of which the food passes during the course
of digestion. From its universal occurrence in the groups mentioned we
conclude that it is a structure of a very archaic type, once
characteristic of ancestral Gnathostomata; a hint as to its
morphological significance is given by its method of development.[15] In
an early stage of development the intestinal rudiment is coiled into a
spiral and it is by the fusion together of the turns that the spiral
valve arises. The only feasible explanation of this peculiar method of
development seems to lie in the assumption that the ancestral
gnathostome possessed an elongated, coiled intestine which subsequently
became shortened with a fusion of its coils. In the higher fishes the
spiral valve has disappeared--being still found, however, in a reduced
condition in _Amia_ and _Lepidosteus_, and possibly as a faint vestige
in one or two Teleosts (certain _Clupeidae_[16] and _Salmonidae_[17]).
In the majority of the Teleosts the absence of spiral valves is coupled
with a secondary elongation of the intestinal region, which in extreme
cases (_Loricariidae_) may be accompanied by a secondary spiral coiling.
The terminal part of the alimentary canal--the cloaca--is characterized
by the fact that into it open the two kidney ducts. In Teleostomes the
cloaca is commonly flattened out, so that the kidney ducts and the
alimentary canal come to open independently on the outer surface.
The lining of the alimentary canal is throughout the greater part of its
extent richly glandular. And at certain points local enlargements of the
secretory surface take place so as to form glandular diverticula. The
most ancient of these as indicated by its occurrence even in _Amphioxus_
appears to be the _liver_, which, originally--as we may assume--mainly a
digestive gland, has in the existing Craniates developed important
excretory and glycogen-storing functions. Arising in the embryo as a
simple caecum, the liver becomes in the adult a compact gland of very
large size, usually bi-lobed in shape and lying in the front portion of
the splanchnocoele. The stalk of the liver rudiment becomes drawn out
into a tubular bile duct, which may become subdivided into branches, and
as a rule develops on its course a pocket-like expansion, the
gall-bladder. This may hang freely in the splanchnocoele or may be, as
in many Selachians, imbedded in the liver substance.
The pancreas also arises by localized bulging outwards of the intestinal
lining--there being commonly three distinct rudiments in the embryo. In
the Selachians the whitish compact pancreas of the adult opens into the
intestine some little distance behind the opening of the bile duct, but
in the Teleostomes it becomes involved in the liver outgrowth and mixed
with its tissue, being frequently recognizable only by the study of
microscopic sections. In the Dipnoans the pancreatic rudiment remains
imbedded in the wall of the intestine: its duct is united with that of
the liver.
_Pyloric Caeca._--In the Teleostomi one or more glandular diverticula
commonly occur at the commencement of the intestine and are known as the
pyloric caeca. There may be a single caecum (crossopterygians,
_Ammodytes_ amongst Teleosts) or there may be nearly two hundred
(mackerel). In the sturgeons the numerous caeca form a compact gland. In
several families of Teleosts, on the other hand, there is no trace of
these pyloric caeca.
In Selachians a small glandular diverticulum known as the _rectal gland_
opens into the terminal part of the intestine on its dorsal side.
_Coelomic Organs._--The development of the mesoderm in the restricted
sense (mesothelium) as seen in the fishes (lamprey, _Lepidosiren_,
_Protopterus_, _Polypterus_) appears to indicate beyond doubt that the
mesoderm segments of vertebrates are really enterocoelic pouches in
which the development of the lumen is delayed. Either the inner, or both
inner and outer (e.g. _Lepidosiren_) walls of the mesoderm segment pass
through a myoepithelial condition and give rise eventually to the great
muscle segments (myomeres, or myotomes) which lie in series on each side
of the trunk. In the fishes these remain distinct throughout life. The
fins, both median and paired, obtain their musculature by the ingrowth
into them of muscle buds from the adjoining myotomes.
[Illustration: From Gegenbaur, _Untersuchungen zur vergleich. Anat. der
Wirbeltiere_, by permission of Wilhelm Engelmann.
FIG. 10.--View of _Torpedo_ from the dorsal side: the electric organs are
exposed.
I, Fore-brain.
II, Mesencephalon.
III, Cerebellum.
IV, Electric lobe.
br, Common muscular sheath covering branchial clefts (on the left side
this has been removed so as to expose the series of branchial sacs).
f, Spiracle.
o.e, Electric organ, on the left side the nerve-supply is shown.
o, Eye.
t, Sensory tubes of lateral line system.]
_Electrical Organs._[18]--It is characteristic of muscle that at the
moment of contraction it produces a slight electrical disturbance. In
certain fishes definite tracts of the musculature show a reduction of
their previously predominant function of contraction and an increase of
their previously subsidiary function of producing electrical
disturbance; so that the latter function is now predominant.
In the skates (_Raia_) the electrical organ is a fusiform structure
derived from the lateral musculature of the tail; in _Gymnotus_--the
electric eel--and in _Mormyrus_ it forms an enormous structure
occupying the place of the ventral halves of the myotomes along nearly
the whole length of the body; in _Torpedo_ it forms a large, somewhat
kidney-shaped structure as viewed from above lying on each side of the
head and derived from the musculature of the anterior visceral arches.
In _Torpedo_ the nerve-supply is derived from cranial nerves VII. IX.
and the anterior branchial branches of X.
The electric organ is composed of prismatic columns each built up of a
row of compartments. Each compartment contains a lamellated electric
disc representing the shortened-up and otherwise metamorphosed muscle
fibre. On one face (ventral in _Torpedo_, anterior in _Raia_) of the
electric disc is a gigantic end-plate supplied by a beautiful,
dichotomously branched, terminal nervous arborization.
The development of the mesoderm of the head region is too obscure for
treatment here.[19] The ventral portion of the trunk mesoderm gives rise
to the splanchnocoel or general coelom. Except in the Myxinoids the
anterior part of the splanchnocoel becomes separated off as a
pericardiac cavity, though in adult Selachians the separation becomes
incomplete, the two cavities being in communication by a
pericardio-peritoneal canal.
_Nephridial System._---The kidney system in fishes consists of
segmentally arranged tubes leading from the coelom into a longitudinal
duct which opens within the hinder end of the enteron--the whole forming
what is known as the _archinephros_ (Lankester) or _holonephros_
(Price). Like the other segmented organs of the vertebrate the
archinephros develops from before backwards. The sequence is, however,
not regular. A small number of tubules at the head end of the series
become specially enlarged and are able to meet the excretory needs
during larval existence (_Pronephros_): the immediately succeeding
tubules remain undeveloped, and then come the tubules of the rest of the
series which form the functional kidney of the adult (_Mesonephros_).
The kidney tubules subserve the excretory function in two different
ways. The wall of the tubule, bathed in blood from the posterior
cardinal vein, serves to extract nitrogenous products of excretion from
the blood and pass them into the lumen of the tubule. The open ciliated
funnel or nephrostome at the coelomic end of the tubule serves for the
passage outwards of coelomic fluid to flush the cavity of the tubule.
The secretory activity of the coelomic lining is specially concentrated
in certain limited areas in the neighbourhood of the nephrostomes, each
such area ensheathing a rounded mass depending into the coelom and
formed of a blood-vessel coiled into a kind of skein--a glomerulus. In
the case of the pronephros the glomeruli are as a rule fused together
into a single glomus. In the mesonephros they remain separate and in
this case the portion of coelom surrounding the glomerulus tends to be
nipped off from the general coelom--to form a Malpighian body. The
separation may be incomplete--the Malpighian coelom remaining in
connexion with the general coelom by a narrow peritoneal canal. The
splanchnocoelic end of this is usually ciliated and is termed a
peritoneal funnel: it is frequently confused with the nephrostome.
_Mesonephros._--The kidney of the adult fish is usually a compact gland
extending over a considerable distance in an anteroposterior direction
and lying immediately dorsal to the coelomic cavity.
Peritoneal funnels are present in the adult of certain Selachians (e.g.
_Acanthias_, _Squatina_), though apparently in at least some of these
forms they no longer communicate with the Malpighian bodies or tubules.
The kidneys of the two sides become fused together posteriorly in
_Protopterus_ and in some Teleosts. The mesonephric ducts undergo fusion
posteriorly in many cases to form a median urinary or urinogenital
sinus. In the Selachians this median sinus is prolonged forwards into a
pair of horn-like continuations--the sperm sacs. In Dipnoans the sinus
becomes greatly dilated and forms a large, rounded, dorsally placed
cloacal caecum. In Actinopterygians a urinary bladder of similar
morphological import is commonly present.
_Gonads._--The portion of coelomic lining which gives rise to the
reproductive cells retains its primitive relations most nearly in the
female, where, as a rule, the genital cells are still shed into the
splanchnocoele. Only in Teleostomes (_Lepidosteus_ and most Teleosts)
the modification occurs that the ovary is shut off from the
splanchnocoele as a closed cavity continuous with its duct.
In a few Teleosts (_Salmonidae_, _Muraenidae_, _Cobitis_) the ovary is
not a closed sac, its eggs being shed into the coelom as in other
groups.
The appearance of the ovary naturally varies greatly with the character
of the eggs.
The portion of coelomic lining which gives rise to the male genital
cells (testis) is in nearly, if not quite, all cases, shut off from the
splanchnocoele. The testes are commonly elongated in form. In
Dipneusti[20] (_Lepidosiren_ and _Protopterus_) the hinder portion of
the elongated testis has lost its sperm-producing function, though the
spermatozoa produced in the anterior portion have to traverse it in
order to reach the kidney. In _Polypterus_[21] the testis is continued
backwards as a "testis ridge," which appears to correspond with the
posterior vesicular region of the testis in _Lepidosiren_ and
_Protopterus_. Here also the spermatozoa pass back through the cavities
of the testis ridge to reach the kidney duct. In the young Teleost[22]
the rudiment of the duct forms a backward continuation of the testis
containing a network of cavities and opening as a rule posteriorly into
the kidney duct. It is difficult to avoid the conclusion that the testis
duct of the Teleost is for the most part the equivalent morphologically
of the posterior vesicular region of the testis of _Polypterus_ and the
Dipneusti.
_Relations of Renal and Reproductive Organs._ (1) _Female._--In the
Selachians and Dipnoans the oviduct is of the type (Müllerian duct)
present in the higher vertebrates and apparently representing a
split-off portion of the archinephric duct. At its anterior end is a
wide funnel-like coelomic opening. Its walls are glandular and secrete
accessory coverings for the eggs. In the great majority of Teleosts and
in _Lepidosteus_ the oviduct possesses no coelomic funnel, its walls
being in structural continuity with the wall of the ovary. In most of
the more primitive Teleostomes (Crossopterygians, sturgeons, _Amia_) the
oviduct has at its front end an open coelomic funnel, and it is
difficult to find adequate reason for refusing to regard such oviducts
as true Müllerian ducts. On this interpretation the condition
characteristic of Teleosts would be due to the lips of the oviduct
becoming fused with the ovarian wall, and the duct itself would be a
Müllerian duct as elsewhere.
[Illustration: From _Arch. zool, expérimentale_, by permission of
Schleicher Frères.
FIG. 11.--Urino-Genital Organs of the right side in a male _Scyllium_.
(After Borcea.)
m.n. 1, Anterior (genital) portion of mesonephros
with its coiled duct.
m.n. 2, Posterior (renal) portion of mesonephros.
s.s, Sperm sac.
T, Testis.
u, "Ureter" formed by fusion of collecting tubes of renal portion of
mesonephros.
u.g.s, Urino-genital sinus;
v.s, Vesicula seminalis.]
A departure from the normal arrangement is found in those Teleosts
which shed their eggs into the splanchnocoele, e.g. amongst
_Salmonidae_, the smelt (_Osmerus_) and capelin (_Mallotus_) possess a
pair of oviducts resembling Müllerian ducts while the salmon possesses
merely a pair of genital pores opening together behind the anus. It
seems most probable that the latter condition has been derived from
the former by reduction of the Müllerian ducts, though it has been
argued that the converse process has taken place. The genital pores
mentioned must not be confused with the _abdominal pores_, which in
many adult fishes, particularly in those without open peritoneal
funnels, lead from coelom directly to the exterior in the region of
the cloacal opening. These appear to be recent developments, and to
have nothing to do morphologically with the genitourinary system.[23]
(2) _Male._--It seems that primitively the male reproductive elements
like the female were shed into the coelom and passed thence through the
nephridial tubules. In correlation probably with the greatly reduced
size of these elements they are commonly no longer shed into the
splanchnocoele, but are conveyed from the testis through covered-in
canals to the Malpighian bodies or kidney tubules. The system of
covered-in canals forms the testicular network, the individual canals
being termed vasa efferentia. In all probability the series of vasa
efferentia was originally spread over the whole length of the elongated
testis (cf. _Lepidosteus_), but in existing fishes the series is as a
rule restricted to a comparatively short anteroposterior extent. In
Selachians the vasa efferentia are restricted to the anterior end of
testis and kidney, and are connected by a longitudinal canal ending
blindly in front and behind. The number of vasa efferentia varies and in
the rays (_Raia_, _Torpedo_) may be reduced to a single one opening
directly into the front end of the mesonephric duct. The anterior
portion of the mesonephros is much reduced in size in correlation with
the fact that it has lost its renal function. The hinder part, which is
the functional kidney, is considerably enlarged. The primary tubules of
this region of the kidney have undergone a modification of high
morphological interest. Their distal portions have become much
elongated, they are more or less fused, and their openings into the
mesonephric duct have undergone backward migration until they open
together either into the mesonephric duct at its posterior end or into
the urinogenital sinus independently of the mesonephric duct. The
mesonephric duct is now connected only with the anterior part of the
kidney, and serves merely as a vas deferens or sperm duct. In
correlation with this it is somewhat enlarged, especially in its
posterior portion, to form a vesicula seminalis.
The morphological interest of these features lies in the fact that
they represent a stage in evolution which carried a little farther
would lead to a complete separation of the definitive kidney
(_metanephros_) from the purely genital anterior section of the
mesonephros (_epididymis_), as occurs so characteristically in the
Amniota.
Dipneusti.--In _Lepidosiren_[24] a small number (about half a dozen) of
vasa efferentia occur towards the hind end of the vesicular part of the
testis and open into Malpighian bodies. In _Protopterus_ the vasa
efferentia are reduced to a single one on each side at the extreme hind
end of the testis.
[Illustration: Graham Kerr, _Proc. Zool. Soc. London_.
FIG. 12.--Diagram illustrating Connexion between Kidney and Testis in
Various Groups of Fishes.
A, Distributed condition of _vasa efferentia_ (_Acipenser_,
_Lepidosteus_).
B, _Vasa efferentia_ reduced to a few at the hind end (_Lepidosiren_).
C, Reduction of vasa efferentia to a single one posteriorly
(_Protopterus_).
D, Direct communication between testis and kidney duct (_Polypterus_,
Teleosts).
c.f, Nephrostome leading from Malpighian coelom into kidney tubule.
T1, Functional region of testis.
T2, Vesicular region of testis.
WD, Mesonephric duct.]
Teleostomi.--In the actinopterygian Ganoids a well-developed testicular
network is present; e.g. in _Lepidosteus_[25] numerous vasa efferentia
arise from the testis along nearly its whole length and pass to a
longitudinal canal lying on the surface of the kidney, from which in
turn transverse canals lead to the Malpighian bodies. (In the case of
_Amia_ they open into the tubules or even directly into the mesonephric
duct.) In the Teleosts and in _Polypterus_ there is no obvious connexion
between testis and kidney, the wall of the testis being continuous with
that of its duct, much as is the case with the ovary and its duct in the
female. In all probability this peculiar condition is to be
explained[26] by the reduction of the testicular network to a single vas
efferens (much as in _Protopterus_ or as in _Raia_ and various anurous
Amphibians at the front end of the series) which has come to open
directly into the mesonephric duct (cf. fig. 12).
_Organs of the Mesenchyme._--In vertebrates as in all other Metazoa,
except the very lowest, there are numerous cell elements which no longer
form part of the regularly arranged epithelial layers, but which take
part in the formation of the packing tissue of the body. Much of this
forms the various kinds of connective tissue which fill up many of the
spaces between the various epithelial layers; other and very important
parts of the general mesenchyme become specialized in two definite
directions and give rise to two special systems of organs. One of these
is characterized by the fact that the intercellular substance or matrix
assumes a more or less rigid character--it may be infiltrated with salts
of lime--giving rise to the supporting tissues of the skeletal system.
The other is characterized by the intercellular matrix becoming fluid,
and by the cell elements losing their connexion with one another and
forming the characteristic fluid tissue, the blood, which with its
well-marked containing walls forms the blood vascular system.
_Skeletal System._--The skeletal system may be considered under three
headings--(1) the chordal skeleton, (2) the cartilaginous skeleton and
(3) the osseous skeleton.
1. _Chordal Skeleton._--The most ancient element of the skeleton appears
to be the _notochord_--a cylindrical rod composed of highly vacuolated
cells lying ventral to the central nervous system and dorsal to the gut.
Except in _Amphioxus_--where the condition may probably be secondary,
due to degenerative shortening of the central nervous system--the
notochord extends from a point just behind the infundibulum of the brain
(see below) to nearly the tip of the tail. In ontogeny the notochord is
a derivative of the dorsal wall of the archenteron. The outer layer of
cells, which are commonly less vacuolated and form a "chordal
epithelium," soon secretes a thin cuticle which ensheaths the notochord
and is known as the primary sheath. Within this there is formed later a
secondary sheath, like the primary, cuticular in nature. This secondary
sheath attains a considerable thickness and plays an important part in
strengthening the notochord. The notochord with its sheaths is in
existing fishes essentially the skeleton of early life (embryonic or
larval). In the adult it may, in the more primitive forms (Cyclostomata,
Dipneusti), persist as an important part of the skeleton, but as a rule
it merely forms the foundation on which the cartilaginous or bony
vertebral column is laid down.
2. _Cartilaginous or Chondral Skeleton._--(A) Vertebral column.[27] In
the embryonic connective tissue or mesenchyme lying just outside the
primary sheath of the notochord there are developed a dorsal and a
ventral series of paired nodules of cartilage known as _arcualia_ (fig.
13, d.a, v.a). The dorsal arcualia are commonly prolonged upwards by
supradorsal cartilages which complete the _neural arches_ and serve to
protect the spinal cord. The ventral arcualia become, in the tail region
only, also incorporated in complete arches--the _haemal arches_. In
correlation with the flattening of the body of the fish from side to
side the arches are commonly prolonged into elongated neural or haemal
spines.
The relations of the arcualia to the segmentation of the body, as
shown by myotomes and spinal nerves, is somewhat obscure. The
mesenchyme in which they arise is segmental in origin (sclerotom),
which suggests that they too may have been primitively segmental, but
in existing fishes there are commonly two sets of arcualia to each
body segment.
In gnathostomatous fishes the arcualia play a most important part in
that cartilaginous tissue derived from them comes into special
relationships with the notochord and gives rise to the vertebral column
which functionally replaces this notochord in most of the fishes. This
replacement occurs according to two different methods, giving rise to
the different types of vertebral column known as chordacentrous and
arcicentrous.
(a) Chordacentrous type. An incipient stage in the evolution of a
chordacentrous vertebral column occurs in the Dipneusti, where cartilage
cells from the arcualia become amoeboid and migrate into the substance
of the secondary sheath, boring their way through the primary sheath
(fig. 13, C). They wander throughout the whole extent of the secondary
sheath, colonizing it as it were, and settle down as typical stationary
cartilage cells. The secondary sheath is thus converted into a cylinder
of cartilage. In Selachians exactly the same thing takes place, but in
recent forms development goes a step further, as the cartilage cylinder
becomes broken into a series of segments, known as vertebral centra. The
wall of each segment becomes much thickened in the middle so that the
notochord becomes constricted within each centrum and the space occupied
by it is shaped like the cavity of a dice-box. When free from notochord
and surrounding tissues such a cartilaginous centrum presents a deep
conical cavity at each end (_amphicoelous_).
[Illustration: From Wiedersheim, _Grundriss der vergleichenden
Anatomie_, by permission of Gustav Fischer.
FIG. 13.--Diagrammatic transverse sections to illustrate the morphology
of the vertebral column.
A, Primitive conditions as seen in any young embryo.
B, Condition as it occurs in Cyclostomata, sturgeons, embryos of bony
Actinopterygians.
C, Condition found in Selachians and Dipnoans.
D and E, Illustrating the developmental process in bony
Actinopterygians and higher vertebrates.
c, Centrum.
d.a, Dorsal arcualia.
n.a, Neural arch.
nc, Notochord.
nc.ep, Chordal epithelium.
n.sp, Neural spine.
sh.1, Primary sheath.
sh.2, Secondary sheath.
sk.l, Connective tissue.
tr.p, Transverse process.
v.a, Ventral arcualia.]
A secondary modification of the centrum consists in the calcification
of certain zones of the cartilaginous matrix. The precise arrangement
of these calcified zones varies in different families and affords
characters which are of taxonomic importance in palaeontology where
only skeletal structures are available (see SELACHIANS).
(b) Arcicentrous type. Already in the Selachians the vertebral column is
to a certain extent strengthened by the broadening of the basis of the
arcualia so as partially to surround the centra. In the Teleostomes,
with the exceptions of those ganoids mentioned, the expanded bases of
the arcualia undergo complete fusion to form cartilaginous centra which,
unlike the chordacentrous centra, lie outside the primary sheath (figs.
13, D and E). In these forms no invasion of the secondary sheath by
cartilage cells takes place. The composition of the groups of arcualia
which give rise to the individual centrum is different in different
groups. The end result is an amphicoelous or biconcave centrum in
general appearance much like that of the Selachian.
In _Lepidosteus_ the spaces between adjacent centra become filled by a
secondary development of intervertebral cartilage which then splits in
such a way that the definitive vertebrae are _opisthocoelous_, i.e.
concave behind, convex in front.
_Ribs._--In the Crossopterygians a double set of "ribs" is present on
each side of the vertebral column, a ventral set lying immediately
outside the splanchnocoelic lining and apparently serially homologous
with the haemal arches of the caudal region, and a second set passing
outwards in the thickness of the body wall at a more dorsal level. In
the Teleostomes and Dipnoans only the first type is present; in the
Selachians only the second. It would appear that it is the latter which
is homologous with the ribs of vertebrates above fishes.
_Median Fin Skeleton._--the foundation of the skeleton of the median
fins consists of a series of rod-like elements, the radialia, each of
which frequently is segmented into three portions. In a few cases the
radialia correspond segmentally with the neural and haemal arches
(living Dipnoans, _Pleuracanthus_ tail region) and this suggests that
they represent morphologically prolongations of the neural and haemal
spines. That this is so is rendered probable by the fact that we must
regard the evolution of the system of median fins as commencing with a
simple flattening of the posterior part of the body. It is only natural
to suppose that the edges of the flattened region would be at first
supported merely by prolongations of the already existing spinous
processes. In the Cyclostomes (where they are branched) and in the
Selachians, the radialia form the main supports of the fin, though
already in the latter they are reinforced by a new set of fin rays
apparently related morphologically to the osseous or placoid skeleton
(see below).
The series of radialia tends to undergo the same process of local
concentration which characterizes the fin-fold as a whole. In its
extreme form this leads to complete fusion of the basal portions of a
number of radialia (dorsal fins of _Holoptychius_ and various
Selachians, and anal fin of _Pleuracanthus_). In view of the identity
in function it is not surprising that a remarkable resemblance exists
between the mechanical arrangements (of skeleton, muscles, &c.), of
the paired and unpaired fins. The resemblance to paired fins becomes
very striking in some of the cases where the basal fusion mentioned
above takes place (_Pleuracanthus_).
[Illustration: _Trans. Roy. Soc. Edinburgh._
FIG. 14.--Chondrocranium of a young Lepidosiren, showing the suspension
of the lower jaw by the upper portion of the mandibular arch. (After
Agar.)
H, Hyoid arch.
M, Mandibular arch.
o.a, Occipital arch.
ot, Auditory capsule.
q, Quadrate = upper end of mandibular arch.
tr, Trabecula.
The palatopterygoid bar (p.pt) is represented by a faint vestige which
disappears before the stage figured.]
(B) _Chondrocranium[28]._--In front of the vertebral column lies the
cartilaginous trough, the chondrocranium, which protects the brain. This
consists of a praechordal portion--developed out of a pair of lateral
cartilaginous rods--the _trabeculae cranii_--and a parachordal portion
lying on either side of the anterior end of the notochord. This arises
in development from a cartilaginous rod (parachordal cartilage) lying
on each side of the notochord and possibly representing a fused row of
dorsal arcualia. The originally separate parachordals and trabeculae
become connected to form a trough-like, primitive cranium, complete or
nearly so laterally and ventrally but open dorsally. With the primitive
cranium there are also connected cartilaginous capsules developed round
the olfactory and auditory organs. There also become fused with the
hinder end of the cranium a varying number of originally distinct neural
arches.
[Illustration: _A._ After W. K. Parker, _Trans. Zool. Soc. London_.
_B._ After Gegenbaur, _Untersuchungen zur verg. Anat. der Wirbeltiere_,
by permission of Wilhelm Engelmann.
_C._ After Hubrecht, Brown's _Tierreich_, by permission of Gustav
Fischer.
FIG. 15.--Chondrocranium, &c. of _Scyllium_ (A), _Notidanus cinereus_
(B) and _Chimaera_ (C).
Br.A, Branchial arches. olf, Olfactory capsule.
c.h, Ceratohyal. ot, Auditory capsule.
e.p.l, Ethmopalatine ligament. p.pt, Palato-pterygoid bar.
Hm, Hyomandibular. p.s.l, Prespiracular ligament.
M, Meckel's cartilage. r, Rostrum.]
o, Orbit.
(C) _Visceral Arches._--The skeleton of the visceral arches consists
essentially of a series of half-hoops of cartilage, each divided in the
adult into a number of segments and connected with its fellow by a median
ventral cartilage. The skeleton of arches I. and II. (mandibular and
hyoidean) undergoes modifications of special interest (figs. 14 and 15).
The lower portion of the mandibular arch becomes greatly thickened to
support the lower or hinder edge of the mouth. It forms the primitive
lower jaw or "Meckel's cartilage." Dorsal to this an outgrowth arises from
the anterior face of the arch which supports the upper or anterior margin
of the mouth: it is the primitive upper jaw or palato-pterygoquadrate
cartilage. The portion of the arch dorsal to the palato-pterygo-quadrate
outgrowth may form the suspensorial apparatus of the lower jaw, being
fused with the cranium at its upper end. This relatively primitive
con-arrangement (_protostylic_, as it may be termed) occurs in Dipneusti
among fishes (cf. fig. 14). More usually this dorsal part of the
mandibular arch becomes reduced, its place being occupied by a ligament
(pre-spiracular) uniting the jaw apparatus to the chondrocranium, the
upper jaw being also attached to the chondrocranium by the ethmopalatine
ligament situated more anteriorly. The main attachment, however, of the
jaws to the chondrocranium in such a case, as holds for the majority of
fishes, is through the enlarged dorsal segment of the hyoid arch
(hyomandibular) which articulates at its dorsal end with the
chondrocranium, while its ventral end is attached to the hinge region of
the jaw by stout ligamentous bands. A skull in which the jaws are
suspended in this manner is termed a hyostylic skull (e.g. _Scyllium_ in
fig. 15).
[Illustration: FIG. 16.--Fore-limb of _Ceratodus_.]
In _Notidanus_ (fig. 15, B) there is a large direct articulation of
the upper jaw to the chondrocranium in addition to the indirect one
through the hyomandibular: such a skull is amphistylic. In
_Heterodontus_ the upper jaw is firmly bound to the cranium throughout
its length, while in Holocephali (fig. 15, C) complete fusion has
taken place, so that the lower jaw appears to articulate directly with
the cranium ("auto stylic" condition). In Dipneusti[29] (_Lepidosiren_
and _Protopterus_) the cartilaginous upper jaw never develops (except
in its hinder quadrate portion) beyond the condition of a faint
rudiment, owing doubtless to its being replaced functionally by
precociously developed bone.
(D) _Appendicular Skeleton._--The skeleton of the free part of the limb
is attached to the limb girdle which lies embedded in the musculature of
the body. Each limb girdle is probably to be looked upon as consisting,
like the skeleton of the visceral arches, of a pair of lateral
half-hoops of cartilage. While in _Pleuracanthus_ the lateral halves are
distinct (and segmented like the branchial arches), in living Selachians
generally the two halves are completely fused ventrally with one
another. The part of the girdle lying dorsal to the articulation of the
limb is termed scapular in the case of the pectoral limb, iliac in the
case of the pelvic, while the ventral portions are known respectively as
coracoid and ischio-pubic.
[Illustration: FIG. 17.--a, Skeleton of pectoral limb of
_Pleuracanthus_. (From Gegenbauer, after Frisch.) b, Skeleton of
pectoral limb of _Acanthias_. (After Gegenbauer.)]
In most Teleostomes the primitive pelvic girdle does not develop; in
the Dipneusti it is represented by a median unpaired cartilage.
The skeleton of the free limb is probably seen in its most archaic form
amongst existing fishes in the biserial archipterygium of _Ceratodus_
(fig. 16). This is indicated by the relative predominance of this type
of fin amongst the geologically more ancient fishes. The biserial
archipterygium consists of a segmented axial rod, bearing a praeaxial
and a postaxial series of jointed rays.
In _Protopterus_ and _Lepidosiren_ the limbs are reduced and the
lateral rays have less (_Protopterus_) or more (_Lepidosiren_)
completely disappeared.
[Illustration: From Budgett, _Trans. Zool. Soc. London_, xvi, part vii.
From Wiedersheim's _Verg. Anat. der Wirbeltiere_, by permission of
Gustav Fischer.
FIG. 18.--Skeleton of Pectoral Limb of _Polypterus_. a, 30 mm. larva. b,
Adult.]
[Illustration: From Wiesdersheim's _Verg. Anat. der Wirbeltiere_, by
permission of Gustav Fischer.
FIG. 19.--Skeleton of Pectoral Fin of _Amia_.]
In such an archaic Selachian as _Pleuracanthus_ the fin is clearly of
the biserial archipterygial type, but the lateral rays are reduced
(pectoral) or absent (pelvic) (fig. 17, a) on one side of the axis. In a
typical adult Selachian the pectoral fin skeleton has little apparent
resemblance to the biserial archipterygium--the numerous outwardly
directed rays springing from a series of large basal cartilages (_pro_-,
_meso_- and _metapterygium_). The condition in the young (e.g. fig. 17,
b, _Acanthias_) hints strongly, however, at the possibility of the fin
skeleton being really a modified biserial archipterygium, and that the
basal cartilages represent the greatly enlarged axis which has become
fixed back along the side of the body. In Crossopterygians
(_Polypterus_) the highly peculiar fin skeleton (fig. 18) while still in
the embryonic cartilaginous stage is clearly referable to a similar
condition. In the Actinopterygians--with the increased development of
dermal fin rays--there comes about reduction of the primitive limb
skeleton. The axis becomes particularly reduced, and the fin comes to be
attached directly to the pectoral girdle by a number of basal pieces
(Teleosts) probably representing vestigial rays (cf. fig. 19).
Views on the general morphology of the fin skeleton are strongly
affected by the view held as to the mode of evolution of the fins. By
upholders of the lateral fold hypothesis the type of fin skeleton
described for _Cladoselache_[30] is regarded as particularly
primitive. It is, however, by no means clear that the obscure basal
structures figured (Fig. 20) in this fin do not really represent the
pressed back axis as in _Pleuracanthus_.
The pelvic fin skeleton, while built obviously on the same plan as the
pectoral, is liable to much modification and frequently degeneration.
[Illustration: From Bashford Dean, Mem. _N.Y. Acad. of Science_.
FIG. 20.--Skeleton of Pectoral Fin of _Cladoselache_.]
[Illustration: FIG. 21.--Placoid elements of a male Thorn-back, _Raia
clavata_.]
_Osseous or Bony Skeleton._--The most ancient type of bony skeleton
appears to be represented in the _placoid_ elements such as are seen in
the skin of the Selachian (fig. 21). Each placoid element consists of a
spine with a broadly expanded base embedded in the dermis. The base is
composed of bone: the spine of the somewhat modified bone known as
dentine. Ensheathing the tip of the spine is a layer of extremely hard
enamel formed by the inner surface of the ectoderm which originally
covered it. Such typical placoid scales are well seen on any ordinary
skate. In the groups of fishes above the Selachians, the coating of
placoid elements shows various modifications. The spines disappear,
though they may be present for a time in early development. The bony
basal plates tend to undergo fusion--in certain cases they form a
continuous bony cuirass (various Siluroids, trunk-fishes) formed of
large plates jointed together at their edges. More usually the plates
are small and regular in size. In Crossopterygians and _Lepidosteus_ and
in many extinct forms the scales are of the ganoid type, being
rhomboidal and having their outer layer composed of hard glistening
ganoine. In other Teleostomes the scales are as a rule thin, rounded and
overlapping--the so-called cycloid type (fig. 22, A); where the
posterior edge shows toothlike projections the scale is termed ctenoid
(fig. 22, B). In various Teleosts the scales are vestigial (eel); in
others (as in most electric fishes) they have completely disappeared.
_Teeth._--Certain of the placoid elements belonging to that part of the
skin which gives rise to the lining of the stomodaeum have their spines
enlarged or otherwise modified to form teeth. In the majority of fishes
these remain simple, conical structures: in some of the larger sharks
(_Carcharodon_) they become flattened into trenchant blades with
serrated edges: in certain rays (_Myliobatis_) they form a pavement of
flattened plates suited for crushing molluscan shells. In the young
_Neoceratodus_[31] there are numerous small conical teeth, the bases of
which become connected by a kind of spongework of bony trabeculae. As
development goes on a large basal mass is formed which becomes the
functional tooth plate of the adult, the original separate denticles
disappearing completely. In the other two surviving Dipnoans, similar
large teeth exist, though here there is no longer trace in ontogeny of
their formation by the basal fusion of originally separate denticles. In
the Selachians the bony skeleton is restricted to the placoid elements.
In the Teleostomes and the Dipnoans the original cartilaginous skeleton
becomes to a great extent unsheathed or replaced by bony tissue. It
seems highly probable that the more deeply seated osseous elements
occurring in these as in the higher groups arose in the course of
evolution by the spreading inwards of bony trabeculae from the bases of
the placoid elements. Such a method has been demonstrated as occurring
in individual development in the case of certain of the more
superficially placed bones.[32]
[Illustration: FIG. 22.--A, Cycloid Scale of _Scopelus resplendens_
(magn.). B, Ctenoid Scale of _Lethrinus_ (magn.).]
The placoid element with its cap of enamel secreted by the ectoderm is
probably originally derived from a local thickening of the basement
membrane which with the external cuticle may be looked on as the most
ancient skeletal structure in the Metazoa. The basal plate appears to
have been a later development than the spine; in the palaeozoic
_Coelolepidae_[33] the basal plate is apparently not yet developed.
Only a brief summary can be given here of the leading features in the
osteology of fishes. Care must be taken not to assume that bony elements
bearing the same name in fishes and in other groups, or even in the
various sub-divisions of the fishes, are necessarily strictly
homologous. In all probability bony elements occupying similar positions
and described by the same anatomical name have been evolved
independently from the ancestral covering of placoid elements.
_Teleostei._--It will be convenient to take as the basis of our
description the bony skeleton of such a Teleostean fish as the salmon.
In the vertebral column all the cartilaginous elements are replaced by
bone. The haemal spines of the turned-up tip of the tail are flattened
(hypural bones) and serve to support the caudal fin rays.
In _Argyropelecus_ and in one or two deep-sea forms the vertebral column
remains cartilaginous.
[Illustration: From Parker & Haswell's _Text-book of Zoology_, by
permission of Messrs. Macmillan & Co., Ltd.
FIG. 23.--One of the radialia of the salmon, consisting of three
segments, ptg¹, ptg², ptg³, and supporting a dermal fin ray. _D.F.R._]
Apart from the ossification of the radialia which takes place in the
adults of bony fishes there exist special supporting structures in the
fins (paired as well as median) of all the gnathostomatous fishes and
apparently in nature independent of the cartilaginous skeleton. These
are known as dermal fin-rays.[34] Morphologically they are probably to
be looked on (like placoid elements) as local exaggerations of the
basement membrane.
In their detailed characters two main types of dermal fin-ray may be
recognized. The first of these are horny unjointed rays and occur in
the fins of Selachians and at the edge of the fins of Teleostomes
(well seen in the small posterior dorsal or "adipose" fin,
particularly in Siluroids). The second type of dermal fin-ray is
originally arranged in pairs and forms the main supports of the fin in
the adult Teleost (fig. 23). The members of each pair are in close
contact except proximally where they separate and embrace the tip of
one of the radialia. The fin-rays of this second type are frequently
branched and jointed: in other cases they form unbranched rigid
spines.
In the angler or fishing-frog (_Lophius_) the anterior rays of the
dorsal fin become greatly elongated to form small fishing-rods, from
which depend bait-like lures for the attraction of its prey.
[Illustration: From Wiedersheim, _Verg. Anat. der Wirbeltiere_, by
permission of Gustav Fischer.
FIG. 24.--Chondrocranium of Salmon, seen from the right side.
alsph, Alisphenoid. orbsph, Orbitosphenoid.
basocc, Basioccipital. proot, Prootic.
ekteth, Lateral ethmoid. psph, Parasphenoid.
epiot, Epiotic. ptero, Pterotic.
exocc, Exoccipital. socc, Supra occipital.
fr, Frontal. sphot, Sphenotic.
opisth, Opisthotic. vo, Vomer.]
In the skull of the adult salmon it is seen that certain parts of the
chondrocranium (fig. 24) have been replaced by bone ("cartilage bones")
while other more superficially placed bones ("membrane bones") cover its
surface (fig. 25). Of cartilage bones four are developed round the
foramen magnum--the basioccipital, supraoccipital and two exoccipitals.
In front of the basioccipital is the basisphenoid with an alisphenoid on
each side. The region (presphenoidal) immediately in front of the
basisphenoid is unossified, but on each side of it an orbitosphenoid is
developed, the two orbitosphenoids being closely approximated in the
mesial plane and to a certain extent fused, forming the upper part of
the interorbital septum. In the anterior or ethmoidal portion of the
cranium the only cartilage bones are a pair of lateral ethmoids lying
at the anterior boundary of the orbit. A series of five distinct
elements are ossified in the wall of the auditory or otic capsule, the
prootic and opisthotic more ventrally, and the sphenotic, pterotic and
epiotic more dorsally. The roof of the cranium is covered in by the
following dermal bones--parietals (on each side of the supraoccipital),
frontals, dermal ethmoid and small nasals, one over each olfactory
organ. The floor of the cranium on its oral aspect is ensheathed by the
large parasphenoid and the smaller vomer in front of and overlapping it.
The cartilaginous lower jaw is ossified posteriorly to form the
articular (fig. 25) with a small membrane bone, the angular, ventral to
it, but the main part of the jaw is replaced functionally by a large
membrane bone which ensheaths it--the dentary--evolved in all
probability by the spreading outwards of bony tissue from the bases of
the placoid elements (teeth) which it bears. The original upper jaw
(palatopterygoid bar) is replaced by a chain of bones--palatine in
front, then pterygoid and mesopterygoid, and posteriorly metapterygoid
and quadrate, the latter giving articulation to the articular bone of
the lower jaw. These representatives of the palatopterygoid bar no
longer form the functional upper jaw. This function is performed by
membrane bones which have appeared external to the palatopterygoid
bar--the premaxilla and maxilla--which carry teeth--and the small
scale-like jugal behind them. The quadrate is suspended from the skull
as in the Selachians (hyostylic skull) by the upper portion of the hyoid
arch--here represented by two bones--the hyomandibular and symplectic.
The ventral portion of the hyoid arch is also represented by a chain of
bones (stylohyal, epihyal, ceratohyal, hypohyal and the ventral unpaired
basihyal), as is also each of the five branchial arches behind it. In
addition to the bony elements belonging to the hyoid arch proper a
series of membrane bones support the opercular flap. Ventrally there
project backwards from the ceratohyal a series of ten overlapping
branchiostegal rays, while more dorsally are the broader interopercular,
subopercular and opercular.
[Illustration: From Wiedersheim, _Verg. Anat. der Wirbeltiere_, by
permission of Gustav Fischer.
FIG. 25.--Complete Skull of Salmon from left side.
art, Articular. op, Opercular.
branchiost, Branchiostegal. pal, Palatine.
dent, Dentary. par, Parietal.
epiot, Epiotic. pmx, Premaxilla.
eth, Dermal ethmoid. preop, Preopercular.
fr, Frontal. pt, Pterygoid.
hyom, Hyomandibular. pter, Pterotic.
intop, Interopercular. Quad, Quadrate.
Jug, Jugal. socc, Supraoccipital.
mpt, Mesopterygoid. sphot, Sphenotic.
mtpt, Metapterygoid. subop, Subopercular.
mx, Maxilla. sympl, Symplectic.
nas, Nasal. Zunge, Tongue.]
In addition to the bones already enumerated there is present a ring of
circumorbital bones, a preopercular, behind and external to the
hyomandibular and quadrate, and squamosal, external to the hinder end of
the auditory capsule.
In the salmon, pike, and various other Teleosts, extensive regions of
the chondrocranium persist in the adult, while in others (e.g. the
cod) the replacement by bone is practically complete. Bony elements
may be developed in addition to those noticed in the salmon.
In the sturgeon the chondrocranium is ensheathed by numerous membrane
bones, but cartilage bones are absent. In the Crossopterygians[35] the
chondrocranium persists to a great extent in the adult, but portions
of it are replaced by cartilage bones--the most interesting being a
large sphenethmoid like that of the frog. Numerous membrane bones
cover the chondrocranium externally. In the Dipneusti[36] the
chondrocranium is strengthened in the adult by numerous bones. One of
the most characteristic is the great palatopterygoid bone which
develops very early by the spreading of ossification backwards from
the tooth bases, and whose early development probably accounts for the
non-development of the palatopterygoid cartilage.
_Appendicular Skeleton._--The primitive pectoral girdle, which in the
Dipneusti is strengthened by a sheath of bone, becomes in the
Teleostomes reduced in size (small scapula and coracoid bones) and
replaced functionally by a secondary shoulder girdle formed of
superficially placed membrane bones (supraclavicular and cleithrum or
"clavicle," with, in addition in certain cases, an infraclavicular and
one or two postclavicular elements), and connected at its dorsal end
with the skull by a post-temporal bone.
The pelvic girdle is in Teleostomes completely absent as a rule.
The skeleton of the free limb undergoes ossification to a less or
greater extent in the Teleostomes.
In _Polypterus_ the pectoral fin (fig. 18, B) shows three
ossifications in the basal part of the fin--pro-, meso- and
metapterygium. Of these the metapterygium probably represents the
ossified skeletal axis: while the propterygium and also the numerous
diverging radials probably represent the lateral rays of one side of
the archipterygium.
In the _Teleostomes_ the place of the pelvic girdle is taken
functionally by an element apparently formed by the fusion of the
basal portions of several radials.
_Vascular System._--The main components of the blood vascular system in
the lower vertebrates are the following: (1) a single or double dorsal
aorta lying between the enteron and notochord; (2) a ventral vessel
lying beneath the enteron; and (3) a series of paired hoop-like aortic
arches connecting dorsal and ventral vessels round the sides of the
pharynx. The blood-stream passes forwards towards the head in the
ventral vessel, dorsalwards through the aortic arches, and tailwards in
the dorsal aorta.
The dorsal aorta is single throughout the greater part of its extent,
but for a greater or less extent at its anterior end (_circulus
cephalicus_) it consists of two paired aortic roots. It is impossible to
say whether the paired or the unpaired condition is the more primitive,
general morphological conditions being in favour of the latter, while
embryological evidence rather supports the former. The dorsal aorta,
which receives its highly oxygenated blood from the aortic arches, is
the main artery for the distribution of this oxygenated blood.
Anteriorly the aortic roots are continued forwards as the dorsal carotid
arteries to supply the head region. A series of paired,
segmentally-arranged arteries pass from the dorsal aorta to supply the
muscular body wall, and the branches which supply the pectoral and
pelvic fins (subclavian or brachial artery, and iliac artery) are
probably specially enlarged members of this series of segmental vessels.
Besides these paired vessels a varying number of unpaired branches pass
from dorsal aorta to the wall of the alimentary canal with its glandular
diverticula (coeliac, mesenteric, rectal).
The ventral vessel undergoes complicated changes and is represented in
the adults of existing fishes by a series of important structures. Its
post-anal portion comes with the atrophy of the post-anal gut to lie
close under the caudal portion of the dorsal aorta and is known as the
caudal vein. This assumes a secondary connexion with, and drains its
blood into, the posterior cardinal veins (see below). In the region
between cloaca and liver the ventral vessel becomes much branched or
even reticular and--serving serving to convey the food-laden blood from
the wall of the enteron to the capillary network of the liver--is known
as the hepatic portal vein. The short section in front of the liver is
known as the hepatic vein and this conveys the blood, which has been
treated by the liver, into a section of the ventral vessel, which has
become highly muscular and is rhythmically contractile. This enlarged
muscular portion, in which the contractility--probably once common to
the main vessels throughout their extent--has become concentrated,
serves as a pump and is known as the heart. Finally the precardiac
section of the ventral vessel--the ventral aorta--conveys the blood from
heart to aortic arches.
In addition to the vessels mentioned a large paired vein is developed in
close relation to the renal organ which it serves to drain. This is the
posterior cardinal. An anterior prolongation (anterior cardinal) serves
to drain the blood from the head region. From the point of junction of
anterior and posterior cardinal a large transverse vessel leads to the
heart (_ductus Cuvieri_).
[Illustration: From Boas, _Lehrbuch der Zoologie_, by permission of
Gustav Fischer.
Fig. 26.--Diagram to illustrate the condition of the Conus in an
Elasmobranch (A), _Amia_ (B) and a typical Teleost (C).
a, Atrium. v,v´, Valves.
b.a, Bulbus aortae. v.a, Ventral aorta.
c.a, Conus arteriosus. vt, Ventricle.]
s.v, Sinus venosus.
_Heart._--Originally a simple tube curved into a somewhat S-shape, the
heart, by enlargements, constrictions and fusions of its parts, becomes
converted into the complex, compact heart of the adult. In this we
recognize the following portions--(1) _Sinus venosus_, (2) _Atrium_, (3)
_Ventricle_. A fourth chamber, the _conus arteriosus_, the enlarged and
contractile hinder end of the ventral aorta, is also physiologically a
part of the heart. The sinus venosus receives the blood from the great
veins (ductus Cuvieri and hepatic veins). It--like the atrium which it
enters by an opening guarded by two lateral valves--has thin though
contractile walls. The atrium is as a rule single, but in the Dipnoans,
in correlation with the importance of their pulmonary breathing, it is
incompletely divided into a right and a left auricle. In Neoceratodus
the incomplete division is effected by the presence of a longitudinal
shelf projecting into the atrial cavity from its posterior wall. The
opening of the sinus venosus is to the right of this shell, that of the
pulmonary vein to the left. In _Prototerus_ and _Lepidosiren_ a nearly
complete septum is formed by the fusion of trabeculae, there being only
a minute opening in it posteriorly. The atrium opens by a wide opening
guarded by two or more flap valves provided with chordae tendineae into
the ventricle.
The ventricle, in correspondence with it being the main pumping
apparatus, has its walls much thickened by the development of muscular
trabeculae which, in the lower forms separated by wide spaces in which
most of the blood is contained, become in the Teleostomes so enlarged as
to give the wall a compact character, the spaces being reduced to small
scattered openings on its inner surface. In the Dipnoans the ventricle,
like the atrium, is incompletely divided into a right and left
ventricle. In _Ceratodus_ this is effected by an extension of the
interauricular shelf into the ventricle. In _Lepidosiren_ the separation
of the two ventricles is complete but for a small perforation
anteriorly, the heart in this respect showing a closer approximation to
the condition in the higher vertebrates than is found in any Amphibians
or in any reptiles except the Crocodilia. The conus arteriosus is of
interest from the valvular arrangements in its interior to prevent
regurgitation of blood from ventral aorta into ventricle. In their
simplest condition, as seen e.g. in an embryonic Selachian, these
arrangements consist of three, four or more prominent longitudinal
ridges projecting into the lumen of the conus, and serving to obliterate
the lumen when jammed together by the systole of the conus. As
development goes on each of these ridges becomes segmented into a row of
pocket valves with their openings directed anteriorly so that
regurgitation causes them to open out and occlude the lumen by their
free edges meeting. Amongst the Teleostomes the lower ganoids show a
similar development of longitudinal rows of valves in the conus. In
_Amia_ (fig. 26, B), however, the conus is shortened and the number of
valves in each longitudinal row is much reduced. This leads to the
condition found in the Teleosts (fig. 26, O), where practically all
trace of the conus has disappeared, a single circle of valves
representing a last survivor of each row (save in a few exceptional
cases, e.g. _Albula_, _Tarpen_, _Osteoglossum_, where two valves of each
row are present).
[Illustration: After Newton Parker, from _Trans. of the Royal Irish
Academy_, vol. xxx.
FIG. 27.--Venous System of _Protopterus_, as seen from ventral side.
a, Atrium. k, Kidney.
ac, Anterior cardinal. l, Liver.
an.v, Anastomotic vein. ov.v, Ovarian veins.
c, Intestine. p, Pericardium.
c.v, Caudal vein. p.c.v, Left posterior cardinal.
f.v, Femoral vein. p.v´, Parietal veins.
g.b, Gall-bladder. r.p.v, Renal portal.
h.v, Hepatic vein. s, Stomach.
i.j.v, Inferior jugular vein. s.b.v, Subclavian.]
i.v.c, Posterior vena cava.
In Front of the conus vestige of the Teleost there is present a thick
walled _bulbus aortae_ differing from the conus in not being
rhythmically contractile, its walls being on the contrary richly
provided with elastic tissue.
The Dipnoans[37] show an important advance on the conus as in atrium and
ventricle. The conus has a characteristic spiral twist. Within it in
_Neoceratodus_ are a number of longitudinal rows of pocket valves. One
of these rows is marked out by the very large size of its valves and by
the fact that they are not distinct from one another but even in the
adult form a continuous, spirally-running, longitudinal fold. This ridge
projecting into the lumen of the conus divides it incompletely into two
channels, the one beginning (i.e. at its hinder end) on the _left_ side
and ending in front _ventrally_, the other beginning on the _right_ and
ending _dorsally_. In _Protopterus_ a similar condition occurs, only in
the front end of the conus a second spiral fold is present opposite the
first and, meeting this, completes the division of the conus cavity into
two separate parts. The rows of pocket valves which do not enter into
the formation of the spiral folds are here greatly reduced.
These arrangements in the conus of the Dipnoans are of the highest
morphological interest, pointing in an unmistakable way towards the
condition found in the higher lung-breathing vertebrates. Of the two
cavities into which the conus is partially divided in the Dipneusti the
one which begins posteriorly on the right receives the (venous) blood
from the right side of the heart, and ending up anteriorly dorsal to the
other cavity communicates only with aortic arches V. and VI. In the
higher vertebrates this cavity has become completely split off to form
the root of the pulmonary arteries, and a result of aortic arch V.
receiving its blood along with the functionally much more important VI.
(the pulmonary arch) from this special part of the conus has been the
almost complete disappearance of this arch (V.) in all the higher
vertebrates.
_Arterial System._--There are normally six aortic arches laid down
corresponding with the visceral arches, the first (mandibular) and
second (hyoidean) undergoing atrophy to a less or greater extent in
post-embryonic life. Where an external gill is present the aortic arch
loops out into this, a kind of short-circuiting of the blood-stream
taking place as the external gill atrophies. As the walls of the clefts
assume their respiratory function the aortic arch becomes broken into a
network of capillaries in its respiratory portion, and there is now
distinguished a ventral afferent and a dorsal efferent portion of each
arch. Complicated developmental changes, into which it is unnecessary to
enter,[38] may lead to each efferent vessel draining the two sides of a
single cleft instead of the adjacent walls of two clefts as it does
primitively. In the Crossopterygians and Dipnoans as in the higher
vertebrates the sixth aortic arch gives off the pulmonary artery to the
lung. Among the Actinopterygians this, probably primitive, blood-supply
to the lung (swimbladder) persists only in _Amia_.
[Illustration: FIG. 28.--Venous System of Polypterus 30 mm. larva
(dorsal view).
a.c.v, Anterior cardinal vein.
d.C, Ductus Cuvieri.
h.v, Hepatic vein.
i.j.v, Inferior jugular vein.
ir.v, Inter-renal vein.
l.v, Lateral cutaneous vein.
p.c.v, Posterior cardinal vein.
p.n, Pronephros.
p.v, Pulmonary vein.
s, Subclavian vein.
s.v, Sinus venosus.
th, Thyroid.
v, Vein from pharyngeal wall.
* Anterior portion of left posterior cardinal vein.]
_Venous System._--The most interesting variations from the general plan
outlined have to do with the arrangements of the posterior cardinals. In
the Selachians these are in their anterior portion wide and sinuslike,
while in the region of the kidney they become broken into a sinusoidal
network supplied by the postrenal portion now known as the renal portal
vein. In the Teleostomes the chief noteworthy feature is the tendency to
asymmetry, the right posterior cardinal being frequently considerably
larger than the left and connected with it by transverse anastomotic
vessels, the result being that most of the blood from the two kidneys
passes forwards by the right posterior cardinal. The Dipnoans (fig. 27)
show a similar asymmetry, but here the anterior end of the right
posterior cardinal disappears, being replaced functionally by a new
vessel which conveys the blood from the right posterior cardinal direct
to the sinus venosus instead of to the outer end of the ductus Cuvieri.
This new vessel is the posterior vena cava which thus in the series of
vertebrates appears for the first time in the Dipneusti.
_Pulmonary Veins._--In _Polypterus_ (fig. 28) the blood is drained from
the lungs by a pulmonary vein on each side which unites in front with
its fellow and opens into the great hepatic vein behind the heart. In
the Dipnoans the conjoined pulmonary veins open directly into the left
section of the atrium as in higher forms. In the Actinopterygians with
their specialized air-bladder the blood passes to the heart via
posterior cardinals, or hepatic portal, or--a probably more primitive
condition--directly into the left ductus Cuvieri (_Amia_).
_Lymphatics._--More or less irregular lymphatic spaces occur in the
fishes as elsewhere and, as in the Amphibia, localized muscular
developments are present forming lymph hearts.
_Central Nervous System._--The neural tube shows in very early stages an
anterior dilated portion which forms the rudiment of the brain in
contradistinction to the hinder, narrower part which forms the spinal
cord. This enlargement of the brain is correlated with the increasing
predominance of the nerve centres at the anterior end of the body which
tend to assume more and more complete control over those lying behind.
_Spinal Cord._--A remarkable peculiarity occurs in the sun fishes
(_Molidae_), where the body is greatly shortened and where the spinal
cord undergoes a corresponding abbreviation so as to be actually shorter
than the brain.
_Brain._--It is customary to divide the brain into three main regions,
fore-, mid-, and hind-brain, as in the most familiar vertebrates there
is frequently seen in the embryo a division of the primitive brain
dilatation into three vesicles lying one behind the other. A
consideration of the development of the brain in the various main groups
of vertebrates shows that these divisions are not of equal importance.
In those archaic groups where the egg is not encumbered by the presence
of a large mass of yolk it is usual for the brain to show in its early
stages a division into two main regions which we may term the primitive
fore-brain or cerebrum and the primitive hind-brain or rhombencephalon.
Only later does the hinder part of the primitive fore-brain become
marked off as mid-brain. In the fully developed brain it is customary to
recognize the series of regions indicated below, though the boundaries
between these regions are not mathematical lines or surfaces any more
than are any other biological boundaries:--
Rhombencephalon (Hind-brain) / Myelencephalon (Medulla oblongata).
\ Metencephalon (Cerebellum).
/ Mesencephalon (Mid-brain).
Cerebrum (Primitive Fore-brain) < Thalamencephalon (Diencephalon).
\ [Hemispheres (Telencephalon).]
The myelencephalon or medulla oblongata calls for no special remark,
except that in the case of _Torpedo_ there is a special upward bulging
of its floor on each side of the middle line forming the electric lobe
and containing the nucleus of origin of the nerves to the electric
organ.
[Illustration: A and B from Wiedersheim, by permission of Gustav
Fischer.
FIG. 29.--Brain of _Scyllium_ (A), _Salmo_ (B) and _Lepidosiren_ (C).
The three figures are not drawn to the same scale.
cer, Cerebellum.
c.h, Cerebral hemisphere.
th, Thalamencephalon.
f.b, Primitive fore-brain (in B the line points to the thickened
wall of the fore-brain, the so-called "basal ganglia").
G.p, Pineal body.
m.b, Roof of mid-brain, optic lobes, _tectum opticum_.
o.l, Olfactory lobe.
IV.v, Fourth ventricle.]
The cerebellum occurs in its simplest form in lampreys and Dipnoans
(fig. 29, C), where it forms a simple band-like thickening of the
anterior end of the roof of the hind-brain. In Selachians it is very
large and bulges upwards, forming a conspicuous organ in a dorsal view
of the brain (fig. 29, A). In Teleosts (fig. 29, B) the cerebellum is
also large. It projects back as a great tongue-like structure over the
roof of the fourth ventricle, while in front it dips downwards and
projects under the roof of the mid-brain forming a highly characteristic
_valvula cerebelli_. A _valvula cerebelli_ occurs also in ganoids, while
in the Crossopterygians a similar extension of the cerebellum projects
backwards into the IV. ventricle or cavity of the hind-brain (fig. 30).
The mesencephalon is a conspicuous structure in the fishes from its
greatly developed roof (_tectum opticum_) which receives the end pencils
of the optic nerve. Normally it projects upwards as a pair of large
optic lobes, but in the Dipnoans (fig. 29, C) the lateral thickening is
not sufficiently great to cause obvious lateral swellings in external
view.
[Illustration: FIG. 30.--Median Longitudinal Section through the brain
of _Lepidosiren_ and _Polypterus_. In the upper figure (_Lepidosiren_)
the habenular ganglion and hemisphere are shown in outline though not
actually present in a median section.
a.c, Anterior commissure. par, Paraphysis.
cer, Cerebellum. pin, Pineal body.
d.s, Dorsal sac. p.c, Posterior commissure.
g.h, Habenular ganglion. s.v, Saccus vasculosus.
h.c, Habenular commissure. t.o, Tectum opticum.
i.g, Infundibular gland. v.III, Third ventricle.
l.p, Lateral plexus. v.IV, Fourth ventricle.
o.c, Optic chiasma. vel, Velum transversum.]
pall, Pallium.
The thalamencephalon is one of the most interesting parts of the brain
from its remarkable uniformity throughout the Vertebrata. Even in
_Amphioxus_ the appearance of a sagittal section strongly suggests
vestiges of a once present thalamencephalon.[39] The roof--like that of
the myelencephalon--remains to a great extent membranous, forming with
the closely applied _pia mater_ a vascular roof to the III. ventricle.
Frequently a transverse fold of the roof dips down into the III.
ventricle forming the _velum transversum_ (fig. 30).
The side walls of the thalamencephalon are greatly thickened forming the
_thalamus_ (epithalamus and hypothalamus), while a ganglionic thickening
of the roof posteriorly on each side forms the _ganglia habenulae_ which
receive olfactory fibres from the base of the hemisphere. The habenular
ganglia are unusually large in the lampreys and are here strongly
asymmetrical, the right being the larger.
The floor of the thalamencephalon projects downwards and backwards as
the infundibulum. The side walls of this are thickened to form
characteristic _lobi inferiores_, while the blind end develops glandular
outgrowths (infundibular gland, fig. 30) overlaid by a rich development
of blood sinuses and forming with them the _saccus vasculosus_. The
optic chiasma, where present, is involved in the floor of the
thalamencephalon and forms a large, upwardly-projecting ridge. Farther
forwards on the floor or anterior wall is the anterior commissure (see
below).
Passing forwards from the mid-brain (cf. fig. 30) a series of
interesting structures are found connected with the roof of the
primitive fore-brain, viz.--posterior commissure (intercalary region),
pineal organ, habenular commissure with anterior parietal organ, dorsal
sac (= pineal cushion), _velum transversum_, paraphysis. The posterior
commissure is situated in the boundary between thalamencephalon and
mid-brain. It is formed of fibres connecting up the right and left
sides of the tectum opticum (?). The habenular or superior commissure
situated farther forwards connects the two ganglia habenulae. In the
immediate neighbourhood of these ganglia there project upwards two
diverticula of the brain-roof known as the pineal organ and the
parapineal (or anterior parietal) organ. The special interest of these
organs[40] lies in the fact that in certain vertebrates one (parapineal
in _Sphenodon_ and in lizards) or both (_Petromyzon_) exhibit
histological features which show that they must be looked on as visual
organs or eyes. In gnathostomatous fishes they do not show any definite
eye-like structure, but in certain cases (_Polyodon_, _Callichthys_,
&c.) the bony plates of the skull-roof are discontinuous over the pineal
organ forming a definite parietal foramen such as exists in lizards
where the eye-like structure is distinct. It is also usual to find in
the epithelial wall of the pineal organ columnar cells which show
club-shaped ends projecting into the lumen (exactly as in the young
visual cells of the retina[41]) and are prolonged into a root-like
process at the other end. Definite nerve fibres pass down from these
parietal organs to the brain. It is stated that the fibres from the
pineal organ pass into the posterior commissure, those of the parapineal
organ into the habenular commissure.
The facts mentioned render it difficult to avoid the conclusion that
these organs either have been sensory or are sensory. Possibly they
represent the degenerate and altered vestiges of eye-like organs present
in archaic vertebrates, or it may be that they represent the remains of
organs not eye-like in function but which for some other reason lay
close under the surface of the body. It would seem natural that a
diverticulum of brain-tissue exposed to the influence of light-rays
should exhibit the same reaction as is shown frequently elsewhere in the
animal kingdom and tend to assume secondarily the characters of a visual
organ. The presence of the rod-like features in the epithelial cells is
perhaps in favour of the latter view. In evolution we should expect
these to appear before the camera-like structure of a highly developed
eye, while in the process of degeneration we should expect these fine
histological characters to go first.
Selachians.--No parapineal organ is present. The pineal body (except
in _Torpedo_ where it is absent) is in the form of a long slender tube
ending in front in a dilated bulb lying near the front end of the
brain in close contact with, or enclosed in, a definite foramen in the
cranial roof.
Holocephali and Crossopterygii.--Here also the pineal body is long and
tubular: at its origin it passes dorsalwards or slightly backwards
behind the large dorsal sac.
Actinopterygian Ganoids resemble Selachians on the whole. In _Amia_ a
parapineal organ is present, and it is said to lie towards the left
side and to be connected by a thick nerve with the _left_ habenular
ganglion (cf. _Petromyzon_, article CYCLOSTOMATA). This is adduced to
support the view that the pineal and parapineal bodies represent
originally paired structures.
Teleostei.--A parapineal rudiment appears in the embryo of some forms,
but in the adult only the pineal organ is known to exist. This is
usually short and club-shaped, its terminal part with much folded wall
and glandular in character. In a few cases a parietal foramen occurs
(_Callichthys_, _Loricaria_, &c.).
Dipneusti.--The pineal organ is short and simple. No parapineal organ
is developed.
The dorsal sac is formed by that part of the roof of the
thalamencephalon lying between the habenular commissure and the region
of the velum. In some cases a longitudinal groove is present in which
the pineal organ lies (Dipneusti). In the Crossopterygians the dorsal
sac is particularly large and was formerly mistaken for the pineal
organ.
The _velum transversum_ is a transverse, inwardly-projecting fold of the
roof of the primitive fore-brain in front of the dorsal sac. To those
morphologists who regard the hemisphere region or telencephalon as a
primitively unpaired structure the velum is an important landmark
indicating the posterior limit of the telencephalon. Those who hold the
view taken in this article that the hemispheres are to be regarded as
paired outpushings of the side wall of the primitive fore-brain
attribute less morphological importance to the velum. Physiologically
the velum is frequently important from the plexus of blood-vessels which
passes with it into the III. ventricle.
In _Petromyzon_ and _Chimaera_ the velum is not developed. In Dipnoans
there are present in its place _paired_ transverse folds which are
probably merely extensions backwards of the lateral plexuses.
The Paraphysis is a projection from the roof of the primitive fore-brain
near its anterior end. It is well seen in Dipnoans[42] (_Lepidosiren_
and _Protopterus_) where in the larva (exactly as in the urodele larva)
it forms a blindly ending tube sloping upwards and forwards between the
two hemispheres. In the adult it becomes mixed with the two lateral
plexuses and is liable to be confused with them. In the other
groups--except the Teleosts where it is small (_Anguilla_) or absent
(most Teleosts)--the paraphysis is by no means such a definite
structure, but generally there is present a more or less branched and
divided diverticulum of the brain wall, frequently glandular, which is
homologized with the paraphysis. The morphological significance of the
paraphysis is uncertain. It may represent the remains of an ancient
sense organ, or it may simply represent the last connexion between the
brain and the external ectoderm from which it was derived.
An important derivative of the primitive fore-brain is seen in the pair
of cerebral hemispheres which in the higher vertebrates become of such
relatively gigantic dimensions. The hemispheres appear to be primitively
associated with the special sense of smell, and they are prolonged
anteriorly into a pair of olfactory lobes which come into close relation
with the olfactory organ. From a consideration of their adult relations
and of their development--particularly in those groups where there is no
disturbing factor in the shape of a large yolk sac--it seems probable
that the hemispheres are primitively paired outpushings of the lateral
wall of the primitive fore-brain[43]--in order to give increased space
for the increased mass of nervous matter associated with the olfactory
sense. They are most highly developed in the Dipneusti amongst fishes.
They are there (cf. fig. 29, C) of relatively enormous size with thick
nervous floor (corpus striatum) and side walls and roof (pallium)
surrounding a central cavity (lateral ventricle) which opens into the
third ventricle. At the posterior end of the hemisphere a small area of
its wall remains thin and membranous, and this becomes pushed into the
lateral ventricle by an ingrowth of blood-vessel to form the huge
lateral plexus ( = _plexus hemisphaerium_). In this great size of the
hemispheres[44] and also in the presence of a rudimentary cortex in the
Dipnoi we see, as in many other features in these fishes, a distinct
foreshadowing of conditions occurring in the higher groups of
vertebrates. The Cyclostomes possess a distinct though small pair of
hemispheres. In the Selachians the relatively archaic _Notidanidae_[45]
possess a pair of thick-walled hemispheres, but in the majority of the
members of the group the paired condition is obscured (fig. 29, A).
In the Teleostomes the mass of nervous matter which in other groups
forms the hemispheres does not undergo any pushing outwards except as
regards the small olfactory lobes. On the contrary, it remains as a
great thickening of the lateral wall of the thalamencephalon (the
so-called basal ganglia), additional space for which, however, may be
obtained by a considerable increase in length of the fore-brain region
(cf. fig. 30, A) or by actual involution into the third ventricle
(_Polypterus_).[46] The great nervous thickenings of the
thalamencephalic wall bulge into its cavity and are covered over by the
thin epithelial roof of the thalamencephalon which is as a consequence
liable to be confused with the pallium or roof of the hemispheres with
which it has nothing to do: the homologue of the pallium as of other
parts of the hemisphere is contained within the lateral thickening of
the thelamencephalic wall, not in its membranous roof.[47]
Associated with the parts of the fore-brain devoted to the sense of
smell (especially the corpora striata) is the important system of
bridging fibres forming the anterior commissure which lies near the
anterior end of the floor, or in the front wall, of the primitive
fore-brain. It is of great interest to note the appearance in the
_Dipnoans_ (_Lepidosiren_ and _Protopterus_) of a corpus callosum (cf.
fig. 30 B) lying dorsal to the anterior commissure and composed of
fibres connected with the pallial region of the two hemispheres.
_Sense Organs._--The olfactory organs are of special interest in the
Selachians, where each remains through life as a widely-open, saccular
involution of the ectoderm which may be prolonged backwards to the
margin of the buccal cavity by an open oronasal groove, thus retaining a
condition familiar in the embryo of the higher vertebrates. In Dipnoans
the olfactory organ communicates with the roof of the buccal cavity by
definite posterior nares as in the higher forms--the communicating
passage being doubtless the morphological equivalent of the oronasal
groove, although there is no direct embryological evidence for this. In
the Teleostomes the olfactory organ varies from a condition of great
complexity in the Crossopterygians down to a condition of almost
complete atrophy in certain Teleosts (Plectognathi).[48]
The _eyes_ are usually of large size. The lens is large and spherical
and in the case of most Teleostomes accommodation for distant vision is
effected by the lens being pulled bodily nearer the retina. This
movement is brought about by the contraction of smooth muscle fibres
contained in the _processus falciformis_, a projection from the choroid
which terminates in contact with the lens in a swelling, the _campanula
Halleri_. In _Amia_ and in Teleosts a network of capillaries forming the
so-called choroid gland surrounds the optic nerve just outside the
retina. As a rule the eyes of fishes have a silvery, shining appearance
due to the deposition of shining flakes of guanin in the outer layer of
the choroid (_Argentea_) or, in the case of Selachians, in the inner
layers (_tapetum_). Fishes which inhabit dark recesses, e.g. of caves or
of the deep sea, show an enlargement, or, more frequently, a reduction,
of the eyes. Certain deep-sea Teleosts possess remarkable telescopic
eyes with a curious asymmetrical development of the retina.[49]
The otocyst or auditory organ agrees in its main features with that of
other vertebrates. In Selachians the otocyst remains in the adult open
to the exterior by the _ductus endolymphaticus_. In _Squatina_[50] this
is unusually wide and correlated; with this the calcareous otoconia are
replaced by sand-grains from the exterior. In Dipnoans (_Lepidosiren_
and _Protopterus_) curious outgrowths arise from the ductus
endolymphaticus and come to overlie the roof of the fourth ventricle,
recalling the somewhat similar condition met with in certain Amphibians.
In various Teleosts the swimbladder enters into intimate relations
with the otocyst. In the simplest condition these relations consist in
the prolongation forwards of the swimbladder as a blindly ending tube
on either side, the blind end coming into direct contact either with
the wall of the otocyst itself or with the fluid surrounding it
(perilymph) through a gap in the rigid periotic capsule. A wave of
compression causing a slight inward movement of the swimbladder wall
will bring about a greatly magnified movement of that part of the wall
which is not in relation with the external medium, viz. the part in
relation with the interior of the auditory capsule. In this way the
perception of delicate sound waves may be rendered much more perfect.
In the Ostariophysi (Sagemehl), including the _Cyprinidae_, the
_Siluridae_, the _Characinidae_ and the _Gymnotidae_, a
physiologically similar connexion between swimbladder and otocyst is
brought about by the intervention of a chain of auditory ossicles
(Weberian ossicles) formed by modification of the anterior
vertebrae.[51]
_Lateral Line Organs._[52]--Epidermal sense buds are scattered about in
the ectoderm of fishes. A special arrangement of these in lines along
the sides of the body and on the head region form the highly
characteristic sense organs of the lateral line system. In _Lepidosiren_
these organs retain their superficial position; in other fishes they
become sunk beneath the surface into a groove, which may remain open
(some Selachians), but as a rule becomes closed into a tubular channel
with openings at intervals. It has been suggested that the function of
this system of sense organs is connected with the perception of
vibratory disturbances of comparatively large wave length in the
surrounding medium.
_Peripheral Nerves._--In the Cyclostomes the dorsal afferent and ventral
efferent nerves are still, as in _Amphioxus_, independent, but in the
gnathostomatous fishes they are, as in the higher vertebrates, combined
together into typical spinal nerves.
As regards the cranial nerves the chief peculiarities of fishes relate
to (1) the persistence of the branchial clefts and (2) the presence of
an elaborate system of cutaneous sense organs supplied by a group of
nerves (_lateralis_) connected with a centre in the brain which develops
in continuity with that which receives the auditory nerve. These points
may be exemplified by the arrangements in Selachians (see fig. 31). I.,
II., III., IV. and VI. call for no special remark.
[Illustration: From Bridge, _Cambridge Natural History_, vol. vii.
"Fishes" (by permission of Macmillan & Co., Ltd.). After Wiedersheim,
_Grundriss der vergleichenden Anatomie_ (by permission of Gustav
Fischer).
FIG. 31.--Diagram of Cranial nerves of a Fish. Cranial nerves and
branchial clefts are numbered with Roman figures. Trigeminus black;
Facialis dotted; Lateralis oblique shading; Glossopharyngeal
cross-hatched; Vagus white.
bucc, Buccal.
c, Commissure between pre- and postauditory parts of lateralis
system.
d.r, Dorsal roots of spinal nerves.
g.g, Gasserian ganglion.
gn.g, (Geniculate) ganglion of VII.
hy, Hyomandibular.
l.n.X, Lateralis vagi.
m, Motor branches of hy.
md, Mandibular.
md.ex, External mandibular.
mk.c, Meckel's cartilage.
mx, Maxillary.
oc, Occipitospinal.
ol.o, Olfactory organ.
op.p, Ophthalmicus profundus.
op.s, Ophthalmicus superficialis.
pn, Palatine.
pq., Palatopterygo-quadrate cartilage.
s, Spiracle.
st, Supra-temporal branch of lateralis system.
t.a, Lateralis centre in brain.
v.n, Visceral nerve.
v.r, Ventral roots.]
_Trigeminus_ (V.).--The _ophthalmicus profundus_ branch (op.p.)--which
probably is morphologically a distinct cranial nerve--passes forwards
along the roof of the orbit to the skin of the snout. As it passes
through the orbit it gives off the long ciliary nerves to the eyeball,
and is connected with the small ciliary ganglion (also connected with
III.) which in turn gives off the short ciliary nerves to the eyeball.
The _ophthalmicus superficialis_ (cut short in the figure) branch passes
from the root ganglion of V. (Gasserian ganglion), and passes also over
the orbit to the skin of the snout. It lies close to, or completely
fused with, the corresponding branch of the lateralis system.
The main trunk of V. branches over the edge of the mouth into the
_maxillary_ (mx.) and _mandibular_ (md.) divisions, the former, like the
two branches already mentioned, purely sensory, the latter
mixed--supplying the muscles of mastication as well as the teeth of the
lower jaw and the lining of the buccal floor.
The main trunk of the _Facialis_ (VII.) bifurcates over the spiracle
into a pre-spiracular portion--the main portion of which passes to the
mucous membrane of the palate as the palatine (pnVII.)--and a
postspiracular portion, the hyomandibular (hy.) trunk which supplies the
muscles of the hyoid arch and also sends a few sensory fibres to the
lining of the spiracle, the floor of mouth and pharynx and the skin of
the lower jaw. Combined with the main trunk of the facial are branches
belonging to the _lateralis_ system.
_Lateralis Group of Nerves._--The _lateralis_ group of nerves are
charged with the innervation of the system of cutaneous sense organs and
are all connected with the same central region in the medulla. A special
sensory area of the ectoderm becomes involuted below the surface to form
the otocyst, and the nerve fibres belonging to this form the auditory
nerve (VIII.). Other portions of the _lateralis_ group become mixed up
with various other cranial nerves as follows:
(a) Facial portion.
(1) _Ophthalmicus superficialis_ (op.s.VII.): passes to lining of nose
or to the lateral line organs of the dorsal part of snout.
(2) _Buccal_ (bucc.VII): lies close to maxillary division of V. and
passes to the sensory canals of the lower side of the snout.
(3) _External mandibular_ (md.ex.): lies in close association with the
mandibular division of V., supplies the sensory canals of the lower jaw
and hyoid region.
_Lateralis vagi_ (l.n.X.) becomes closely associated with the vagus. It
supplies the lateral line organs of the trunk.
In the lamprey and in Dipnoans the _lateralis vagi_ loses its
superficial position in the adult and comes into close relation with the
notochord.
In Actinopterygians and at least some Selachians a _lateralis_ set of
fibres is associated with IX., and in the former fishes a conspicuous
trunk of _lateralis_ fibres passes to some or all (_Gadus_) of the fins.
This has been called the _lateralis accessorius_ and is apparently
connected with V., VII., IX., X. and certain spinal nerves.[53]
_Vagus Group_ (IX., X., XI.).--The _glossopharyngeus_ (IX.) forks over
the first branchial cleft (pretrematic and post-trematic branches) and
also gives off a palatine branch (pn.IX.). In some cases (various
Selachians, Ganoids and Teleosts) it would seem that IX. includes a few
fibres of the _lateralis_ group.
Vagus (X.) is shown by its multiple roots arising from the medulla and
also by the character of its peripheral distribution to be a compound
structure formed by the fusion of a number of originally distinct
nerves. It consists of (1) a number of branchial branches (X.¹ X.² &c.),
one of which forks over each gill cleft behind the hyobranchial and
which may (Selachians) arise by separate roots from the medulla; (2) an
intestinal branch (v.n.X.) arising behind the last branchial and
innervating the wall of the oesophagus and stomach and it may be even
the intestine throughout the greater part of its length (_Myxine_).
The _accessorius_ (XI.) is not in fishes separated as a distinct nerve
from the vagus.
With increased development of the brain its hinder portion, giving rise
to the vagus system, has apparently come to encroach on the anterior
portion of the spinal cord, with the result that a number of spinal
nerves have become reduced to a less or more vestigial condition. The
dorsal roots of these nerves disappear entirely in the adult, but the
ventral roots persist and are to be seen arising ventrally to the vagus
roots. They supply certain muscles of the pectoral fins and of the
visceral arches and are known as spino-occipital nerves.[54]
These nerves are divisible into an anterior more ancient set--the
occipital nerves--and a posterior set of more recent
origin--(occipito-spinal nerves). In Selachians 1-5 pairs of occipital
nerves alone are recognizable: in Dipnoans 2-3 pairs of occipital and
2-3 pairs of occipito-spinal: in Ganoids 1-2 pairs occipital and 1-5
pairs occipito-spinal; in Teleosts finally the occipital nerves have
entirely disappeared while there are 2 pairs of occipito-spinal. In
Cyclostomes no special spino-occipital nerves have been described.
The fibres corresponding with those of the _Hypoglossus_ (XII.) of
higher vertebrates spring from the anterior spinal nerves, which are
here, as indeed in Amphibia, still free from the cranium.
_Sympathetic._--The sympathetic portion of the nervous system does not
in fishes attain the same degree of differentiation as in the higher
groups. In Cyclostomes it is apparently represented by a fine plexus
with small ganglia found in the neighbourhood of the dorsal aorta and on
the surface of the heart and receiving branches from the spinal nerves.
In Selachians also a plexus occurs in the neighbourhood of the cardinal
veins and extends over the viscera: it receives visceral branches from
the anterior spinal nerves. In Teleosts the plexus has become condensed
to form a definite sympathetic trunk on each side, extending forwards
into the head and communicating with the ganglia of certain of the
cranial nerves. (J. G. K.)
V. DISTRIBUTION IN TIME AND SPACE
The origin of Vertebrates, and how far back in time they extend, is
unknown. The earliest fishes were in all probability devoid of hard
parts and traces of their existence can scarcely be expected to be
found. The hypothesis that they may be derived from the early
Crustaceans, or Arachnids, is chiefly based on the somewhat striking
resemblance which the mailed fishes of the Silurian period
(Ostracodermi) bear to the Arthropods of that remote time, a
resemblance, however, very superficial and regarded by most
morphologists as an interesting example of mimetic resemblance--whatever
this term may be taken to mean. The minute denticles known as conodonts,
which first appear in the Ordovician, were once looked upon as teeth of
Cyclostomes, but their histological structure does not afford any
support to the identification and they are now generally dismissed
altogether from the Vertebrates. As a compensation the Lower Silurian of
Russia has yielded small teeth or spines which seem to have really
belonged to fishes, although their exact affinities are not known
(_Palaeodus_ and _Archodus_ of J. V. Rohon).
It is not until we reach the Upper Silurian that satisfactory remains of
unquestionable fishes are found, and here they suddenly appear in a
considerable variety of forms, very unlike modern fishes in every
respect, but so highly developed as to convince us that we have to
search in much earlier formations for their ancestors. These Upper
Silurian fishes are the _Coelolepidae_, the _Ateleaspidae_, the
_Birkeniidae_, the _Pteraspidae_, the _Tremataspidae_ and the
_Cephalaspidae_, all referred to the Ostracophori. The three last types
persist in the Devonian, in the middle of which period the Osteolepid
Crossopterygii, the Dipneusti and the Arthrodira suddenly appear. The
most primitive Selachian (_Cladoselache_), the Acanthodian Selachians
(_Diplacanthidae_), the Chimaerids (_Ptyctodus_), and the Palaeoniscid
ganoids (_Chirolepis_) appear in the Upper Devonian, along with the
problematic _Palaeospondylus_.
In the Carboniferous period, the Ostracophori and Arthrodira have
disappeared, the Crossopterygii and Dipneusti are still abundant, and
the Selachians (_Pleuracanthus_, Acanthodians, truesharks) and
Chondrostean ganoids (_Palaeoniscidae_ and _Platysomidae_) are
predominant. In the Upper Permian the Holostean ganoids
(_Acanthophorus_) make their appearance, and the group becomes dominant
in the Jurassic and the Lower Cretaceous. In the Trias, the
Crossopterygii and Dipneusti dwindle in variety and the _Ceratodontidae_
appear; the Chondrostean and Holostean ganoids are about equally
represented, and are supplemented in the Jurassic by the first,
annectant representatives of the Teleostei (_Pholidophoridae_,
_Leptolepidae_). In the latter period, the Holostean ganoids are
predominant, and with them we find numerous Cestraciont sharks, some
primitive skates (_Squatinidae_ and _Rhinobatidae_), Chimaerids and
numerous Coelacanthid crossopterygians.
The fish-fauna of the Lower Cretaceous is similar to that of the
Jurassic, whilst that of the Chalk and other Upper Cretaceous formations
is quite modern in aspect, with only a slight admixture of Coelacanthid
crossopterygians and Holostean ganoids, the Teleosteans being abundantly
represented by _Elopidae_, _Albulidae_, _Halosauridae_, _Scopelidae_ and
_Berycidae_, many being close allies of the present inhabitants of the
deep sea. At this period the spiny-rayed Teleosteans, dominant in the
seas of the present day, made their first appearance.
With the Eocene, the fish-fauna has assumed the essential character
which it now bears. A few Pycnodonts survive as the last representatives
of typically Mesozoic ganoids, whilst in the marine deposits of Monte
Bolca (Upper Eocene) the principal families of living marine fishes are
represented by genera identical with or more or less closely allied to
those still existing; it is highly remarkable that forms so highly
specialized as the sucking-fish or remoras, the flat-fish
(_Pleuronectidae_), the Pediculati, the Plectognaths, &c., were in
existence, whilst in the freshwater deposits of North America
_Osteoglossidae_ and _Cichlidae_ were already represented. Very little
is known of the freshwater fishes of the early Tertiaries. What has been
preserved of them from the Oligocene and Miocene shows that they
differed very slightly from their modern representatives. We may
conclude that from early Tertiary times fishes were practically as they
are at present. The great hiatus in our knowledge lies in the period
between the Cretaceous and the Eocene.
At the present day the Teleosteans are in immense preponderance,
Selachians are still well represented, the Chondrostean ganoids are
confined to the rivers and lakes of the temperate zone of the northern
hemisphere (_Acipenseridae_, _Polyodontidae_), the Holostean ganoids are
reduced to a few species (_Lepidosteus_, _Amia_) dwelling in the fresh
waters of North America, Mexico and Cuba, the Crossopterygians are
represented by the isolated group _Polypteridae_, widely different from
any of the known fossil forms, with about ten species inhabiting the
rivers and lakes of Africa, whilst the Dipneusti linger in Australia
(_Neoceratodus_), in South America (_Lepidosiren_), and in tropical
Africa (_Protopterus_). The imperfections of the geological record
preclude any attempt to deal with the distribution in space as regards
extinct forms, but several types, at present very restricted in their
habitat, once had a very wide distribution. The _Ceratodontidae_, for
instance, of which only one species is now living, confined to the
rivers of Queensland, has left remains in Triassic, Rhaetic, Jurassic
and Cretaceous rocks of Europe, North America, Patagonia, North and
South Africa, India and Australia; the _Amiidae_ and _Lepidosteidae_
were abundant in Europe in Eocene and Miocene times; the
_Osteoglossidae_, now living in Africa, S.E. Asia and South America,
occurred in North America and Europe in the Eocene.
In treating of the geographical distribution of modern fishes, it is
necessary to distinguish between fresh-water and marine forms. It is,
however, not easy to draw a line between these categories, as a large
number of forms are able to accommodate themselves to either fresh or
salt water, whilst some periodically migrate from the one into the
other. On the whole, fishes may be roughly divided into the following
categories:--
I. Marine fishes. A. shore-fishes; B. pelagic fishes; C. deep-sea
fishes.
II. Brackish-water fishes.
III. Fresh-water fishes.
IV. Migratory fishes. A. anadromous (ascending fresh waters to spawn);
B. catadromous (descending to the sea to spawn).
About two-thirds
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