Report on the Radiolaria Collected by H.M.S. Challenger During the Years 1873-1876, First Part: Porulosa (Spumellaria and Acantharia)

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Title: Report on the Radiolaria Collected by H.M.S. Challenger During the Years 1873-1876, First Part: Porulosa (Spumellaria and Acantharia)

Author: Ernst Haeckel

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Transcriber's note:

Text enclosed by underscores is in italics (_Actissa_).

Text enclosed by hash marks is in "gesperrt"
(wide-spaced) type (#Larcoidea#).

Page numbers enclosed by curly braces (example: {25}) have
been incorporated to facilitate the use of the Index in
Part II.

Some typographical errors in the printed work have been
corrected. They are listed at the end of the text.

The Addenda & Errata (Second Part, pp. 1763-4) have been
applied without further comment.

REPORT ON THE SCIENTIFIC RESULTS OF THE VOYAGE OF H.M.S. CHALLENGER
DURING THE YEARS 1873-76

Under the Command of Captain George S. Nares, R.N., F.R.S.
and the Late Captain Frank Tourle Thomson, R.N.

Prepared Under the Superintendence of
the Late Sir C. Wyville Thomson, KNT., F.R.S., &c.
Regius Professor of Natural History in the University of Edinburgh
Director of the Civilian Scientific Staff on Board
and Now of
John Murray
One of the Naturalists of the Expedition

ZOOLOGY--VOL. XVIII.

FIRST PART

Published by Order of Her Majesty's Government

Printed for Her Majesty's Stationary Office
and Sold by
London:--Eyre & Spottiswoode, East Harding Street, Fetter Lane
Edinburgh:--Adam & Charles Black
Dublin:--Hodges, Figgis, & Co.
1887

Price (in Two Parts, with a Volume of Plates) £5, 10s.

CONTENTS.

REPORT on the RADIOLARIA collected by H.M.S. CHALLENGER during the years
1873-1876.

By ERNST HAECKEL, M.D., Ph.D., Professor of Zoology in the University of
Jena.

FIRST PART.--PORULOSA.

(#SPUMELLARIA AND ACANTHARIA.#)

EDITORIAL NOTES.

The Report on the RADIOLARIA by Professor Ernst Haeckel of Jena occupies
the whole of the present Volume, the text being bound up in Two Separate
Parts and the Plates in a Third Part. The Report forms Part XL. of the
Zoological Series of Reports on the Scientific Results of the Expedition,
and is the largest single Report of the series which has up to this time
been published.

The Manuscript of the Systematic Part was written by Professor Haeckel in
the English language, and was received by me in instalments on the 12th
August 1884, 13th July and 4th December 1885, and 3rd June 1886. The
Introduction was written in German and was translated into the English
language by Mr. W. E. Hoyle of the Challenger Editorial Staff; the German
text being received in instalments between the 15th July 1886, and the 25th
January 1887.

The Challenger Naturalists found the representatives of this group of
animals to be universally distributed throughout ocean waters, and their
dead remains to be nearly equally widely distributed over the floor of the
ocean, the relative abundance and the species differing, however, with
change of locality, and their abundance or variety being intimately
connected with some of the most interesting and intricate problems of
general oceanography.

It was a fortunate circumstance that so distinguished a Naturalist, with
such an intimate knowledge of the RADIOLARIA, should have been willing to
undertake the laborious examination and description of the extensive
collections made during the Expedition. Professor Haeckel has devoted ten
years of his life to this work, and this Report sets forth the results of
his labours, on the conclusion of which he will be congratulated by all
Naturalists. The entire literature of the RADIOLARIA (from 1834 to 1884) is
completely recorded, and the older species (both living and fossil)
redescribed, so that the Report is a complete Monograph, which will be an
invaluable aid to all future Investigators.

CHALLENGER OFFICE, 32 QUEEN STREET,
EDINBURGH, _1st February 1887._

THE

VOYAGE OF H.M.S. CHALLENGER.

#ZOOLOGY.#

Report on the RADIOLARIA collected by H.M.S. Challenger during the Years
1873-76. By ERNST HAECKEL, M.D., Ph.D., Professor of Zoology in the
University of Jena.

PREFACE.

The significance of the Radiolaria in regard to the relations of life in
the ocean has been increased in a most unexpected manner by the discoveries
of the Challenger. Large swarms of these delicate Rhizopoda were found not
only at the surface of the open ocean but also in its different
bathymetrical zones. Thousands of new species make up the wonderful
Radiolarian ooze, which covers large areas of the deep-sea bed, and was
brought up from abysses of from 2000 to 4000 fathoms by the sounding
machine of the Challenger. They open a new world to morphological
investigation.

When ten years ago (in the autumn of 1876) I accepted the enticing
invitation of Sir Wyville Thomson to undertake the investigation of these
microscopic creatures, I hoped to be able to accomplish the task with some
degree of completeness within a period of from three to five years, but the
further my investigations proceeded the more immeasurable seemed the range
of forms, like the boundless firmament of stars. I soon found myself
compelled to decide between making a detailed study of a selection of
special forms or giving as complete a survey as possible of the varied
forms of the whole class; and I decided upon the latter course, having
regard both to the general plan of the Challenger Reports, and to the
interests of our acquaintance with the class as a whole. I must, however,
confess at the close of my work that my original intention is far from
having been fulfilled. The extraordinary extent and varied difficulties of
the undertaking must excuse the many deficiencies.

The special examination of the Challenger collection was for the most part
completed in the summer of 1881; I collected its results in my Entwurf
eines Radiolarien-Systems auf Grund von Studien der Challenger-Radiolarien
(Jenaische Zeitschr. f. Naturw., Bd. xv., 1881). Since the manuscript of
this preliminary communication was completed only a few days before my
departure for Ceylon, and since I was unable to correct the proofs myself,
several errors have crept into the Prodromus Systematis Radiolarium
included in it. These have been corrected in the following more extensive
working out of it. Even at that time I had distinguished 630 genera and
more than 2000 species; but on the revision of these, which I undertook
immediately on my return from India, this number was considerably
increased. The total number of forms here described amounts to 739 genera
and 4318 species; of these 3508 are new, as against 810 previously
described. In spite of this large number, however, and in spite of the
astonishing variety of the new and marvellous forms, the riches of the
Challenger collection are by no means exhausted. A careful and patient
worker who would devote a second decade to the work, would probably
increase the number of new forms (especially of the smaller ones) by more
than a thousand; but for a really complete examination, the lifetime of one
man would not suffice.

The richest source of the Challenger material is the Radiolarian ooze of
the central Pacific Ocean (Stations 265 to 274). This remarkable deep-sea
mud consists for the greater part of well-preserved siliceous shells of
Polycystina (SPUMELLARIA and NASSELLARIA). Not less important, however,
especially for the study of the ACANTHARIA and PHÆODARIA, are the wonderful
preparations stained with carmine and mounted in Canada balsam on the spot
by Dr. John Murray. One such preparation (_e.g._, from Station 271) often
contains twenty or thirty, sometimes even fifty new species. In many of
these preparations the individual parts of the unicellular organism are so
well preserved that they show clearly the characteristic peculiarities of
the legions and orders. Since the material for these preparations was taken
with the tow-net, not only from the surface of the sea but also from
different bathymetrical zones, it furnishes valuable conclusions regarding
the chorology, as well as the physiology and morphology of the group. For
many new discoveries I am indebted to the study of such preparations, of
which I have examined about a thousand from 168 different Stations (compare
§ 240). In addition to these about 100 bottles were handed to me,
containing partly bottom-deposits, partly tow-net gatherings.

Sir Wyville Thomson, who directed the investigations of the Challenger with
so much devotion, and only partly saw its results, has laid me under a deep
debt of obligation; not less is this the case, however, with his successor,
Dr. John Murray. I am especially indebted to both gentlemen for the freedom
they have allowed me in the carrying out of my work, and especially for the
permission to include a description of all known Radiolaria in the
Challenger Report, which has thus become a second edition many times
enlarged of my Monograph published in 1862. Since all previous literature
of the subject has been consulted and critically revised, it is hoped that
this Report will form a useful foundation for future investigations. All
names of sufficiently described Radiolaria published during the first half
century of our knowledge of the class (from 1834 to 1884), are inserted in
alphabetical order in the index at the end of this work.

In addition to the treasures of the Challenger, my own collection of
Radiolaria has yielded many new forms whose description is here included.
On my journeys to the Mediterranean (an account of which is given in the
introduction to my Monograph of the Medusæ), I have given special attention
to these delicate microscopic organisms for more than thirty years. Besides
the various points on the Mediterranean, the Atlantic Ocean at the Canaries
(in the winter of 1866-67) yielded many interesting new forms; whilst my
voyage across the Indian Ocean, from Aden to Bombay, in November 1881,
thence to Ceylon and back by Socotra in March 1882, was still more
productive. In particular, some extended excursions which I had the
opportunity of making from Belligemma and Matura (at the southern extremity
of Ceylon) gave me an insight into the rich treasures of the Indian Ocean.

Most important, however, as regards the knowledge of the Indian Radiolaria,
are the collections which Captain Heinrich Rabbe of Bremen has so
beautifully preserved during his many voyages through that region. In the
neighbourhood of Madagascar and the Cocos Islands more especially, and also
in the Sunda Archipelago, he met with large swarms of Radiolaria, among
which were many new and remarkable forms. These were of special value for
completing the chorology, and the more so since the course of the
Challenger in the Indian Ocean lay very far to the southwards. I will
therefore take this opportunity of repeating my best thanks to Captain
Rabbe for the friendly donation of his valuable collection.

The Radiolarian fauna of the North Atlantic Ocean, which was previously but
little known and only slightly increased by the investigations of the
Challenger, received a valuable increase from the interesting collections
made by Dr. John Murray on various expeditions to the Færöe Islands (on the
"Knight Errant" in 1880 and on the "Triton" in 1882). A large number of new
Radiolaria were captured in the Færöe Channel, partly at the surface of the
Gulf Stream, partly at various depths, and the proof was thus furnished
that at certain points in the North Atlantic Ocean Radiolaria are very
richly developed. I am further indebted to Dr. John Murray for the free use
of this important material as well as for much other assistance in the
carrying out of my work. Another rich source of Radiolaria I found in the
alimentary canal of pelagic animals from all seas. Medusæ, Siphonophoræ,
Salpæ, Pteropoda, Heteropoda, Crustacea, &c., which live partly at the
surface of the sea and partly at various depths, and swallow large masses
of Radiolaria, often contain numbers of their shells well-preserved in
their intestine. The alimentary canal of Fishes and Cephalopods too, which
live upon these pelagic animal frequently contains considerable quantities
of siliceous shells; and another newly discovered source has been found in
the coprolites of the Jurassic period, which consist largely of Radiolarian
skeletons.

In the investigation of this complicated system of organisms, I have
endeavoured on the one hand to give accurately the forms and dimensions of
the species observed, and on the other hand to present a survey of the
relationships of the different genera and families; and in this I have
striven especially to combine the phylogenetic aims of the natural system
with the essentially artificial divisions of a practical classification.
Being, however, a conscientious supporter of the theory of descent, I can
of course lay no stress upon the value of the categories, which are here
distinguished as Legions, Orders, Families, Genera, &c. All these
artificial systematic grades I regard as of merely relative value; and from
the same cause I attach no importance to the distinction of all the species
here described; many of them are probably only developmental stages, and
like my predecessors I have determined their boundaries on subjective
grounds. In the systematic working out of so much material one always runs
the risk of doing either too much or too little in the way of creating
species; but in the light of the theory of descent this danger is of no
consequence.

In the carrying out of this extensive task the friendly aid of Dr. Reinhold
Teuscher of Jena was of the greatest benefit to me; at my request he was at
the trouble of making a large number of accurate drawings with the camera
lucida, and he also undertook a long series, amounting to some 8000,
accurate micrometric measurements, which were of the greatest value in the
attempt to settle the important question of the constancy of the various
species; I have alluded to this in a note at the conclusion of the Report
(p. 1760). My best thanks are due to Dr. Teuscher for the patient and
careful manner in which he discharged these tedious tasks.

The figures of new species of Radiolaria (about 1600 in number) which
appear in the atlas of one hundred and forty plates accompanying this
Report, were nearly all drawn with the camera lucida, partly by Mr. Adolph
Giltsch and partly by myself. The names of the genera which appear at the
bottom of the plates have in many cases been changed since they were
printed off, as may be seen from the explanations which accompany them. Had
it been possible to complete the examination of the material before the
plates were commenced this might have been avoided, and in many cases a
better selection of figures might have been made. All the drawings have
been made upon the stone by the practised hand of Mr. Adolph Giltsch, in
his usual masterly manner, and his lithographic work, which has lasted
fully ten years, is the more valuable since he has himself microscopically
studied the greater part of the species figured. The fact that the atlas
presents so full a picture of the marvellous wealth of form of the
Radiolaria is especially due to his lively interest in the work, to his
unwearying care, and to his morphological acuteness. May it be the means of
inducing many naturalists to study more deeply this inexhaustible kingdom
of microscopic life, whose endless variety of wonderful forms justifies the
saying--_Natura in minimis maxima_.

CONTENTS.

FIRST PART.

GENERAL INTRODUCTION-- PAGE

I. Anatomical Section (§§ 1-140), i
Chapter I. The Unicellular Organism, i
" II. The Central Capsule, xxiv
" III. The Extracapsulum, li
" IV. The Skeleton, lxviii

II. Biogenetical Section (§§ 141-200), xciii
Chapter V. Ontogeny (Individual Development), xciii
" VI. Phylogeny (Genealogical Development), ci

III. Physiological Section (§§ 201-225), cxxviii
Chapter VII. Vegetative Functions, cxxviii
" VIII. Animal Functions, cxl

IV. Chorological Section (§§ 226-250), cxlvi
Chapter IX. Geographical Distribution, cxlvi
" X. Geological Distribution, clxiv

V. Bibliographical Section (§§ 251-254), clxxvi

SYSTEMATIC PART, 1

I. Subclass PORULOSA, 6
Legion I. SPUMELLARIA vel PERIPYLEA, 6
Order 1. Colloidea, 10
" 2. Beloidea, 28
" 3. Sphæroidea, 50
" 4. Prunoidea, 284
" 5. Discoidea, 402
" 6. Larcoidea, 599

Legion II. ACANTHARIA vel ACTIPYLEA, 716
Order 7. Actinelida, 728
" 8. Acanthonida, 740
" 9. Sphærophracta, 795
" 10. Prunophracta, 859

SECOND PART.

II. Subclass OSCULOSA, 889
Legion III. NASSELLARIA vel MONOPYLEA, 889
Order 11. Nassoidea, 895
" 12. Plectoidea, 898
" 13. Stephoidea, 931
" 14. Spyroidea, 1015
" 15. Botryodea, 1103
" 16. Cyrtoidea, 1126

Legion IV. PHÆODARIA vel CANNOPYLEA, 1521
Order 17. Phæocystina, 1542
" 18. Phæosphæria, 1590
" 19. Phæogromia, 1642
" 20. Phæoconchia, 1710

Note on the Dimensions and Measurements, 1760

ADDENDA, 1761

ERRATA, 1763

INDEX, 1765

GENERAL INTRODUCTION.

----

ANATOMICAL SECTION.

A SKETCH OF OUR KNOWLEDGE OF THE ORGANISATION OF THE RADIOLARIA IN THE YEAR
1884.

----

CHAPTER I.--THE UNICELLULAR ORGANISM.

(§§ 1-50.)

1. _Definition of the Radiolaria._--RADIOLARIA are marine Rhizopoda, whose
unicellular body always consists of two main portions, separated by a
membrane; an inner _Central capsule_ (with one or more nuclei) and an
_Extracapsulum_ (the external calymma, which has no nucleus, and the
pseudopodia); the endoplasm of the former and the exoplasm of the latter
are connected by openings in the capsule-membrane. The central capsule is
partly the general central organ of the Radiolarian cell, partly the
special organ of reproduction, since its intracapsular protoplasm, along
with the nuclei embedded in it, serves for the formation of flagellate
spores. The extracapsulum is partly the general organ for intercourse with
the outer world (by means of the pseudopodia), partly the special organ of
protection (calymma) and nutrition (sarcomatrix). The majority of
Radiolaria develop also a skeleton for support and protection, which
presents the utmost variety of form, and is generally composed of silica,
sometimes of an organic substance (acanthin). The Radiolarian cell usually
leads an isolated existence (Monozoa _vel_ Monocyttaria); only in a small
minority (of one legion) are the unicellular organisms united in colonies
or coenobia (Polyzoa _vel_ Polycyttaria).

The extent of the Radiolaria, as limited by the above definition, which I
have made as compact as possible, differs in several important respects
from that allowed to the group by all previous diagnoses. The shortest
expression of its scope might perhaps be:--Rhizopoda with central capsule
and calymma; for the most important character of the Radiolaria, and that
by which they are distinguished from all other Rhizopoda, is the
differentiation of the unicellular body into two principal parts of equal
importance and their separation by a constant capsule-membrane.

2. _The Two Subclasses of the Radiolaria._--The systematic catalogue of the
Radiolaria, which forms the second part of this Report, and is brought up
to the year {ii}1884, contains 20 orders, 85 families, 739 genera, and 4318
species. The consideration that but a small proportion of the ocean his yet
been investigated renders it likely, however, that even this large number
does not include the half of the recent species. The great progress which
our knowledge of the organisation of the Radiolaria has made, by means of
comparative study, renders it possible to arrange this enormous mass of
forms in four main divisions or legions, and these are again related in
pairs, so that two divisions of the highest rank or subclasses are
constituted, the _Porulosa_ (or _Holotrypasta_) and _Osculosa_ (or
_Merotrypasta_).

The division of the Radiolaria into two subclasses and four legions (or
principal orders), I sought to establish in 1883 in a communication on
the Orders of the Radiolaria (Sitzb. Jena Gesellsch. Med. u. Naturwiss.,
February 16, 1883). As a believer in the theory of descent, I regard all
the systematic arrangements of specialists as artificial, and all their
divisions as subjective abstractions, and hence I shall be guided in the
establishment of such groups as subclasses, legions, orders, &c., by
purely practical considerations, especially by the desire to give as
ready a survey as possible of the complex multitude of forms (compare §§
154 to 156).

3. _Porulosa or Holotrypasta._--The subclass Porulosa or Holotrypasta
includes the two legions, PERIPYLEA or SPUMELLARIA, and ACTIPYLEA or
ACANTHARIA, which agree in the following constant and important
characters:--(1) The _Central Capsule_ is primitively a sphere, and retains
this homaxon form in the majority of the species. (2) The _Membrane_ of the
central capsule is everywhere perforated by very numerous minute pores, but
possesses no larger principal aperture (osculum). (3) The _Pseudopodia_
radiate in all directions and in great numbers from the central capsule,
passing through its pores. (4) The _Equilibrium_ of the floating
unicellular body is in most Porulosa pantostatic (indifferent) or
polystatic (plural-stable), since a vertical axis is either absent, or, if
present, has its two poles similarly constituted. (5) The _Ground-forms_ of
the skeleton are therefore almost always either spherotypic or
isopolar-monaxon, very rarely zygotypic. The two legions of the Porulosa
are distinguished mainly by the skeleton of the SPUMELLARIA (or PERIPYLEA)
being siliceous, never centrogenous, nor composed of acanthin, whilst in
the ACANTHARIA (or ACTIPYLEA) it is always centrogenous and made up of
acanthin; hence in the former the nucleus is always central, in the latter
always excentric.

4. _Osculosa or Merotrypasta._--The subclass Osculosa or Merotrypasta
includes the two legions MONOPYLEA or NASSELLARIA, and CANNOPYLEA or
PHÆODARIA, which agree in the following constant and important
characters:--(1) The Central Capsule is originally monaxon (ovoid or
spheroidal) and retains this ground-form in most of the species. (2) The
_Membrane_ of the central capsule possesses a single large principal
aperture (osculum) at the basal pole of the vertical main axis. (3) The
_Pseudopodia_ radiate from a stream of sarcode which passes out from the
central capsule only on one side, namely, through the principal aperture.
(4) The _Equilibrium_ of the floating body is {iii}monostatic or unistable,
since the two poles of the principal axis are always more or less different
from each other. (5) The _Ground-forms_ of the skeleton are, therefore, for
the most part grammotypic (centraxon) or zygotypic (centroplan), rarely
spherotypic. The two legions of the Osculosa are distinguished chiefly by
the principal opening (osculum) being closed by a porous plate (porochora
with its podoconus) in the NASSELLARIA (or MONOPYLEA), and by a radiate
cover (operculum with its astropyle) in the PHÆODARIA (or CANNOPYLEA).

5. _The four Legions of Radiolaria._--The four principal groups of
Radiolaria, to which we have given the name "legions," are natural units,
since the most important peculiarities in the structure of the central
capsule are quite constant within the limits of the same legion, and since
all the forms in the same legion may be traced without violence to the same
phylogenetic stem. The four legions are, however, related to each other, in
so far as they all exhibit those characters which distinguish the
Radiolaria from other Protista. The two which compose the Porulosa (§ 3)
seem somewhat more nearly related to each other than to the two which make
up the Osculosa (§ 4). When, however, the attempt is made to bring them all
into a phylogenetic relationship, it undoubtedly appears that the
SPUMELLARIA (or PERIPYLEA) are the primitive stem, out of which the other
three have been developed as independent branches. All three have been
derived, probably independently, from the most ancient stem-form of the
SPUMELLARIA, the spherical _Actissa_.

6. _Peripylea or Spumellaria._--Those Radiolaria which we call "PERIPYLEA"
on account of the constitution of their central capsule, or "SPUMELLARIA"
on account of the nature of their skeleton, are separated from the other
three legions of the class by the combination of the following constant
characters:--(1) The _Membrane_ of the central capsule is single and evenly
perforated all over by innumerable fine pore-canals, but without any larger
principal opening (osculum). (2) The _Nucleus_ always lies centrally in the
SPUMELLARIA monozoa and is serotinous, for it divides only at a later
period into the nuclei of the spores; in the SPUMELLARIA polyzoa it is
precocious, and divides early into many small nuclei. (3) The _Pseudopodia_
are exceedingly numerous and distributed evenly over the whole surface of
the central capsule. (4) The _Calymma_ contains no phæodium. (5) The
_Skeleton_ is seldom wanting, is never centrogenous, and is always
siliceous. (6) The _Ground-form_ of the central capsule is originally
spherical (often modified); that of the skeleton is also spherical or, in
the majority of cases, derived in different ways from the sphere.

7. _Actipylea or Acantharia._--These Radiolaria which we call "ACTIPYLEA"
on account of the constitution of their central capsule, or "ACANTHARIA"
from the formation of their skeleton, are separated from the other three
legions by the combination of the following constant characters:--(1) The
_Membrane_ of the central capsule is single and {iv}perforated by numerous
fine pore-canals, which are regularly distributed in series or groups, but
without a larger principal opening (osculum). (2) The _Nucleus_ is always
excentric and generally precocious, since it divides early by a peculiar
process of budding into numerous small nuclei. (3) The _Pseudopodia_ are
very numerous and distributed regularly in groups (or series united into a
network). (4) The _Calymma_ contains no phæodium. (5) The _Skeleton_ is
generally present, always centrogenous, and composed of acanthin. (6) The
_Ground-form_ of the central capsule is originally spherical (often
modified), that of the skeleton polyaxon (often modified).

8. _Monopylea or Nassellaria._--Those Radiolaria which we call "MONOPYLEA"
from the formation of their central capsule, or "NASSELLARIA" from the
nature of their skeleton, are distinguished from the other three legions of
the class by the combination of the following constant characters:--(1) The
_Membrane_ of the central capsule is single, and has only one large
principal opening (osculum) at the basal pole of the vertical main axis;
this osculum is closed by a perforated lid (porochora or operculum porosum)
from which there arises within the central capsule a peculiar cone of
threads or pseudopodia (podoconus). (2) The _Nucleus_ is usually excentric
and is always serotinous, since it only divides at a comparatively late
period into spore-nuclei. (3) The _Pseudopodia_ are not very numerous and
arise by division of a single stem or bundle of threads of sarcode, which
issues from the porochora. (4) The _Calymma_ contains no phæodium. (5) The
_Skeleton_ (very rarely absent) is never centrogenous, but always
extracapsular and siliceous. (6) The _Ground-form_ of the central capsule
is always monaxon (with a vertical allopolar main axis), originally ovoid,
often modified; that of the skeleton is also generally monaxon, often
modified (triradial or bilateral).

9. _Cannopylea or Phæodaria._--Those Radiolaria which we call "CANNOPYLEA"
from the constitution of their central capsule, or "PHÆODARIA" on account
of their peculiar phæodium, are distinguished from the other three legions
by the combination of the following characters:--(1) The _Membrane_ of the
central capsule is double, consisting of a strong outer and delicate inner
capsule, and has only one principal opening (osculum) at the basal pole of
the vertical main axis; this osculum is closed by a radiate cover
(astropyle or operculum radiatum), from the centre of which arises an
external tubular spout (proboscis). Occasionally a few small accessory
openings (parapylæ) are present besides the principal opening. (2) The
_Nucleus_ lies centrally or subcentrally in the capsule (in the vertical
main axis), and is serotinous, inasmuch as it only divides at a late period
into spore-nuclei. (3) The _Pseudopodia_ are usually very numerous and
arise from a thick sarcomatrix, formed by the spreading out of a thick stem
of sarcode, which issues from the astropyle. (4) The _Calymma_ always
contains a phæodium or peculiar voluminous excentric mass of pigment. (5)
The _Skeleton_ (very rarely absent) is never centrogenous, always
extracapsular and formed of a silicate of carbon. (6) The {v}_Ground-form_
of the central capsule is always monaxon (with a vertical allopolar main
axis) and generally spheroidal; that of the skeleton is very varied.

10. _Synopsis of the Subclasses and Legions:--_

11. _Individuality of the Radiolaria._--Like other Protozoa the Radiolaria
are unicellular organisms, the whole fully developed organisation of which
falls under the category of a single cell, both morphologically and
physiologically. Since this view is based upon the composition of the
individual body out of two different morphological elements, nucleus and
protoplasm, it is at once justified in the case of the majority of
Radiolaria, in which the plasmatic body encloses only a single nucleus (the
so-called "Binnen-Bläschen"); such is the case in all the SPUMELLARIA
monozoa, NASSELLARIA and PHÆODARIA. This aspect of the case might appear
doubtful in those Radiolaria in which the simple primary cell-nucleus
divides early into numerous small secondary nuclei, as is the case in the
SPUMELLARIA polyzoa and most ACANTHARIA. Strictly speaking, the
multinucleate central capsule should in such cases be regarded as a
syncytium; but since the individual unity of the unicellular organism is as
clearly defined in these precocious multinuclear Radiolaria as in the
ordinary serotinous forms, the former must be considered unicellular
Rhizopods just as are the latter. This mode of regarding {vi}the case is
the more necessary, inasmuch as the early division of the nucleus has no
further influence upon the organisation. Just as in many other classes of
the Protista there are monozootic (solitary) and polyzootic (social) forms,
so also in the Radiolaria there are in addition to the ordinary monozootic
or monobious forms certain families in which colonies or coenobia are
formed by the association of individuals; this distinction may be expressed
by the terms "Monocyttaria" and "Polycyttaria."

The unicellular nature of the Radiolaria was first established by Richard
Hertwig in 1879 (L. N. 33),[1] and brought into conformity with our
present histiological knowledge and the new reform of the cell-theory.
Huxley, however, who was in 1851 the first to examine living Radiolaria
accurately, declared _Thalassicolla nucleata_ to be a unicellular
Protozoon, and the individual central capsules of _Sphærozoum punctatum_
to be cells, but, owing to the then condition of the cell-theory, he was
unable to give a conclusive demonstration of this view. Later, when
Johannes Müller in 1858 and myself in 1862 recognised the peculiar
"yellow cells" which occur in large numbers in many Radiolaria as true
nucleated cells, it appeared impossible any longer to maintain the
unicellular nature of the Radiolaria; also the great complication which I
showed to exist in the structure of _Thalassicolla_ appeared to
contradict it. Only after Cienkowski (1871) and Brandt (1881) had shown
that the "yellow cells" do not belong to the Radiolarian organism, but
are symbiotic unicellular algæ, was it possible to revive and demonstrate
anew the unicellular nature of the Radiolaria.

12. _Morphological Individuality._--From the morphological standpoint the
individuality of the unicellular elementary organism is obvious in the
ordinary solitary Radiolaria (Monobia), and is to be so regarded that the
whole body with all its constituent parts, and not merely the central
capsule, is to be regarded as _a cell_. Naturally the xanthellæ or yellow
cells (§§ 76, 90), which as independent algæ live in symbiosis with many
Radiolaria, must be excluded. The unicellular organisation of the
Radiolaria is further to be distinguished from that of the other Protista,
inasmuch as an internal membrane (capsule-membrane) separates the central
(medullary) from the peripheral (cortical) portion. In the coenobia of the
social Radiolaria (or Polycyttaria), the morphological individuality
persists only as regards the medullary portions of the aggregated cells
(the individual central capsules), while the cortical portions fuse
completely to form a common extracapsulum. Hence in these SPUMELLARIA
polyzoa two different stages of morphological individuality must be
distinguished, the _Cell_ as a _Morphon of the first stage_, and the
_Coenobium_ as a _Morphon of the second stage_.

13. _Physiological Individuality._--From the physiological standpoint also
the individuality of the unicellular organism is immediately obvious in the
case of the ordinary solitary Radiolaria (Monobia); as in other Protista it
fulfils all the functions of life by itself alone. This physiological
individuality of the monobious Radiolarian cell is furthermore not
influenced by the xanthellæ, which live as independent algæ in symbiosis
with many Radiolaria; even though these often by the production of starch
assist in the {vii}nourishment of the Radiolaria, yet they are by no means
indispensable to them. On the other hand, the physiological individuality
offers more complicated relations in the social Radiolaria (Polycyttaria)
which live united in colonies or coenobia. Here the actual _Bion_ (or the
fully developed physiological individual) is not represented by the
individual cells, but by the whole multicellular coenobium, which in each
species has a definite form and size. In these coenobia, which are usually
spherical or cylindrical jelly-like masses, several millimeters in
diameter, numerous cells are so intimately united that only their medullary
portions (the central capsule with the endoplasm) remain independent; the
cortical portions (calymma and exoplasm) on the contrary uniting into a
common extracapsulum. This discharges, as a whole, the functions of
locomotion, sensation, and inception of nutriment, while the separate
central capsules act in the main only as reproductive organs (forming
spores) and partly also as the central organs of metastasis (digestion).
Each coenobium may also be regarded as a polycyttarium, _i.e._, a
"multicellular Radiolarian," whose numerous central capsules represent so
many sporangia or spore-capsules.

On this head compare the section in my monograph of 1862 (L. N. 16),
entitled Die Organisation der Radiolarien-Colonien; _Polyzoen_ oder
_Polycyttarien?_ (pp. 116 to 126); and also R. Hertwig, Zur Histologie
der Radiolarien, 1876 (L. N. 26, p. 23).

14. _Monocyttaria_ and _Polycyttaria_.--In the majority of the Radiolaria
each unicellular organism passes its individual life in an isolated
condition (as a Monocyttarium). Only in a part of the SPUMELLARIA numerous
unicellular individuals are united into societies which are regarded as
coenobia or colonies (Polycyttaria). This is the case in three different
families belonging to the PERIPYLEA, in the Collozoida (without a skeleton,
Pl. 3), the Sphærozoida (with a Beloid skeleton, Pl. 4), and the
Collosphærida (with a Sphæroid skeleton, Pls. 5-8). All three families of
Polycyttaria (or social Radiolaria), agree in their mode of forming
colonies, since the central capsules of the social individuals remain
separate and lie in a common jelly-like mass, which is formed by the fusion
of their extracapsulum. The chief part of the voluminous colonies, which
attain a diameter of several millimetres (sometimes more than 1 cm.), and
are generally spherical, ellipsoidal or cylindrical, consists therefore of
the jelly-like calymma, and this is penetrated by a sarcoplegma, to whose
meshes all the individual organisms contribute by means of the pseudopodia,
which radiate from their sarcomatrix. A further peculiarity in which the
social SPUMELLARIA differ from the solitary consists in the fact that the
former are precocious and the latter serotinous in the division of the
nucleus (§ 64). Whilst in the solitary or monozootic SPUMELLARIA the middle
of the central capsule is occupied by the simple nucleus, and this divides
only at a late period (immediately before the formation of spores) into the
numerous spore nuclei, in the colonial or polyzootic SPUMELLARIA this
division takes place very early, and the middle of each central capsule is
usually occupied by an oil-globule.

{viii}The colonial Radiolaria were described as early as the year 1834 by
Meyen, the first investigator of the class, under the name _Sphærozoum_,
and, as _Palmellaria_, compared with the gelatinous colonies of the
Nostochineæ. The first accurate observations upon their structure were,
however, made in 1851 by Huxley, who described examples of all three
families under the name _Thalassicolla punctata_. More extended,
however, were the investigations of Johannes Müller, who in his
fundamental work (1858) divided the whole class Radiolaria into
_Solitaria_ and _Polyzoa_. The _Radiolaria solitaria_ he divided into
Thalassicolla, Polycystina and Acanthometra, the _Radiolaria polyzoa_
into Sphærozoa (without a shell) and Collosphæra (with a shell). The
most accurate delineation of the Polycyttaria was given by Hertwig in his
beautiful memoir, Zur Histologie der Radiolarien (1876). Quite recently,
however (1886), since the completion of my manuscript upon the Challenger
Radiolaria, a very complete Monograph of the Polycyttaria has appeared by
Karl Brandt, Die colonie-bildenden Radiolarien (Sphærozoen) des Golfes
von Neapel und der angrenzenden Meeres-Abschnitte (276 pp., 8 pls.,
Berlin). It contains in particular most valuable contributions to the
physiology and histology.

15. _The Central Capsule and Extracapsulum._--The special peculiarity of
the unicellular Radiolarian organism, by which it is clearly distinguished
from all other Rhizopoda (and indeed from most other Protista), is its
differentiation into two separate chief constituents, the central capsule
and extracapsulum, and the formation of a special membrane which separates
them. This, the capsule-membrane, is not to be compared with an ordinary
cell-membrane, as an external layer, but rather to be regarded as an
internal differentiated product. The extracapsulum or external (cortical)
portion of the body is in most Radiolaria more voluminous than the central
capsule or inner (medullary) portion. The exoplasm of the former (the
cortical or extracapsular protoplasm) is emphatically different from the
endoplasm of the latter (the medullary or intracapsular protoplasm).
Besides the most important vital processes are distributed by division of
labour so completely between them that they appear most distinctly
co-ordinated. The central capsule is on the one hand the general central
organ of the "cell-soul" for the discharge of its sensory and motor
functions (comparable to a ganglion-cell), on the other hand the special
organ of reproduction (sporangium). The extracapsulum, also, is not less
significant, since on the one hand its calymma acts as a protecting
envelope to the central capsule, as a support to the pseudopodia, and a
foundation for the skeleton or a matrix for the development of the shell,
and on the other hand its pseudopodia are of the utmost importance as
peripheral organs of movement and sensation as well as of nutrition and
respiration. The _central capsule_ and the _extracapsulum_ are therefore to
be regarded both morphologically and physiologically as the two
_characteristic co-ordinated principal parts_ of the unicellular
Radiolarian organism.

In most of the more modern delineations of the Radiolaria the central
capsule is regarded as the "cell proper" and its membrane as the
"cell-wall." The following facts are opposed to the correctness of this
interpretation:--1. In most Radiolaria the exoplasm is clearly different
from {ix}the endoplasm, and the former is more voluminous than the
latter. 2. In all Radiolaria the division of labour is so carried out
between the central capsule and the extracapsulum, that the physiological
significance and independence of both principal parts of the cell is
almost equally great. 3. It is only in the ACANTHARIA that the formation
of the skeleton takes place within the central capsule; in all the other
three legions it is quite independent of it.

16. _The Malacoma and Skeleton._--Whilst the division of the unicellular
organism into central capsule and extracapsulum is undoubtedly the most
important character of the Radiolarian organism, the development of a
skeleton of peculiar and most varied form is of very striking significance.
This skeleton is _always a secondary product of the cell_, but is always
anatomically so independent, and so clearly marked off from the soft parts
or malacoma, that it seems advisable to regard both separately in a general
morphological survey. The skeleton stands in a different relation to each
of the two principal constituents of the malacoma. Only in the ACANTHARIA
is it centrogenous and developed from the central capsule outwards. In the
other three legions the skeleton never arises in the centre of the capsule;
in the NASSELLARIA and PHÆODARIA it is always extracapsular; in the
SPUMELLARIA it is also outside the central capsule originally, but
afterwards becomes often surrounded by it, and finally lies in most cases
partly within and partly without the central capsule. The chemical basis of
the skeleton in the ACANTHARIA is the curious acanthin (an organic
substance allied to chitin), in the PHÆODARIA a silicate of carbon, and in
the NASSELLARIA and SPUMELLARIA silica.

17. _Ground-Forms of the Radiolaria (Promorphology)._--The ground-forms of
the Radiolaria exhibit a greater variety than those of any other class in
the organic world, greater indeed than is to be found in all the remaining
groups together. For every conceivable ground-form which can be defined in
the system of promorphology is actually present in the Radiolaria; their
skeleton exhibits, as it were, in material existence, certain geometrical
ground-forms which are found in no other organisms. The cause of this
unexampled richness in different forms lies chiefly in the static relations
of the Radiolaria, which swim freely in the sea, partly also in the
peculiar plasticity of their protoplasm and the material of their
skeletons.

Regarding the general system of ground-forms compare my Generelle
Morphologie (1866, Bd. i. pp. 375-552; Bd. iv., Allgemeine
Grundformenlehre). The ground-forms there proposed and systematically
defined have, however, found but little acceptance (chiefly, no doubt,
owing to the difficult and complicated nomenclature); but having now,
twenty years after their publication, anew carefully revised and
critically studied them, I can find no sufficient reason for abandoning
the principles there adopted. On the contrary the study of the Challenger
Radiolaria during the last ten years, with its incomparable wealth of
forms, has only confirmed the accuracy of my system of ground-forms. The
customary treatment of these in zoological and botanical handbooks (such
as those of Claus and Sachs) is quite insufficient.

{x}18. _The Principal Groups of Geometrical Ground-Forms._--The great
variety of the geometrical ground-forms which are actually realised in the
variously shaped bodies of the Radiolaria, renders it desirable to classify
these in as small a number as possible of principal groups and a larger
number of subdivisions. As extensive principal groups four at least must be
distinguished; the _Centrostigma_ or Sphærotypic, the _Centraxonia_ or
Grammotypic, the _Centroplana_ or Zygotypic, and the _Acentrica_ or Atypic.
The natural centre of the body, about which all its parts are regularly
arranged, is in the first group a point (stigma), in the second a straight
line (principal axis), in the third a plane (sagittal plane), in the fourth
a centre is of course wanting.

19. _The Centrostigma or Sphærotypic Ground-Forms._--The first group of
geometrical ground-forms, here distinguished as sphærotypic or the
centrostigma, is undoubtedly the most important among the Radiolaria,
inasmuch as if these be considered monophyletic, it must be the original
one from which all the other ground-forms have been derived. The common
character of all these sphærotypic ground-forms is that their natural
centre is a point (stigma); thus there is no single principal axis (or
protaxon) such as is characteristic of the two following groups. The
sphærotypic ground-forms are subdivided into two important smaller groups,
the _spheres_ (Homaxonia) and the _endospherical polyhedra_ (Polyaxonia).
The spherical ground-forms, fully developed in the central capsule and
calymma of _Actissa_ and the #Sphæroidea# as well as in many ACANTHARIA,
present no different axes; all possible axes passing through the centre of
the body are equal (Homaxonia). In the endospherical polyhedra, on the
contrary, numerous axes (three at least) may be distinguished, which are
precisely equal to each other and different from all the remaining axes
(Polyaxonia). If the extremities of these axes, or the poles, which are all
equidistant from the common centre, be united by straight lines, a
polyhedral figure is produced whose angles all lie in the surface of the
sphere. According as the poles of the axes are at equal, subequal, or at
different distances from each other, we may divide the endospherical
polyhedra into regular, subregular and irregular. (See Gener. Morphol., Bd.
i. pp. 404-416.)

20. _The Centraxonia or Grammotypic Ground-Forms._--The second principal
group of organic ground-forms, here called grammotypic or centraxonia, is
characterised by the fact that a straight line (gramma) or a single
principal axis (protaxon) forms the natural centre of the body. This
important and extensive group is divided into two subgroups, those with one
axis (Monaxonia) and those with crossed axes (Stauraxonia); in the latter
different secondary transverse or cross-axes may be distinguished, but not
in the former. In the Monaxonia, therefore, every transverse section of the
body perpendicular to the principal axis is a circle, in the Stauraxonia,
on the contrary, a polygon. The Monaxonia are further subdivided into two
groups, in one of which the two poles of the principal axis {xi}are equal
and similar (Isopolar), in the other of which they are different
(Allopolar); in the former the two halves of the body, which are separated
by the equatorial plane (or the largest transverse plane, perpendicular to
the principal axis), are equal, in the latter unequal. Among the isopolar
uniaxial ground-forms (Monaxonia isopola) may be mentioned the ellipsoidal,
spheroidal, lenticular, &c.; to the allopolar uniaxial forms (Monaxonia
allopola) belong the conical, hemispherical, ovoid, &c. In the same way the
pyramidal ground-forms with crossed axes are divisible into two groups,
according as the two poles of the principal axis are equal or not. The
ground-form of the former is the double pyramid, that of the latter the
single pyramid. Both the double and the single pyramids may again be
subdivided, each into two important lesser groups, the regular and the
amphithect. In the first division the equatorial plane of the double and
the basal plane of the single pyramid is a regular polygon (square, &c.),
whilst in the other division it is an elongated or amphithect polygon
(rhombus, &c.); the crossed axes are equal in the former, unequal in the
latter. (See Gener. Morphol., Bd. i. pp. 416-494.)

21. _The Centroplana or Zygotypic Ground-Forms._--The third principal group
of ground-forms includes those which are bilaterally symmetrical in the
ordinary sense, or zeugitic or zygotypic; the natural centre of their body
is a plane. These forms are the only ones in which the distinction between
right and left is possible, since their body is divided by the median plane
(planum sagittale) into two symmetrical halves (right and left). In all
these zeugites the position of every part is determined by three axes
perpendicular to each other, and of these three dimensive axes two are
allopolar, one is isopolar. The two unlike poles of the principal (or
longitudinal) axis are the oral and aboral, the two unlike poles of the
sagittal (or vertical) axis are the dorsal and ventral; the two similar
poles of the frontal (or transverse) axis, however, are the right and left.
This important group of zeugitic or bilateral forms may also be divided
into two clearly distinct lesser groups, the _Amphipleura_ and the
_Zygopleura_. In the Amphipleura (or bilaterally radial ground-forms) the
"radial two-sided" body is produced by modification of a regular pyramid
(as _Spatangus_ from _Echinus_), and hence is composed of several (not less
than three) antimeres. In the Zygopleura (or bilaterally symmetrical
ground-forms) on the other hand, the bodies consist of two antimeres (as in
all the higher animals, Vertebrata, Arthropoda, &c.). (See Gener. Morphol.,
Bd. i. pp. 495-527.)

22. _The Acentrica or Atypic Ground-Forms._--Among the acentrica or
anaxonia are included all those ground-forms which are absolutely
irregular, and in which neither a definite centre nor constant axes can be
distinguished (_e.g._, most Sponges). These quite irregular ground-forms
are very rare among the Radiolaria, but nevertheless there may be referred
to them the amoeboid central capsule of some #Colloidea# (_Collodastrum_,
p. 27, Pl. 3, figs. 4, 5) among the SPUMELLARIA, the irregular shells of
many Collosphærida {xii}(Pl. 8, fig. 2), and the absolutely irregular
shells of the Phorticida and Soreumida among the #Larcoidea#. (See Gener.
Morphol., Bd. i. p. 400.)

23. _The Subsidiary Groups of Geometrical Ground-Forms._--The four natural
principal groups of ground-forms, which have just been defined according to
the nature of the centre of their bodies, may be divided again into
numerous subsidiary groups, defined by the relations of the constant axes
and the two poles of each axis, as well as by the number of the axes and
the differentiation of the secondary with respect to the principal axis.
The most important of these subsidiary groups into which the principal ones
are immediately divided are the following:--(1) The _Centrostigma_ (or
sphærotypic) are divided into spheres (Homaxonia) and endospherical
polyhedra (Polyaxonia). (2) The _Centraxonia_ (or grammotypic) into
uniaxial (Monaxonia) and those with crossed axes (Stauraxonia); among the
former of these may be distinguished the isopolar (phacotypic) and the
allopolar (conotypic); among the latter the double and single pyramids. (3)
The _Centroplana_ (or bilaterals) are divided into amphipleura (or
bilaterally radial) and zygopleura (or bilaterally symmetrical). (4) The
_Acentrica_ (or Anaxonia) or absolutely irregular ground-forms, present no
special subdivisions.

For a complete system of the geometrical ground-forms and their relation
to promorphological classification, see Gener. Morphol., Bd. i. pp.
555-558.

24. _The Spherical or Homaxon Ground-Form._--The spherical is the only
absolutely regular ground-form, since only in it are all axes which pass
through the centre equal; it is very often realised among the Radiolaria,
especially in the SPUMELLARIA and ACANTHARIA, where it furnishes the common
original ground-form, but it is often to be seen in the shells of many
PHÆODARIA (in most #Phæosphæria#); on the other hand, it is never found
among the NASSELLARIA. Geometrical spheres, in the strict sense of the
term, are only to be found among the SPUMELLARIA and ACANTHARIA, namely, in
the central capsule of many #Collodaria# (Pls. 1, 2) and all #Sphæroidea#
(Pls. 11-30) as well as many Acanthometra and Acanthophracta (Pls.
128-138). Nevertheless, speaking generally, one includes those central
capsules and skeletons which have been distinguished here as endospherical
polyhedra. (On these ground-forms see Gener. Morphol., Bd. i. pp. 404-406.)

25. _The Endospherical Polyhedral Ground-Form._--The endospherical
polyhedron or polyaxon ground-form naturally follows the spherical or
homaxon. Under it are included all polyhedra whose angles fall in the
surface of a sphere; this ground-form is especially common among the
SPUMELLARIA, especially in the shells of #Sphæroidea#, but is also found
among the ACANTHARIA (especially in the Astrolophida and #Sphærophracta#),
as well as among the #Phæosphæria# (in most genera of the Orosphærida,
Sagosphærida, and Aulosphærida). Strictly speaking, all those
lattice-shells which have {xiii}been incorrectly called "spherical" belong
to this category, for they are none of them true spheres in the geometrical
sense (like the central capsules of the #Sphæroidea#), but rather
endospherical polyhedra, whose angles are indicated by the nodal points of
the lattice shell, or the radial spines which spring from them. These
endospherical polyhedra may be divided into three groups, the regular,
subregular, and irregular. Of _regular polyhedra_, properly so-called, it
may be shown geometrically that only five can exist, namely, the regular
tetrahedron, cube, octahedron, dodecahedron, and icosahedron. All these are
actually manifested among the Radiolaria, although but seldom. Much more
common are the _subregular endospherical polyhedra_, _e.g._, spherical
lattice-shells with regular hexagonal meshes of equal size; they are never
exactly equal nor perfectly regular, but the divergences are so
insignificant that they escape superficial observation (Pl. 20, figs. 3, 4;
Pl. 26, figs. 1-3). On the contrary in the _irregular endospherical
polyhedra_ the meshes of the lattice-sphere are more or less different in
size and often in form also (Pl. 28, figs. 4, 8; Pl. 30, figs. 4, 6). The
five truly regular polyhedra require separate notice on account of their
importance. (See Gener. Morphol., Bd. i. p. 406.)

26. _The Regular Icosahedral Ground-Form._--The ground-form whose
geometrical type is the regular icosahedron (bounded by twenty equilateral
triangles) is rarely exemplified, but it occurs among the PHÆODARIA in the
Circoporid genus _Circogonia_ (Pl. 117, fig. 1), and also in certain
Aulosphærida, but, apparently, only as an accidental variation (_e.g._,
_Aulosphæra icosahedra_). Furthermore, this ground-form may also be assumed
to occur in those #Sphæroidea# whose spherical lattice-shells bear twelve
equidistant radial spines (_e.g._, many species of _Acanthosphæra_,
_Heliosphæra_, and other Astrosphærida); the basal points of these spines
indicate the twelve angles of the regular icosahedron. (See on this head
Gener. Morphol., Bd. i. p. 411.)

27. _The Regular Dodecahedral Ground-Form._--The ground-form whose
geometrical type is the regular dodecahedron (or pentagonal dodecahedron),
bounded by twelve equilateral and equiangular pentagons, is very rarely
found perfectly developed, as in _Circorrhegma dodecahedra_ (Pl. 117, fig.
2). This form is by no means so common among the Radiolaria as in the
pollen grains of plants (_e.g._, _Buchholzia maritima_, _Fumaria spicata_,
_Polygonum amphibium_, &c.). It can, however, be regarded as present in all
those #Sphæroidea# whose spherical lattice-shells bear twenty equal and
equidistant radial spines (_e.g._, many species of _Acanthosphæra_,
_Heliosphæra_, and other Astrosphærida); the basal points of these spines
mark out the twenty angles of the regular pentagonal dodecahedron. (See
Gener. Morphol., Bd. i. p. 412.)

28. _The Regular Octahedral Ground-Form._--The ground-form whose
geometrical type is the regular octahedron (bounded by eight equilateral
triangles), commonly appears among the SPUMELLARIA in the family
Cubosphærida (p. 169, Pls. 21-25). In {xiv}these #Sphæroidea# the typical
ground-form is usually indicated by six equal radial spines, which are
opposed to each other in pairs, and lie in three similar axes perpendicular
to each other; these are the three axes of the tesseral crystallographic
system; one of them is vertical, whilst the other two cross each other at
right angles in its centre. Occasionally, too, the spherical form of the
lattice-shell passes over into that of the regular octahedron (Pl. 22,
figs. 8, 10). The same form recurs in _Circoporus_ (Pl. 117, fig. 6) among
the PHÆODARIA. In the vegetable kingdom it is exhibited by the antheridia
of _Chara_. It is not found in the NASSELLARIA and ACANTHARIA. (See Gener.
Morphol., Bd. i. p. 412.)

29. _The Regular Cubic Ground-Form._--The ground-form whose geometrical
type is that of a die or cube, is actually presented in a very striking
manner by various Radiolaria. Among the SPUMELLARIA it occurs in certain
#Sphæroidea#, _e.g._, in the Astrosphærid genera _Centrocubus_ and
_Octodendron_ (Pl. 18, figs. 1-3); in these the central medullary shell is
a complete cube, bounded by six equal squares, from the eight angles of
which eight equal radial spines project. This form can also be regarded as
present in those #Sphæroidea# whose spherical lattice-shell bears eight
equal and equidistant radial spines (many Astrosphærida). Besides these the
cubic ground-form is to be seen in certain NASSELLARIA of the family
Tympanida, especially in _Lithocubus_ (Pl. 82, fig. 12; Pl. 94, fig. 13),
in many species of _Acrocubus_, _Microcubus_, &c.; the twelve bars of its
lattice-skeleton correspond often exactly to the edges of the cube. (See
Gener. Morphol., Bd. i. p. 413.)

30. _The Regular Tetrahedral Ground-Form._--The ground-form whose
geometrical type is the regular tetrahedron, bounded by four equilateral
triangles, occurs less frequently in the Radiolaria than the other four
regular polyhedra. Among the SPUMELLARIA it is found in the #Beloidea#, and
especially in those members of the Thalassosphærida and Sphærozoida whose
spicules bear four equal branches, diverging at equal angles from a common
centre. Precisely the same structure is seen also among the NASSELLARIA in
some #Plectoidea#, as in _Tetraplagia_ among the Plagonida, and
_Tetraplecta_ among the Plectanida. The skeleton of both these genera
consists of four equal rods, which radiate at equal angles from a common
centre, just as do the axes of the regular tetrahedron. The tetrahedral
form of these #Plectoidea# is the more important and interesting since on
the one hand it is related to the similar spicular form of the #Beloidea#,
and on the other perhaps furnishes the starting point from which _Cortina_
among the NASSELLARIA may be derived (_Plagoniscus_, _Plectaniscus_). (See
Gener. Morphol., Bd. i. p. 415.)

31. _The Isopolar-Monaxon or Phacotypic Ground-Form._--The isopolar
uniaxial or phacotypic ground-form is characterised by the possession of a
vertical main axis with {xv}equal poles, whilst no transverse axes are
differentiated. All horizontal planes which cut the axis at right angles
are circles, and increase in size from the poles towards the equator. The
most important ground-forms of this group are the _phacoid_ (the lens or
oblate spheroid) and the _ellipsoid_ (or prolate spheroid). Phacoids (or
geometrical lenses with blunt margins) are very often presented by the
central capsules of the #Discoidea# and of many ACANTHARIA (Quadrilonchida
and Hexalaspida), but the lattice-shells of many SPUMELLARIA and ACANTHARIA
exhibit the same form, as also do a few PHÆODARIA (_e.g._, _Aulophacus_).
True geometrical ellipsoids are seen in the central capsules of many
#Prunoidea# among the SPUMELLARIA, and of many Amphilonchida and
Belonaspida among the ACANTHARIA. Furthermore, the lattice shells of many
species of these groups retain the same essential form, _e.g._, many
Ellipsida, Druppulida, and Spongurida (Pls. 13-17, and 39), as well as most
Belonaspida. (See Gener. Morphol., Bd. i. p. 422.)

32. _Allopolar-Monaxon or Conotypic Ground-Form._--The allopolar uniaxial
or conotypic ground-form is characterised by the possession of a vertical
main axis whose two poles are unlike, while no transverse axes are
differentiated. All horizontal planes cutting the main axis at right
angles are circles, and decrease more rapidly from the largest plane
towards the basal than towards the apical pole. The most important
ground-forms of this group are the ovoid, the cone, and the hemisphere.
They often occur (and in geometrical perfection) in the egg-shaped central
capsule and podoconus of the NASSELLARIA, as well as in the shells of
several groups of this legion, particularly in the Cyrtocalpida or
Monocyrtida eradiata (Pl. 51, figs. 10-13), and in many Stichocyrtida
eradiata; furthermore, they are also seen among the PHÆODARIA, _e.g._,
certain Challengerida (Pl. 99, figs. 19-22). (See Gener. Morphol., Bd. i.
p. 426.)

33. _The Regular Dipyramidal or Quadrilonchial Ground-Form._--The
ground-forms whose geometrical type is the regular double pyramid are
characterised by a vertical main axis which possesses equal poles, and
which is crossed at its centre by several equal transverse axes. The
horizontal equatorial plane is therefore a regular polygon, and divides the
body into two equal regular pyramids. The simplest and commonest form of
this group is the quadratic octahedron, the ground-form of the quadratic
crystallographic system; its equatorial plane is a square. This regular
dipyramidal ground-form occurs among the SPUMELLARIA in the shells of the
Staurosphærida as well as of many #Discoidea#, in which several equidistant
radial spines or arms lie in the quadratic equatorial plane of the body,
and project from the margin of the lenticular disc (_e.g._, _Sethostaurus_,
Pl. 31; _Histiastrum_, Pl. 46, &c.). It is, however, among the ACANTHARIA
that the most important part is played by this ground-form (and especially
by the quadratic octahedron); it forms the basis of all those
#Acanthometra# and #Acanthophracta# in which twenty radial spines are
disposed according to the Müllerian Law, and in which {xvi}the four
equatorial spines are of equal dimensions (Icosacantha). (See Gener.
Morphol., Bd. i. p. 436-446.)

34. _The Amphithect Dipyramidal or Lentelliptical Ground-Forms._--The
ground-forms whose geometrical type is the lenticular or "triaxial"
ellipsoid, may also be designated amphithect double pyramids; they are
characterised by the possession of a vertical main axis which has similar
poles, and is crossed at its middle by two transverse axes, unequal but
isopolar. The horizontal equatorial plane of the body is therefore an
amphithect or elongated polygon (a rhombus in the simplest case possible),
and divides the whole body into two equal amphithect pyramids. The simplest
and commonest form of this group is the rhombic octahedron, which is also
the ground-form of the rhombic crystallographic system. It plays an
important part in those ACANTHARIA in which twenty radial spines are
disposed according to the Müllerian Law, but in which the two pairs of
equatorial spines are unequal (different geotomical and hydrotomical axes,
see p. 719); to this category belong the Amphilonchida (Pl. 132),
Belonaspida (Pl. 136), Hexalaspida (Pl. 139), and Diploconida (Pl. 140). A
form essentially identical obtains also among the SPUMELLARIA in the
majority of the #Larcoidea#, both in their triaxial lattice-shells, and in
their lentelliptical central capsules, which present geometrically accurate
triaxial ellipsoids, with three unequal isopolar axes at right angles to
each other. (See Gener. Morphol., Bd. i. p. 446-452.)

35. _The Regular Pyramidal Ground-Forms._--The ground-forms whose
geometrical type is the regular pyramid, and which are the most conspicuous
in the Medusæ, Polyps, Corals, and regular Echinoderms (the Radiata of
earlier authors), are almost confined among the Radiolaria to the legion
NASSELLARIA; they occur, however, in the great majority of these, and
especially in those families which may be classed together as "#Cyrtoidea#
triradiata et multiradiata." Strictly speaking, however, almost all these
NASSELLARIA, at all events in their origin, are bilateral or dipleuric,
since the primary sagittal ring with its characteristic apophyses marks out
the sagittal median plane, and further, since the three feet of the basal
tripod are usually divided into an unpaired dorsal (pes caudalis) and two
paired ventral or lateral (pedes pectorales, dexter et sinister). On the
other hand, it is noteworthy, firstly, that among the primitive
#Plectoidea# there are perfectly regular radial forms, without any
indication of an original bilateral symmetry, and secondly, that similar
forms are also very common among the #Cyrtoidea#, probably as secondary
radial forms, developed from primitive bilateral ones. Similar cases also
occur in certain PHÆODARIA (_e.g._, the Medusettida and Tuscarorida, Pls.
100, 120), but they are entirely wanting among the ACANTHARIA and
SPUMELLARIA. The multiradial NASSELLARIA have arisen from the triradial by
the interpolation of three, six, nine, or more interradial and adradial
secondary apophyses between the three primary perradial ones. (See Gener.
Morphol., Bd. i. pp. 459-874.)

{xvii}36. _The Amphithect Pyramidal Ground-Forms._--The ground-forms whose
geometrical type is the amphithect pyramid, are distinguished from the
regular pyramidal forms, just discussed, chiefly by the form of the basal
plane, which is not a regular, but an amphithect or elongated polygon (in
the simplest case a rhombus). Hence in this case the allopolar main axis of
the body is crossed by two transverse axes which are isopolar and at right
angles, but are unequal; they cannot, however be distinguished as sagittal
and frontal axes as is the case in the zeugites. In the animal as well as
in the vegetable kingdom, an important part is played by this ground-form,
_e.g._, in the Ctenophora, where it is the rhombic pyramid. Among the
Radiolaria it is not common, though it is clearly expressed among the
NASSELLARIA in a number of #Stephoidea# (Stephanida and Tympanida), as well
as in many #Spyroidea# (_e.g._, the bipedal Zygospirida). It is very
accurately developed among the PHÆODARIA in the bivalved #Phæoconchia#
(Pls. 121-128), where the two valves of the shell (dorsal and ventral) are
generally exactly alike, their median keels corresponding to the poles of
the sagittal axis. In the slit between the two valves lie the two secondary
openings (right and left) of the tripylean central capsule, corresponding
to the two poles of the frontal axis, and the main axis stands
perpendicularly to both these, its oral pole being indicated by the
astropyle, or principal aperture. (See Gener. Morphol., Bd. i. pp.
479-494.)

37. _The Amphipleural Ground-Forms._--By the term amphipleural ground-forms
are to be understood those usually defined as "bilaterally radial"; their
geometrical type is a half amphithect pyramid. The best known examples of
this form in the animal kingdom are the bilateral five-rayed Echinoderms
(_Spatangus_, _Clypeaster_), in the vegetable kingdom the symmetrical
five-rayed flowers (_Viola_, _Trifolium_). The three dimensive axes have
the same relation as in the zygopleura, to be next discussed, and which
also resemble them in being divisible only by one plane (the sagittal
median plane) into two equal halves. They differ, however, the amphipleural
body not being made up of two antimeres, but of at least three pairs of
antimeres (or three parameres), being therefore primitively radial. Hence
each of the symmetrical halves of the body contains more than one antimere.
Among the Radiolaria this form does not occur in the SPUMELLARIA,
ACANTHARIA, or PHÆODARIA; it is very common, however, among the
NASSELLARIA; many #Cyrtoidea# multiradiata and #Spyroidea# multiradiata
show this bilaterally radial ground-form, inasmuch as the body consists of
two symmetrical halves, and is also composed of numerous (usually three,
six, nine, or more) radial parameres. In the multiradiate Dicyrtida and
Tricyrtida the cephalis (the first joint) is usually bilateral, whilst the
thorax (the second joint) is multiradial. (See Gener. Morphol., Bd. i. pp.
495-506.)

38. _The Zygopleural Ground-Forms._--As zygopleural or dipleural
ground-forms, as opposed to the amphipleural, are classed those zeugites or
centroplana which are known {xviii}as "bilaterally symmetrical" in the
strictest sense of the term. This is the most important ground-form in the
animal kingdom, inasmuch as it obtains almost exclusively among the higher
animals (Vertebrata, Articulata, Mollusca, Vermes). The body consists of
only two antimeres, which correspond to the two symmetrical halves of the
body. Of the three dimensive axes two are allopolar, one isopolar; the oral
pole of the longitudinal main axis is different from the aboral; the dorsal
pole of the sagittal axis is different from the ventral; but the right pole
of the frontal axis is equal to the left. The right antimere is usually
precisely similar to the left (Eudipleura), more rarely it is slightly
dissimilar or asymmetrical (Dysdipleura). Among the Radiolaria this
ground-form is entirely wanting in the Porulosa or Holotrypasta
(SPUMELLARIA and ACANTHARIA), but on the contrary it is very common in the
Osculosa or Merotrypasta (NASSELLARIA and PHÆODARIA). In the NASSELLARIA it
is of special importance, for the typical _Cortina_ (the combination of the
primary sagittal ring with the basal tripod) exhibits the zygopleural
ground-form clearly sketched out; indeed it is usually clearly seen even in
the sagittal ring itself, for its ventral segment is more strongly curved
than the dorsal; its basal (or oral) pole is always different from the
apical (or aboral). Of the three feet of the basal tripod the unpaired
(caudal) one is directed dorsally and backwards, the two paired (pectoral)
ones ventrally and forwards. The majority of the NASSELLARIA may be
regarded as modifications of this original ground-form. Its relation to the
primitively triradiate tripod presents a still unsolved problem, and the
numerous relations of the zygopleural to the multiradiate ground-forms in
the NASSELLARIA are exceedingly complicated. The zygopleural ground-form is
less widely distributed among the PHÆODARIA, though it is very
characteristically developed in the rich and varied group of Challengerida
(Pl. 99). (See Gener. Morphol., Bd. i. pp. 507-527.)

39. _Synopsis of the Geometrical Ground-Forms:--_

Principal Groups of Ground-Forms.
Subsidiary Groups of Ground-Forms.
Geometrical Type.
Examples.

_Tetraplagia_, _Tetraplecta_, &c.

II. CENTRAXONIA. {xix}
The geometrical centre of the body is a straight line (the vertical main
axis).

Constant transverse axes (perpendicular to the main axis) are wanting in
the Monaxonia (which have circular transverse sections); on the contrary
they are differentiated in the Stauraxonia (which have polygonal
transverse sections).
|
+-III. MONAXONIA.
| Uniaxial ground-forms or centraxonia without transverse axes. The
| transverse planes (perpendicular to the main axis) are circles.
| |
| +-8. _Monaxonia isopola._
| | (Spheroids and ellipsoids; both poles of the main axis similar.)
| |
| | Central capsule and lattice-shell of of many #Discoidea# (lenses) and
| | #Prunoidea# (ellipsoids), Belonaspida, &c.
| |
| +-9. _Monaxonia allopola._
| (Cone, ovoid and hemisphere; the two poles of the axis dissimilar.)
|
| Central capsule and lattice-shell of many NASSELLARIA, especially
| the #Cyrtoidea# eradiata (Cyrtocalpida, &c.).
|
+-IV. STAURAXONIA.
Pyramidal ground-forms or centraxonia with transverse axes. The
transverse planes (perpendicular to the main axis) are either regular
or amphithect polygons.
|
+-10. _Dipyramides regulares._
| (Quadratic octahedron, or quadrilonchial forms and regular double
| pyramids.)
|
| ACANTHARIA with twenty radial spines, the four equatorial being
| equal. Multiradial #Discoidea# and Staurosphærida.
|
+-11. _Dipyramides amphithectæ._
| (Rhombic octahedron, lentellipsoid, and amphithect double pyramids.)
|
| ACANTHARIA with twenty radial spines, whose four equatorial spines
| are unequal but paired. Many #Larcoidea#.
|
+-12. _Pyramides regulares._
| (Regular pyramids.)
|
| Many NASSELLARIA (triradial and multiradial). Medusettida and
| Tuscarorida.
|
+-13. _Pyramides amphithectæ._
(Rhombic pyramids.)

#Phæoconchia#. Bipedal #Spyroidea# and #Stephoidea#.

III. CENTROPLANA.
The geometrical centre of the body is a plane (the sagittal plane).
Constant transverse axes (perpendicular to the main axis) are wanting in
the Monaxonia (which have circular transverse sections); on the contrary
they are differentiated in the Stauraxonia (which have polygonal
transverse sections).
|
+-V. BILATERALIA (or ZEUGITA).
Bilateral forms in the general sense, with right and left halves.
|
+-14. _Amphipleura_
| (Bilaterally radial ground-form.)
|
| Many #Cyrtoidea# and #Spyroidea# multiradiata.
|
+-15. _Zygopleura._
(Bilaterally symmetrical ground-form.)

Most NASSELLARIA (primitively at least), many Challengerida.

IV. ACENTRA.
There is no geometrical centre.
|
+-VI. ANAXONIA.
No definite axes can be determined.
|
+-16. _Irregularia._
(Absolutely irregular ground-forms.)

_Collodastrum_, _Collosphæra_, Phorticida, Soreumida.

40. _Mechanical Causes of the Geometrical Ground-Forms._--The great variety
of ground-forms exhibited by the Radiolaria is of special interest, since
in most instances their causes admit of recognition, and since they are so
intimately related to each other that even in the remaining cases the
assumption that they have arisen by purely mechanical _causæ efficientes_
seems justified. In this respect the first rank is taken by statical
conditions, especially the indifferent or stable equilibrium of the whole
organism, which floats freely in the water. With regard to these
fundamental statical relations, three principal groups of ground-forms may
be distinguished, pantostatic, polystatic, and monostatic.

41. _Pantostatic Ground-Forms._--By pantostatic or indifferently stable
ground-forms are meant those in which the centre of gravity coincides with
the centre of the body, so that they are in equilibrium in any given
position. Strictly speaking, the only form which possesses perfectly
indifferent equilibrium is the sphere, that being the only truly homaxon
and perfectly regular form. Nevertheless, in a somewhat wider sense many
Polyaxonia, especially the endospherical polyhedra with very numerous
sides, may be {xx}included in this category. Such indifferently stable
bodies are found among the SPUMELLARIA in many #Collodaria# and
#Sphæroidea#, as well as in the Astrolophida among the ACANTHARIA. On the
contrary they are entirely wanting among the NASSELLARIA and PHÆODARIA,
since their central capsule constantly presents a main axis with a
differentiated basal pole, and determines the position of stable
equilibrium.

42. _Polystatic Ground-Forms._--Those ground forms are defined as
polystatic or multistable in which the body is in equilibrium in several
different positions (though not in an infinite number). The number of these
positions is usually twice as many as that of the constant equal isopolar
axes exhibited by the form. Hence the regular polyhedra have as many
positions of equilibrium as they have angles or sides, the icosahedron
twenty, dodecahedron twelve, octahedron eight, cube six, tetrahedron four.
The isopolar monaxon ground-forms (lens, ellipsoid, cylinder) and the
diplopyramidal ground forms (quadrilonchial and lentelliptical) have two
positions of stable equilibrium, since the two poles of the vertical axis
are equal and similar and the body is divided into equal halves by the
equatorial plane. This is the case in many SPUMELLARIA (especially
#Discoidea#, #Prunoidea#, and #Larcoidea#), as well as in the great
majority of ACANTHARIA. Perhaps the same holds good also in certain
NASSELLARIA (_e.g._, isopolar Tympanida) and PHÆODARIA (_e.g._, isopolar
#Phæosphæria#), though here unistable equilibrium appears to be
necessitated by the constant main axis of the central capsule and the
differentiated basal pole of the main axis.

43. _Monostatic Ground-Forms._--Those ground-forms are classed as
monostatic or unistable in which the body is in equilibrium only in one
position, since the centre of gravity of the body lies in a constant
vertical axis below its centre. This fixed position is only rarely and
exceptionally found among the SPUMELLARIA (_e.g._, in _Xiphostylus_,
_Sphærostylus_, _Lithomespilus_, _Lithapium_) and among the ACANTHARIA
(_e.g._, in _Zygostaurus_ and _Amphibelone_). On the contrary it is quite
usual among the NASSELLARIA and PHÆODARIA (with but few exceptions); for
here a vertical main axis, with a differentiated basal pole, is determined
even by the formation of the central capsule, and usually also by the
corresponding structure of the skeleton. Among the NASSELLARIA this basal
pole, with the porochora of the central capsule, appears always to be the
lower; as also in most #Phæogromia# among the PHÆODARIA. In the peculiar
bivalved #Phæoconchia#, on the other hand, the basal pole with the
cannopyle is directed upwards; as also in the Challengerida and
Tuscarorida. The #Phæosphæria# and #Phæocystina# are probably to a large
extent polystatic. In general unistable equilibrium may be assumed in the
following categories of ground-forms:--(1) Allopolar monaxon (conical and
ovoid); (2) pyramidal (regular and amphithect); (3) Centroplana
(amphipleura and zygopleura); (4) Anaxonia.

{xxi}44. _Principal Axes._--From the foregoing consideration of the
statical conditions and their direct causal connection with the geometrical
ground-forms of the Radiolaria, the great mechanical significance of the
differentiation of definite axes in these unicellular free-swimming
organisms will be manifest. The most important of these is the primary main
axis (axis principalis, or protaxon), which in all cases has a vertical
direction. It is wanting in the Centrostigma (spheres and endospherical
polyhedra), and in the Anaxonia (acentra). It is isopolar in the phacotypic
forms (Monaxonia isopola), and in the double pyramids (Stauraxonia
isopola). It is allopolar in all monastatic ground-forms, in the conotypic
forms (Monaxonia allopola), pyramids (Stauraxonia allopola), and the
Centroplana (or bilateral forms).

45. _Secondary or Transverse Axes._--In contrast to the vertical main axis
all the other constant axes differentiated in the body may be called
"secondary axes," or "transverse axes," since they cross the former at
definite points. All ground-forms whose vertical axis is crossed by a fixed
number of such axes at definite angles may be called "Stauraxonia." They
are divided into two groups, double pyramids and single pyramids; in the
former the two poles of the main axis (or the two halves of the body
separated by the equatorial plane) are similar (Stauraxonia homopola), in
the latter dissimilar (Stauraxonia heteropola). If all the secondary axes
be equal, the stauraxon ground-form is regularly radial. If some of them be
unequal they are arranged in certain relations towards two primary
transverse axes, perpendicular to each other, to which all the other
secondary axes are subsidiary; the ground-forms are then either amphithect
or bilateral. The two primary transverse axes, which may also be designated
"ideal transverse axes" (euthyni), divide the vertical main axis in its
centre; one of them is the sagittal, the other the frontal. These three
dimensive axes give the factors which accurately determine the ground-form
and the dimensions in most Radiolaria; the vertical main axis determines
the length (principal axis); one horizontal transverse axis determines the
thickness (sagittal axis), and the other the breadth (frontal axis). Those
ground-forms in which the transverse axes are isopolar are termed
"amphithect," and those in which the one (frontal or lateral) is isopolar
and the other (sagittal or dorso-ventral) is allopolar, are termed
"bilateral," or better "zeugitic."

46. _Primary and Secondary Ground-Forms._--The geometrical sphere must be
regarded as the original ground-form of the Radiolaria; it being understood
that its monophyletic derivation from a single stem-form, _Actissa_, is
correct. The simplest forms of _Actissa_ (_Procyttarium_, Pl. 1, fig. 1)
are in fact geometrically _perfect spheres_; indeed even the individual
parts which compose their unicellular bodies (nucleolus, nucleus, central
capsule and calymma) are concentric spheres. But in addition the central
capsules of most other SPUMELLARIA, especially the #Sphæroidea#, as well as
of many ACANTHARIA {xxii}are true spheres. Furthermore the simple or
concentrically composed lattice-spheres of #Sphæroidea#, #Sphærophracta#,
and #Phæosphæria# may be regarded as spheres, although strictly speaking
they are endospherical polyhedra. From the primary spherical form of the
Radiolaria all other secondary forms may be derived in the following
order:--1. By the development of a main axis the Monaxonia arise. 2. By the
development of transverse axes the Stauraxonia arise. 3. In both groups
(Monaxonia and Stauraxonia) the two poles (or upper and lower halves of the
body) are at first similar (Isopola). 4. By differentiation in the two
poles or halves of the body (distinction between the basal pole and the
apical) the forms with different poles (Allopola) arise. 5. The transverse
axes of the Stauraxonia are at first equal (regular pyramids and double
pyramids). 6. By differentiation in the transverse axes (distinction
between the sagittal and the frontal axis) the amphithect pyramids and
double pyramids arise. 7. From the amphithect pyramids the Amphipleura
arise by differentiation of both poles of the sagittal axis. 8. The
zygopleural ground-form appears last, as the simplest form of the
Amphipleura.

47. _The Ground-Forms of the Spumellaria._--The SPUMELLARIA, being the
oldest and most primitive Radiolaria, have for the most part either
indifferent or multistable equilibrium; _e.g._, all #Colloidea# and
#Beloidea# which have a spherical central capsule, and also most
#Sphæroidea#. Among these primitive Centrostigma true spheres and
endospherical polyhedra are represented in the utmost variety, and the
regular polyhedra in particular. By the development of a vertical main axis
these Centrostigma have also given rise to very numerous Centraxonia, which
are usually isopolar, very rarely allopolar. Sometimes they are Monaxonia
(circular in transverse section), sometimes Stauraxonia (polygonal in
transverse section). The vertical main axis is longer in the #Prunoidea#,
shorter in the #Discoidea# than any of the other axes. The #Larcoidea# are
distinguished by their lentelliptical or triaxial ellipsoid form; the three
different but isopolar axes corresponding with those of the rhombic
octahedron; but even among the #Sphæroidea#, #Prunoidea#, and #Discoidea#,
this form is sometimes produced by the differentiation of two different
transverse axes at right angles to each other. Whilst these ground-forms
(Centraxonia and Centrostigma) occur in the utmost variety among the
SPUMELLARIA, the centroplanar (or true bilateral) ground-form is entirely
wanting.

48. _The Ground-Forms of Acantharia._--In the small family Astrolophida,
which contains the most archaic forms of the legion (_Actinelius_,
_Astrolophus_), the ACANTHARIA show a direct relation to the most primitive
SPUMELLARIA (_Actissa_), and like these have indifferent equilibrium; their
central capsule is a sphere, their calymma an endospherical polyhedron,
whose angles are indicated by the distal ends of the numerous {xxiii}equal
radial spines. In the great majority of ACANTHARIA, however (all
#Acanthonida# and #Acanthophracta#), twenty radial spines are present,
regularly distributed, according to Müller's icosacanthan law, in five
parallel circles, each containing four crossed spines (p. 717). Usually the
twenty spines are equal, and the ground-form is the quadratic octahedron,
or a regular double pyramid with sixteen sides. But in some groups (the
Amphilonchida and Prunophracta) two opposite equatorial spines are much
more strongly developed than the other eighteen, and therefore the
hydrotomical axis in the equatorial plane is larger than the geotomical
axis (p. 719); the isopolar stauraxonian form passes over into the
allopolar, and the ground-form becomes the rhombic octahedron or the
amphithect double pyramid (compare §§ 33 and 34, and p. 720). The
centroplanar ground-form is entirely wanting in the ACANTHARIA.

49. _The Ground-Forms of the Nassellaria._--The NASSELLARIA all possess
monostatic ground-forms, inasmuch as by the very structure of their
monopylean central capsule a vertical main axis is necessitated, whose
basal pole occupies the porochora. The same arrangement is also for the
most part clearly recognisable in the corresponding structure of the
skeleton, which is generally either centraxon or centroplanar. Among their
manifold skeletal forms different larger groups of ground-forms may be
recognised according as the vertical allopolar main axis is crossed by
differentiated transverse axes or not (Stauraxonia or Monaxonia); the
former are either triradial or multiradial. The triradial, with three
lateral or terminal radial apophyses, constitute the greater part of the
NASSELLARIA, and have probably been derived originally from the triradial
#Plectoidea# (_Triplagia_, _Triplecta_); a more careful examination,
however (especially with reference to the structure of the cortinar
septum), reveals the fact that the ground-form is not strictly regularly
pyramidal (with three equal radii), but amphipleural (with two paired
ventral and one unpaired dorsal radius), and that it usually passes over
into a distinctly zygopleural form. The same holds true of the multiradial
NASSELLARIA, where for the most part three interradial or six adradial
(sometimes more) apophyses are intercalated between the three primary
perradial ones; sometimes here also the ground-form is a quite regular
hexagonal or nonagonal pyramid, but usually it is more or less amphithect
or amphipleural. Among the eradial NASSELLARIA, which have no radial
apophyses, the ground-form is sometimes allopolar monaxon (conical, ovoid,
hemispherical, &c.), sometimes amphithect pyramidal (even in the simplest
Stephanida, _Archicircus_, &c.), or sometimes distinctly zygopleural or
bilateral (many #Plectellaria#).

50. _The Ground-Forms of the Phæodaria._--The PHÆODARIA agree with the
NASSELLARIA in the possession of a primitively centraxon ground-form, and
like them are monostatic, since a vertical main axis whose basal pole
passes through the astropyle is present, owing to the characteristic
structure of their cannopylean central capsule. In {xxiv}the great majority
of PHÆODARIA the spheroidal central capsule also possesses a pair of
parapylæ near the opposite apical pole of the main axis (Tripylea), and
these determine (as the right and left secondary openings) an isopolar
frontal axis. Hence, strictly speaking, in most PHÆODARIA the central
capsule has the geometrical ground-form of the amphithect pyramid (as in
the Ctenophora), with an allopolar vertical main axis, and two unequal, but
isopolar, horizontal transverse axes. In many PHÆODARIA the skeleton also
has this amphithect pyramidal ground-form, _e.g._, the bivalved
#Phæoconchia# and part of the #Phæogromia#. On the contrary, in the rest of
the PHÆODARIA the skeleton exhibits very various geometrical ground-forms,
independent of that of the central capsule. In the #Phæosphæria# it forms
preferably spheres or endospherical polyhedra, as also in the Castanellida
and Circoporida among the #Phæogromia#; among the Circoporida there are
also seen with remarkable distinctness the regular polyhedra (especially
the dodecahedron and icosahedron). Isopolar monaxonia are found among the
Aulosphærida (_Aulatractus_) and Orosphærida; allopolar monaxonia among the
Challengerida (_Lithogromia_). The Medusettida and Tuscarorida show various
forms of regular pyramids (allopolar Stauraxonia); and finally, the
Challengerida are for the most part centroplanar or bilateral. Thus the
PHÆODARIA present a great wealth of different geometrical ground-forms in
the development of their skeleton, not in that of their central capsule.

CHAPTER II.--THE CENTRAL CAPSULE.

51. _Components of the Central Capsule._--In all Radiolaria without
exception, at some period of life or other, the central portion of the soft
body is separated from the peripheral portion by an independent,
anatomically recognisable membrane; this membrane with all its contents is
designated the central capsule, and is the peculiar central organ of the
unicellular body, which distinguishes the Radiolaria most clearly from the
other Rhizopoda. In the great majority of the Radiolaria the volume of the
central capsule is less than that of the surrounding peripheral soft body
which we place in opposition to it as "extracapsulum." The
"capsule-membrane," which separates these two constituents, arises very
early in most Radiolaria, and persists throughout their whole life. In some
species, however, the membrane only appears later, immediately before the
formation of the spores, and hence is absent for a considerable period.
Regarded as a whole, then, the capsule consists of the following
parts:--(1) the capsule-membrane; (2) the enclosed endoplasm, or
intracapsular protoplasm; (3) the nucleus. But in addition, many other
non-essential structures may be enclosed in the central capsule, especially
hyaline spheres (vacuoles), fatty spheres, pigment granules, crystals, &c.

The central capsule was first described in my Monograph in 1862 (pp.
69-82) as the most characteristic component of the Radiolarian organism,
and distinguished from the whole extracapsular {xxv}soft body. The fact
that it has recently been reported as absent by various authors is due to
their having observed young or unripe specimens, before the formation of
the spores. In some species of #Polycyttaria# and ACANTHARIA the membrane
persists only a very short time.

52. _The Primary Form of the Central Capsule._--The form of the central
capsule is originally a geometrical sphere; and if in accordance with our
monophyletic hypothesis all Radiolaria are to be derived from one common
stem-form (_Actissa_, see p. 12), then the central capsule of this common
stem-form must be regarded as perfectly spherical (_Procyttarium_, p. 13,
Pl. 1, fig. 1). Since, further, the enclosed nucleus and the surrounding
calymma of this primitive archaic form must also be spheres, and since the
nucleus lies in the centre of the body, and the protoplasm is evenly
distributed between it and the membrane, it follows that no axes or
excentrically differentiated parts are to be distinguished in this most
primitive Radiolarian. Rather in the primary central capsule all parts are
concentrically and evenly arranged round its centre. This primary spherical
form becomes modified in most Radiolaria into various secondary
ground-forms, which are correlated partly with the structure of the capsule
itself, and partly also with the development of openings in its membrane.
In general the ground-form of the central capsule is polyaxon in the
Porulosa (SPUMELLARIA and ACANTHARIA); but in the Osculosa centraxon forms
are more frequently observed; in the NASSELLARIA the ovoid (allopolar
monaxon) form is predominant, and in the PHÆODARIA the rhomboid or
amphithect pyramid. In these latter, the astropyle indicates the basal pole
of the vertical main axis, whilst the two parapylæ (right and left) mark
the poles of the frontal transverse axis. In the NASSELLARIA the centre of
the porochora corresponds with the basal pole of the main axis, whilst no
transverse axes are originally present.

53. _The Secondary Forms of the Central Capsule._--The original purely
spherical form of the central capsule persists only in the minority of the
Radiolaria, namely, the greater part of the SPUMELLARIA and ACANTHARIA; it
passes over into various other secondary forms in the majority of the
class, in the whole of the NASSELLARIA and PHÆODARIA, and in a considerable
portion of the SPUMELLARIA and ACANTHARIA. These secondary or derived forms
may be divided into two quite distinct groups, which may be designated
endometamorphic and exometamorphic; in the former the cause of the
divergence of the secondary form from the sphere lies in the internal
structure of the central capsule; in the latter it lies in the external
influence exerted by the growth of the skeleton. Obviously the former
series of modifications is more significant than the latter.

54. _The Endometamorphic Forms of the Central Capsule._--The secondary
forms of the central capsule, which are due to internal causes connected
with its growth, are as follows:--

{xxvi}A. _The Ellipsoidal Central Capsule_, with one axis elongated, so
that it becomes the vertical main axis of the body.

_a._ Among the SPUMELLARIA, _Actiprunum_ (p. 14), _Colloprunum_ (p. 25,
Pl. 3, fig. 9), most #Prunoidea# (p. 288).

_b._ Among the ACANTHARIA, many Amphilonchida (p. 782, Pl. 132, figs.
2, 6), and Belonaspida (p. 861).

_c._ Among the NASSELLARIA, many #Plectoidea# (p. 905, Pl. 91, figs. 5,
9), #Stephoidea# (p. 937, Pl. 81, fig. 16), Monocyrtida (Pl. 51, fig.
3), &c.

B. _The Cylindrical Central Capsule_, with considerable elongation of the
vertical main axis, which is several times as long as the horizontal
transverse axis.

_a._ Amongst the SPUMELLARIA, _Collophidium_ (p. 26, Pl. 3, figs. 1-3)
and many #Prunoidea# (_Spongurus_, &c.).

_b._ Among the ACANTHARIA, some Amphilonchida.

C. _The Discoidal, Spheroidal, or Lenticular Central Capsule_, with one
axis shorter than the others, which becomes the vertical main axis.

_a._ Among the SPUMELLARIA, _Actidiscus_ (p. 15), _Collodiscus_ (p.
27), and the large group #Discoidea# (p. 408).

_b._ Among the ACANTHARIA, many Quadrilonchida (p. 768, Pl. 131), and
most Hexalaspida (p. 874).

_c._ Among the NASSELLARIA, certain #Stephoidea# and #Cyrtoidea#.

_d._ Among the great legion PHÆODARIA the spheroidal central capsule is
almost always more or less flattened in the direction of the main axis
(p. 1525, Pls. 101-128).

D. _The Lentelliptical Central Capsule_ (or triaxial ellipsoid), with
three unequal but isopolar axes at right angles to each other, the
sections in all three dimensions of space being ellipses.

_a._ Among the SPUMELLARIA, _Actilarcus_ and the large group
#Larcoidea# (p. 604).

_b._ Among the ACANTHARIA, certain Amphilonchida and Belonaspida.

E. _The Polymorphic, Amoeboid or Irregular Central Capsule._

_a._ Among the SPUMELLARIA, _Collodastrum_ (p. 28, Pl. 3, figs. 4, 5),
and some #Larcoidea#.

55. _The Exometamorphic Forms of the Central Capsule._--The secondary forms
of the central capsule, which are brought about by external causes, chiefly
dependent on the formation of the skeleton, are very various and in many
cases devoid of special interest; in other instances, on the contrary, they
are of great importance, because of the clear relation of cause and effect
which can be traced between the development of the skeleton and of the
capsule. The most important phenomena to be recorded in this connection are
as follows:--

{xxvii}I. SPUMELLARIA.--(A) In many of the #Sphæroidea#, the central
capsule of which is originally enclosed by a simple lattice-sphere, it
puts out protrusions through the meshes of the shell, thus forming
club-shaped processes, corresponding in number with the meshes of the
lattice (Pl. 11, figs. 1, 5; Pl. 20, fig. 1_a_; Pl. 27, fig. 3, &c.). The
whole surface of the spherical capsule may thus be covered with numerous
independent radial clubs of equal size, but usually they unite again
outside the shell to form a simple sphere with smooth surface. (B) In
many #Prunoidea# whose originally ellipsoidal body has become cylindrical
by the marked prolongation of the main axis, the central capsule is
divided by a series of constrictions into segments, which correspond with
the annular constrictions of the skeleton (Pls. 39, 40). (C) In most
#Discoidea# whose lentiform or discoidal shell develops radial arms at
its margin, the central capsule sends out processes into these arms, and
adapts itself to the stellate form of the skeleton (p. 409, Pl. 43, fig.
15; Pl. 47, &c.) (D) In many #Larcoidea# whose growth is originally
lentelliptical, but later spiral or irregular, the central capsule
follows the mode of growth and develops irregular protuberances.

II. ACANTHARIA.--Whilst the central capsule of most ACANTHARIA retains
its primitive spherical form, in a minority of the group it passes over
into various secondary forms, which are directly determined by the growth
of the skeleton; especially common are lappet or club-shaped prominences
which follow the larger radial spines. Hence the central capsule may
assume the form of a violin, with two lobes corresponding to the two
poles of the elongated main axis, as in many Amphilonchida (p. 782, Pl.
132, fig. 10), and the Diploconida (p. 884, Pl. 140). On the other hand
the central capsule becomes cruciform, with four lobes disposed at right
angles, as in Lithoptera and other Quadrilonchida (p. 768, Pl. 131, fig.
10, &c.).

III. NASSELLARIA.--The primitive ellipsoid or ovoid form of the central
capsule persists only in a few NASSELLARIA, such as the simplest and most
archaic forms, the Nassellida, many #Plectoidea#, #Stephoidea#,
Monocyrtida, &c. In the great majority of the NASSELLARIA, on the
contrary, the ellipsoid or ovoid form passes over into a secondary form
which is usually characterised by the presence of lobes, and is obviously
dependent upon the previous development of the skeleton. In many
#Stephoidea# and #Spyroidea# (probably the majority), a bilobed central
capsule is formed (with symmetrically equal right and left lobes), since
the primary vertical sagittal ring interferes with the growth in the
median plane (Pl. 90, figs. 7-10). In other {xxviii}#Spyroidea#, on the
contrary, and the majority of the #Cyrtoidea#, the central capsule forms
at its basis rounded lobes, which protrude and hang down from the meshes
of the cortinar plate; and since this latter has usually three or four
large pores, the capsule similarly develops three or four processes (Pl.
53, fig. 19; Pl. 55, figs. 4-11; Pl. 59, figs. 4-13; Pl. 60, figs. 3-7;
Pl. 65, fig. 1).

56. _The Membrane of the Central Capsule._--The capsule-membrane or
envelope of the central capsule is both morphologically and physiologically
one of the most important parts of the Radiolarian body, for it separates
its two main constituents, the capsule with its nucleus and endoplasm and
the extracapsulum with the calymma and exoplasm. The capsule-membrane is
invariably present at some time or other during the life of the organism,
even though in a few species it may persist only for a short time. It is
characterised in general by its power of resistance to chemical and
physical reagents, and appears to be related to the elastic tissues or
perhaps even more to the chitinous substances. Its thickness is usually
less than 0.0001, though in certain groups it ranges between 0.001 and
0.002, and in many of the larger Radiolaria (such as Collida and PHÆODARIA)
it may attain a thickness of 0.003 to 0.006 or more. In the three legions
SPUMELLARIA, ACANTHARIA, and NASSELLARIA the capsule-membrane is single,
while in the PHÆODARIA it is always double, being composed of a firm outer
and a delicate inner membrane, which are in contact at only few points.
Usually it is quite structureless, except for its apertures; the thicker
membrane showing occasionally a fine concentric lamination. In certain
large #Colloidea# (_e.g._, _Thalassicolla_, Pl. 1, fig. 5_b_) the membrane
is covered on the inner surface by a network of polygonal ridges, and in
some large PHÆODARIA with remarkable small curved rods (Pl. 114, fig. 13).
In all Radiolaria the membrane is perforated by definite openings or pores,
through which the intracapsular and extracapsular protoplasm are in direct
communication. These openings (or "pylae") show very characteristic and
constant differences in the four legions, which have given rise to the
names--PERIPYLEA, ACTIPYLEA, MONOPYLEA, CANNOPYLEA.

The capsule-membrane was first indicated as the most important and
absolutely constant component of all Radiolaria, and as the differential
character of the class, in my Monograph (1862, pp. 69-71). The careful
investigations of R. Hertwig have confirmed this view and at the same
time have yielded the most important conclusions regarding the nature and
systematic significance of the openings in the capsule (_op. cit._, 1879,
pp. 105-107). On the contrary, Karl Brandt has recently propounded the
theory that the capsule-membrane is by no means a constant part of the
Radiolarian organism, but is lacking in certain species of _Collozoum_
and _Sphærozoum_ (1881, p. 392). This contradiction is explained by the
fact that in some #Collodaria# and #Acanthometra# the formation of the
central capsule takes place much later than in the other Radiolaria, in
some {xxix}species indeed only just prior to the development of the swarm
spores. I have recognised the presence of it in all species which I have
investigated (more than a thousand), and even in those in which Brandt
denies its existence. It is often very delicate and may easily be
overlooked, especially when the contents of the capsule are colourless,
but in all cases by the prudent application of staining fluids and other
reagents its presence may be demonstrated. Even in those cases in which
the contour of the capsule was not visible, and its contents appeared to
pass without definite boundary into the matrix of the extracapsulum, it
was possible by the use of appropriate stains or reagents, which would
not penetrate the capsule, or of those solvents which were capable of
dissolving its contents and of causing it to swell up like a distended
bladder, to recognise the existence of the membrane. Those Radiolaria in
which it is truly absent are young animals of species in which the
membrane is only formed immediately before sporification, and persists
but for a short time (_e.g._, species of _Collozoum_, _Sphærozoum_,
_Acanthometra_, _Acanthochiasma_, &c.).

57. _The Capsule-Openings of the Peripylea (or Spumellaria)._--The
capsule-membrane of the PERIPYLEA is generally perforated by extremely fine
and numerous pores, which are distributed at equal distances over the whole
surface, and are precisely alike in all parts of the capsule. Hence the
SPUMELLARIA may be called "Holotrypasta" or "Porulosa"; they agree with the
ACTIPYLEA in being devoid of an osculum or operculum; they are
distinguished from the latter group mainly in that their pores are equally
distributed over the whole surface of the capsule, whilst in the ACTIPYLEA
the pores are disposed in definite groups or lines, separated by large
imporous areas.

The central capsule of the SPUMELLARIA, with its innumerable fine and
evenly distributed pores, must be regarded as the primitive arrangement,
from which the different central capsules of the three other legions have
been developed. The central capsule of the ACTIPYLEA has been derived
from that of the PERIPYLEA by reduction in the number of the pores and
their distribution in definite, regularly disposed areas in the membrane.
The central capsule of the Osculosa is characterised by the formation of
a special main-aperture (osculum) at the basal pole, which is closed in
the MONOPYLEA by the porochora, and in the CANNOPYLEA by the astropyle;
the remaining pores, with the exception of the accessory openings of many
CANNOPYLEA, remain undeveloped in both these legions. In the same way
Hertwig regards the central capsule of the PERIPYLEA as the primitive
form (1879, L. N. 33, p. 107).

58. _The Capsule-Openings of the Actipylea (or Acantharia)._--The
capsule-membrane of the ACTIPYLEA is perforated by very numerous fine
pores, which are regularly distributed over the surface of the central
capsule, and separated by imporous intervals. Hence the ACANTHARIA belong
to the "Holotrypasta" or "Porulosa"; they have neither osculum nor
operculum, and agree in this particular with the PERIPYLEA; but they are
separated from these latter chiefly by the fact that their pores are much
less numerous, and marked off into regularly arranged groups or lines by
imporous intervals. In the PERIPYLEA, on the contrary, the pores are much
more numerous and are evenly distributed over the whole surface of the
capsule.

{xxx}The central capsule of the ACANTHARIA has hitherto been for the most
part confounded with that of the SPUMELLARIA, and no clear distinction
has been drawn in this respect between the two legions of the Porulosa.
Hertwig, who in 1879 first discovered the remarkably different structure
of the Osculosa (NASSELLARIA and PHÆODARIA), recognised no distinction
between the structure of the capsules in the PERIPYLEA and ACTIPYLEA (his
Acanthometrea), and supposed that in both these legions "very fine pores
were evenly distributed in large numbers over the capsule-membrane"
(_loc. cit._, p. 106). I have, however, during the last few years
convinced myself, by the careful comparative investigation of numerous
ACANTHARIA, that in this respect they are quite distinct from the
SPUMELLARIA (with perhaps the exception of the Astrolophida, which are
nearly related to the primitive _Actissa_). The number of pores in the
ACTIPYLEA is usually very much smaller than in the PERIPYLEA, and they
are regularly arranged in groups.

59. _The Capsule-Openings of the Monopylea (or Nassellaria.)_--The
capsule-membrane of the MONOPYLEA always possesses a single large
main-opening, an osculum, which lies at the basal pole of the main axis,
and is closed by a circular perforated lid (operculum porosum). When seen
from the surface this lid appears as a clearly defined porous area
(porochora or area porosa), and forms the horizontal base of a peculiar
cone, which stands vertically in the interior of the capsule and may be
designated the "thread-cone" (podoconus). The NASSELLARIA may hence be
termed "Merotrypasta" or "Osculosa," like the CANNOPYLEA; the structure and
significance of the circular lid (operculum), which closes the main-opening
(osculum) is, however, quite different in the two legions. Whilst the lid
of the CANNOPYLEA (astropyle) is solid, traversed by radial ribs, and only
perforated in its centre by a short tube (proboscis), in the MONOPYLEA the
operculum (porochora) is always perforated by numerous vertical fine pores,
and is in connection with the peculiar internal "pseudopodial cone"
(podoconus, Pl. 51, figs. 5, 13; Pl. 81, fig. 16; Pl. 91, fig. 5; Pl. 98,
fig. 13). The pores are separated by small vertical, highly refractive rods
(opercular rhabdillæ); these become intensely stained by carmine, and are
either evenly distributed over the surface of the porochora or arranged in
definite groups. The outer or distal end of each rod is rounded, sometimes
thickened like a club or split into lobes; the inner or proximal end is
usually pointed, and stands in connection with a myophane thread of the
podoconus (see § 79). The primary circular form of the porochora, in which
the opercular rhabdillæ are evenly distributed in a horizontal plane,
undergoes various secondary modifications in many NASSELLARIA. The
triradial structure of the skeleton, which characterises the majority of
the legion, causes a splitting of the base of the central capsule into
three or four lobes; this division also affects the porochora, which lies
in the centre of the base, so that the rhabdillæ become arranged in three
or four equal circles. If, however, the lobes of the central capsule become
larger and protrude through the three or four collar pores of the cortinar
septum, the central porochora may separate entirely into three or four
elongated tracts, which lie on the axial side of the magnified lobes; the
rhabdillæ are then arranged over the whole surface of {xxxi}these tracts,
on the outer aspect of which run the longitudinal myophane fibrillæ of the
podoconus (compare §§ 79 and 99).

The porous area of the MONOPYLEA was first described by Hertwig in 1879,
and shown to be the characteristic main-opening of the central capsule in
various families belonging to this legion (L. N. 33, pp. 71, 73, 83, 106,
Taf. vii., viii.). According to his view "the capsule-membrane in the
porous area becomes thickened around each pore into a rod, perforated by
a canal," and the intracapsular protoplasm passes outwards through these
fine canals (_loc. cit._, p. 106). I am not able to share this
interpretation, but think rather that I have convinced myself by the
examination of some living NASSELLARIA, and of many well-stained and
preserved preparations in the Challenger collection, that the rods are
_solid_, specially modified portions of the capsular wall, and that the
protoplasm does not pass through them but through pores which lie between
them.

60. _The Capsule-Openings of the Cannopylea (or Phæodaria)._--The
capsule-membrane of the CANNOPYLEA always possesses only a single large
main-opening or osculum, which lies at the basal pole of the vertical main
axis, and is closed by a circular radiated lid (operculum radiatum). This
operculum appears, when seen from the surface, as a sharply defined
stellate area (astropyle), from the middle of which arises a shorter or
longer cylindrical tube, the proboscis. Hence the PHÆODARIA, like the
MONOPYLEA, belong to the "Merotrypasta" or "Osculosa"; the structure and
significance of the circular operculum, which closes the main-opening
(osculum), are, however, quite different in the two legions. Whilst the
operculum of the MONOPYLEA (porochora) is perforated by numerous fine
vertical pores, and connected with the peculiar internal pseudopodial cone
(podoconus), this structure is entirely wanting in the CANNOPYLEA, and
instead of it there is a solid operculum, with radial ribs which originate
at the base of its central tubular mouth; this tube (proboscis) is
cylindrical, often conical at the base, of very variable length and with a
round aperture at either end. In spite of the great difference which the
various families of CANNOPYLEA exhibit in the formation of their skeleton
and its appendages, the constitution of this characteristic stellate
main-opening (astropyle) is always essentially the same; both the stellate
operculum itself, and the proboscis which rises from its centre, show only
slight differences in the various groups. In addition to this large
main-opening most PHÆODARIA possess several small accessory openings
(parapylæ); and usually two of these are present, placed symmetrically
right and left of the aboral pole of the main axis and in the frontal plane
(Pl. 101, figs. 2, 6, 10; Pl. 104, figs. 1, 2_a_). Sometimes there are more
numerous accessory openings (three to six or more) regularly arranged, as
in the two peculiar families, Circoporida and Tuscarorida; occasionally
also there is only a single parapyle, at the aboral pole of the main axis
(_e.g._, in _Tuscaridium_). The parapylæ seem to be quite absent in the
families Challengerida, Medusettida, Castanellida, and perhaps also in
other PHÆODARIA. The form and structure of the small accessory openings
appear to be always the same. The {xxxii}outer capsule-membrane is elevated
in the form of a short cylindrical tube or "apertural ring" (collare
paraboscidis), the external margin of which bends inwards, and at the base
of the ring passes over into the delicate internal capsule membrane. Upon
this apertural ring is situated a longer or shorter "apertural cone"
(paraboscis), which is a tubular, cylindrical or conical, prolongation of
the membrane, open externally.

The peculiar capsule-openings of the PHÆODARIA were first discovered and
carefully described by Hertwig in 1879 (L. N. 33, pp. 95, 107). He found
in all the six genera which he examined _three_ openings, a main-opening
at the basal pole of the main axis and two accessory openings, one on
either side of the apical pole; hence he named the whole group
"TRIPYLEA." This name, however, is not applicable to the numerous
PHÆODARIA mentioned above, which have only a main opening without any
accessory openings, nor to those genera in which the number of the latter
is variable. I have, therefore, replaced Hertwig's designation by the
term "CANNOPYLEA," which has reference to the peculiar tubular form of
the opening. This I find much more developed in many PHÆODARIA than
Hertwig has represented, and I must also, in certain particulars, dissent
from his delineation of the minute structure, although this is in the
main remarkably accurate.

61. _The Nucleus._--The nucleus, enclosed in the central capsule of all
Radiolaria, behaves in every respect like a true cell-nucleus, and thus
lies at the base of the now universal opinion, that the whole Radiolarian
organism, in spite of its varied development and remarkable variations, is
unicellular and remains throughout life a true individual cell. This
important theory is not invalidated by the fact that the nucleus undergoes
peculiar modifications in many groups, and in certain groups presents
appearances seldom or never seen elsewhere.

62. _Uninuclear and Multinuclear Radiolaria (Monocaryotic and
Polycaryotic)._--All Radiolaria present two different conditions in respect
of the behaviour of the nucleus, since in their young stages they are
uninuclear (_monocaryotic_), and in later stages multinuclear
(_polycaryotic_). This is readily explained by the fact that each
individual Radiolarian is developed from a simple unicellular swarm-spore,
and that afterwards, before the formation of swarm-spores, the single
nucleus divides into many small nuclei. Thus in the Radiolaria the nucleus
is pre-eminently the _organ of reproduction and inheritance_. The division
of the originally single nucleus into many small nuclei may take place,
however, at very different periods, so that the Radiolaria may be divided
in this respect into precocious and serotinous.

63. _Serotinous and Precocious Radiolaria._--In the great majority of the
Radiolaria the division of the nucleus takes place only at a late period, a
short time or even immediately before the process of spore formation; it
then breaks up rapidly into numerous small nuclei (always more than one
hundred, sometimes many thousands), and each of these {xxxiii}either
becomes itself the nucleus of a swarm-spore, or by repeated division gives
rise to a group of spore-nuclei. All those Radiolaria which are uninuclear
during the greater part of their existence, and in which the process of
division is late, and takes place rapidly, are called "serotinous" or
late-dividing forms. To this category belong all PHÆODARIA and NASSELLARIA,
as well as all the solitary or monozoic SPUMELLARIA and some ACANTHARIA. On
the other hand, the name "precocious," or early dividing, is applied to
those Radiolaria in which the division of the nucleus takes place very
early, and in which, therefore, the cell is multinuclear during the greater
part of its existence. This is the case in all the social or polyzootic
Radiolaria (Polycyttaria, Pls. 3-8), and also in the great majority of the
ACANTHARIA, both #Acanthometra# and #Acanthophracta#. In the last two
groups, however, there are numerous exceptions, and these are seen in
remarkably large species, characterised by the great size of the central
capsule. From a phylogenetic point of view, the conclusion is allowable
that the precocious forms are secondary, and have arisen by adaptive
modification from the primitive serotinous stem. In the Polycyttaria (or
social SPUMELLARIA, _i.e._, the three families Collozoida, Sphærozoida, and
Collosphærida), the cause of the adaptation lies most probably in the
formation of the colony itself, for all these three families are so closely
related to three corresponding families of serotinous, monozootic
Radiolaria (Thalassicollida, Thalassosphærida, Ethmosphærida), that certain
species of the latter are hardly to be distinguished from isolated
individuals of the former. Perhaps the remarkable formation of the large
central oil-globule, which particularly characterises the Polycyttaria, is
the prime cause of their early nuclear division. In the ACANTHARIA the
cause is most likely to be found in the characteristic _centrogenous
development_ of their acanthin skeleton, whose radial bars first of all
appear in the centre of the capsule. Hence arises directly the excentric
position of the nucleus, which in the archaic stem of ACANTHARIA
(_Actissa?_) was probably central. In any case, but little weight is to be
laid upon the precocious division of the nucleus in the ACANTHARIA in
general, inasmuch as in certain species (both #Acanthometra# and
#Acanthophracta#) the more usual serotinous division persists.

64. _Central and Excentric Nuclei._--The position of the nucleus in the
interior of the central capsule was no doubt primitively central, and this
situation in the geometrical centre of the original spherical central
capsule has been accurately retained in all monozootic SPUMELLARIA; in the
polyzootic families of this legion (Polycyttaria), on the contrary, it is
obscured by the precocious division of the nucleus. In the other three
legions, which may be phylogenetically derived from the SPUMELLARIA, the
position of the nucleus is rarely central, but usually excentric, or at
most subcentral. In the ACANTHARIA (both #Acanthometra# and
#Acanthophracta#) the central position of the nucleus is at once excluded
by the constantly centrogenous development of the skeleton; the nucleus is
therefore always excentric, and may lie at either side; it usually
{xxxiv}divides very early into numerous separate nuclei, which are usually
distributed in the peripheral portions of the central capsule. In the
NASSELLARIA the development of the porochora, and of the podoconus which
stands upon it, brings about the formation of a vertical axis, and in
consequence the central capsule assumes a monaxon form (usually ovoid or
conical); the nucleus then lies in the main axis, but excentrically between
the apex of the podoconus and the aboral pole. In many NASSELLARIA,
however, especially when the podoconus is so large that its apex approaches
the aboral pole of the central capsule, the nucleus is pressed to one side
and lies quite excentrically. The PHÆODARIA exhibit a different
arrangement; the large spheroidal nucleus is always subcentral, so that its
main axis corresponds with that of the concentric spheroidal central
capsule; but since the astropyle always occupies the oral pole of the
latter, and since the distance of the nucleus from this pole is always
somewhat different from its distance from the other, it follows that,
strictly speaking, the nucleus never lies accurately in the geometrical
centre.

65. _Homogeneous and Allogeneous Nuclei._--The nucleus of the Radiolaria
not only exhibits a similar structure and composition, and suffers similar
modifications to those which are found to occur in the case of other
cell-nuclei, but also to some extent shows very peculiar developmental
forms, which are seldom or never found in other cells. In the first place
the nuclei may be divided into homogeneous and allogeneous, the former are
structureless and consist of a uniform mass of nuclein, whilst the latter
are composed of different substances and show various structural relations.
_Homogeneous_ nuclei, whose whole mass is uniform and exhibits no
structural differentiation, are probably always to be found in the
swarm-spores; in the fully developed Radiolarian body they are found only
in the first legion, SPUMELLARIA, and that both in many Monozoa (especially
small #Sphæroidea# and #Prunoidea#) and in the Polyzoa (or Polycyttaria).
The whole mass of these homogeneous nuclei, which are usually spherical or
ellipsoidal, consists of uniform, perfectly clear and transparent nuclein,
and becomes evenly stained by carmine, hæmatoxyline, &c. They may be
readily distinguished by these means from the clear vacuoles or "hyaline
vesicles," which are evenly distributed in the endoplasm of many
Radiolaria, and may be confused with the former. _Allogeneous_ nuclei,
which are always composed of different parts and often show complicated
structural relations, are found developed in the great majority of
Radiolaria. The most important differentiation exhibited by these secondary
forms is the separation of the nuclear mass into a firm nuclear substance
(caryoplasm) and a fluid nuclear juice (caryolymph). In addition in each
nucleus a nucleolus is visible, and often several or many may be seen (see
§§ 67 to 70).

66. _The Form of the Nucleus._--The nucleus of the Radiolaria shows greater
variations in form and structure than are to be found in the majority of
cell-nuclei; {xxxv}exception must, however, be made in the case of many
animal ovicells, which, in their peculiar form and composition, often
recall large Radiolarian nuclei. With respect to the external shape two
main forms may be distinguished, as primary and secondary. The _primary
form_ of the Radiolarian nucleus is the sphere; it occurs not only in most
swarm-spores, but also in most adult forms belonging to the legion
SPUMELLARIA, and in individual instances in other groups; indeed the nuclei
of most SPUMELLARIA, as also the concentric central capsules in which they
lie, are true geometrical spheres. The _secondary forms_ of the nucleus are
found in the majority of adult Radiolaria, and arise from the primary
spherical forms in various ways, either by the elongation or contraction of
one axis, or by the formation of apophyses or processes. The most important
of these secondary forms are as follows:--

1. _Ellipsoidal nuclei_, arising by elongation of one principal axis;
very common among the NASSELLARIA, as well as in many #Prunoidea# and
#Larcoidea# among the SPUMELLARIA; also in several ACANTHARIA.

2. _Discoidal nuclei_, arising by contraction of one principal axis,
sometimes lenticular or spheroidal, biconvex, sometimes shaped like a
disc or coin; especially common in the #Discoidea# among the SPUMELLARIA,
also in some ACANTHARIA; the large nucleus of the PHÆODARIA is always
spheroidal or almost spherical, with a slightly shortened main axis.

3. _Stellate nuclei_, spherical, and armed with evenly distributed radial
club-shaped or conical processes; rare but very characteristic,
especially in the two large Thalassicollida _Thalassopila_ (Pl. 1, fig.
3), and _Thalassophysa_ (Monogr. d. Radiol., Taf. i.); also in some
#Sphærellaria# (Pl. 11, fig. 5).

4. _Amoeboid nuclei_, with unequal processes irregularly arranged, in
certain irregular forms of SPUMELLARIA and ACANTHARIA.

5. _Lobate nuclei_, with several (usually two or three) large ovoid or
pyriform lobes, which protrude into corresponding larger lobes of the
central capsule, in many NASSELLARIA, especially the multiarticulate
#Cyrtoidea# (Pl. 59, figs. 12, 13). The budding nucleus of the ACANTHARIA
is also lobate (Pl. 129, figs. 6-11).

67. _The Nucleus of the Peripylea._--The nucleus of the SPUMELLARIA or
PERIPYLEA shows in certain groups a very primitive arrangement, indeed the
archaic structure from which the various forms of nuclei of other
Radiolaria may be derived; but on the other hand, in other groups it
exhibits very peculiar and remarkable differentiations. In the first place
it may be noted that the monozootic or solitary SPUMELLARIA usually possess
a single serotinous nucleus, which only divides into numerous swarm-spores
at a late period; {xxxvi}whilst, on the contrary, the polyzootic colonial
SPUMELLARIA (or Polycyttaria) are uninuclear only in the young state (Pl.
3, fig. 12), and speedily present numerous small homogeneous nuclei, which
have arisen by precocious division of a single nucleus; these are usually
spherical and 0.008 to 0.012 mm. in diameter. The serotinous nucleus of the
monozootic SPUMELLARIA, in many divisions of this large legion, and
especially in the simply constituted #Sphæroidea#, is a homogeneous sphere
of nuclein, lying in the middle of the central capsule. In many other cases
it assumes the form of a spherical vesicle ("Binnen-Bläschen"), whose fluid
or semi-fluid contents are enclosed by a more or less firm membrane. This
vesicle often contains a single central spherical _nucleolus_ (Pl. 1, figs.
1_l_, 4_l_), but sometimes a variable number of small excentric nucleoli
(Pl. 1, figs. 1_a_, 2_a_). The nuclear membrane is often somewhat thick,
presenting a double contour, and in such cases may even exhibit a fine
radial striation, the expression of minute pores (Pl. 1, fig. 2_a_). In the
colossal nuclei (as much as 1 to 2 mm. in diameter) of certain large
Thalassicollida the nucleolus presents a very remarkable form, becoming
stellate by the protrusion of processes, which may again branch in a
dendritic fashion (as in the common _Thalassicolla nucleata_), or it may
develop into a very long cylindrical thread, which is disposed in
serpentine coils, and in _Thalassophysa pelagica_ passes into the different
cæcal processes of the stellate nucleus. In many #Sphæroidea#, whose
skeleton is composed of numerous concentric lattice spheres, the small
central spherical nucleus lies at first within the innermost of these (the
medullary shell); but afterwards it grows through the meshes of the
lattice-work, and the radiating club-shaped processes thus formed (Pl. 11,
fig. 5) unite with each other outside the medullary shell, and form an
external nuclear sphere which completely encloses the latter. In the
Polysphærida (with several concentric lattice-shells) and in the
Spongosphærida (with spongy lattice-spheres), this process may be several
times repeated, so that eventually the central spherical nucleus attains
considerable dimensions, and encloses two or more concentric lattice-shells
with their radial connecting rods. The nuclear membrane is in these cases
usually penetrated by radial bars, which connect the outermost of the
enclosed shells with the remaining cortical shells which surround the
central capsule. The same remarkable arrangement is also very common among
the #Discoidea#. The small spherical primary nucleus is in such instances
immediately surrounded by the innermost earliest developed lattice-shell,
around which the concentric rings are subsequently deposited; it then grows
out through the meshes, and the processes fuse outside the ring to form a
homogeneous lentiform nucleus (Pl. 43, fig. 15). The same process recurs in
certain #Prunoidea# and #Larcoidea#, whilst in other SPUMELLARIA of these
groups (_e.g._, Pylonida) the lobate processes of the nucleus remain free.

Both the simple serotinous nucleus of the monozootic SPUMELLARIA, and the
numerous precocious nuclei of the Polycyttaria, were first described in
my Monograph in 1862, the former as the "endocyst" ("Binnen-Bläschen"),
the latter as "spherical transparent vesicles" ("Kugelige
{xxxvii}wasserhelle Bläschen"). I was in error, however, in regarding the
latter as identical with the so-called "hyaline spherules" in the central
capsule of many Monozoa, which rather belong to the category of
intracapsular vacuoles (see § 72). The credit of recognising, by the aid
of the modern methods of staining, the distinctness of these two
structures, which may readily be mistaken for each other, and of
demonstrating the true nature both of the serotinous and precocious
nuclei, belongs to Richard Hertwig (1879, L. N. 33).

68. _The Nucleus of the Actipylea._--The nucleus of the ACANTHARIA or
ACTIPYLEA shows very peculiar relations in respect of structure and
division, particularly special forms of lobular budding, which belong to
the characteristic peculiarities of this singular legion, and are not found
among other Radiolaria. The position of the nucleus is _always excentric_,
even in the youngest ACANTHARIA, for the centrogeneous formation of the
skeleton, the constant development of the earliest radial portions of it in
the middle of the central capsule, forces the nucleus from its normal
central position. The majority of the ACANTHARIA, like most Polycyttaria,
are precocious, the primary nucleus early dividing into numerous small
nuclei (see note A below). Nevertheless there are many exceptions to this
rule in different families, _e.g._, _Stauracantha_, _Xiphacantha_,
_Phatnacantha_, and _Pristacantha_ among the #Acanthometra#, and
_Stauraspis_, _Echinaspis_, _Dodecaspis_, and _Phatnaspis_ among the
#Acanthophracta#. In these instances the primary nucleus remains for a long
time as a simple excentric ellipsoidal or irregularly round body, even in
the fully developed stage, and only at a very late period (sometimes just
before the formation of the spores) divides into many small nuclei. Since
this serotinous division of the nucleus takes place in different genera of
very various groups, it can only be decided by further investigations how
widely it is spread among the ACANTHARIA, and upon what circumstances it is
dependent (see note B). The division of the nucleus appears to be
precocious in the majority of this legion, and a number of small nuclei
appear to be early formed by a peculiar process of budding; in most fully
developed ACANTHARIA these are disposed in one or two layers under the
surface of the central capsule, but if their numbers increase to any
considerable extent, the whole space between the skeletal rods becomes
filled with small nuclei; sometimes these are homogeneous, sometimes
vesicular, 0.002 to 0.012 mm. in diameter; usually they are spherical and
have a small nucleolus (compare Pl. 129, figs. 6-11, and note C).

A. The numerous nuclei, which are to be found in the central capsule of
most mature ACANTHARIA, were first described in my Monograph (1862) as
"spherical, transparent vesicles, provided with a small dark granule" (p.
374, Taf. xv. figs. 2, 5; Taf. xvi. figs. 2, 4; Taf. xxi. fig. 7, &c.).
Their more minute constitution and peculiar origin were first accurately
delineated by R. Hertwig (1879, _loc. cit._, pp. 11-24, Taf. i-iii.).

B. The fact that in a number of ACANTHARIA the nucleus does not divide
early as in the majority of the legion, but only at a later period, was
first observed by R. Hertwig in a species of #Acanthometra# (_Xiphacantha
serrata_), and a species of #Acanthophracta# (_Phatnaspis
{xxxviii}mülleri_ = _Haliommatidium mülleri_) (_loc. cit._, pp. 11 and
27). This serotinous division of the nucleus seems, however, to be rather
widely spread in both sublegions of the ACANTHARIA; I have found, not
only in the forms above mentioned, but also in several others belonging
to different genera, a single large excentric nucleus, even in those
individuals in which the skeleton was fully developed.

C. The peculiar mode of nuclear budding, by which these small nuclei
arise, appears to proceed in the following manner (Pl. 129). The
vesicular primary nucleus, which, in consequence of the centrogeneous
development of the skeleton protrudes as it grows into irregular lobes
(Pl. 129, fig. 9), assumes a peculiar concavo-convex form, sometimes that
of a hood or dish, sometimes that of a kidney or sausage. The convex
surface is apposed to the capsule-membrane, while the concave is turned
towards the central star of the skeleton (fig. 6). There is now formed at
the centre of the convex surface of the strong, doubly-contoured, nuclear
membrane, a flask-shaped invagination with a narrow neck and expanded
base; the membrane now becomes disposed in peculiar folds, which at the
narrow aperture of invagination appear as folds, but on the expanded body
of the flask take the form of concentric rings, laid closely side by side
(Pl. 129, fig. 10). The convex bottom of the flask, which is directed
towards the concave proximal side of the nucleus, becomes again
invaginated by a central conical apophysis of the enlarged nucleolus,
which is situated between them. Usually the nucleolus has already become
flattened into a lentiform shape, and upon its distal face a conical
apophysis has been developed, which is divisible into a darker proximal
and clearer distal portion. The tip of the latter appears to be in direct
connection with the nuclear membrane at the centre of the base of the
flask-shaped invagination (figs. 6, 10). At this stage of development the
nucleus of the ACANTHARIA generally presents the characteristic form of a
hood-shaped, concavo-convex vesicle, whose radial axis is also the axis
of the flask-shaped distal invagination, and of the depressed conical
nucleolus, which lies between the latter and the concave side of the
nucleus. After this peculiar invagination has persisted for some time in
connection with the enlarged nucleolus, both disappear, and then a
remarkable growth of lobular processes takes place on the concave
proximal side of the hood or kidney-shaped nucleus; from four to eight
knobs of unequal size usually appear, and their thickened wall encloses a
variable number of small of nucleoli; these are at first few but
afterwards more numerous (fig. 7). Subsequently these knobs or lobes
become completely separated by constriction from the original central
mass of the nucleus, and appear as so many separate independent
"sausage-shaped bodies" in the hollow central capsule (fig. 8). Each of
the bodies now appears, and at first on its convex aspect, to form a
large number of small nucleoli, which either separate by constriction
from it or become free by its breaking up and lie in numbers in the
central capsule. Finally the buds or lobes of the nucleus break up
entirely into such nucleoli, which are evenly distributed in the central
capsule, and become the nuclei of the swarm-spores (fig. 11). Compare R.
Hertwig, L. N. 33, Taf. i.-iii. pp. 19-25.

69. _The Nucleus of the Monopylea._--The nucleus of the mature forms of the
NASSELLARIA or MONOPYLEA is generally simple or lobate, homogeneous or
vesicular and _excentric_, and appears only to divide into numerous small
nuclei just before the formation of the spores. Nevertheless I have
sometimes, though not often, seen in representatives of very various
families of the MONOPYLEA, the central capsule filled with many small
spherical homogeneous nuclei (Pl. 53, fig. 19). Hence all the families of
this legion appear to be serotinous, their simple primitive nucleus
persisting for a long period. It {xxxix}is commonly placed excentrically,
and most usually in the apical or aboral portion of the central capsule,
either between its apex and the podoconus, or quite excentrically on the
dorsal aspect. The simple nucleus of the NASSELLARIA usually appears to be
vesicular and to possess a somewhat firm membrane, clear contents, and a
rather large, dark coloured nucleolus. In many NASSELLARIA the nucleus is
spherical or ellipsoidal (Pl. 53, fig. 11); whilst in many #Stephoidea# and
#Spyroidea#, where the central capsule is constricted by the sagittal ring
and divided into two symmetrical lateral lobes, the nucleus partakes of the
same mode of growth and appears in the middle of the capsule as a
transversely placed ellipsoid or even as a short cylinder (Pl. 90, figs. 7,
9). The most remarkable modification in the form of the nucleus is to be
found in the multi-articulate #Cyrtoidea#. Here it is usually enclosed in
the cephalis and is spherical, ellipsoidal or spheroidal, often flattened
almost into a disc. If now the central capsule increase greatly in size and
put forth three or four clavate lobes which hang down through the pores of
the cortinar septum into the thorax (or even into the succeeding joints),
the nucleus usually undergoes similar modification, and three or four
finger-like apophyses are developed from its base, which project into the
corresponding lobes of the central capsule (Pl. 59, figs. 4, 12, 13).

The numerous small, spherical, homogeneous nuclei which are to be found
in the central capsules of those NASSELLARIA, which are ripe and about to
develop spores, were described in 1862 in my Monograph, as "numerous,
small, transparent, spherical cells" in the case of various #Cyrtoidea#
(_Arachnocorys_, _Lithomelissa_, _Eucecryphalus_, _Eucyrtidium_, &c.)
(_loc. cit._, pp. 302, 305, 309, 321, &c.), and I find them of the same
form and dimensions, but deeply stained with carmine in many preparations
in the Challenger collection. R. Hertwig has delineated them very
accurately in the case of _Tridictyopus_ (1879, _loc. cit._, p. 84, Taf.
vii. fig. 3). He was also the first to recognise the uninucleate
condition of the NASSELLARIA, which is much more frequently observed than
the serotinous multinucleate condition, and he described very clearly the
peculiar lobed nuclei which arise in #Cyrtoidea#, owing to the protrusion
of the nucleus through the cortinar septum (_loc. cit._, p. 85, Taf.
viii. figs. 3-8).

70. _The Nucleus of the Cannopylea._--The nucleus presents the same
remarkable structures in all species of the PHÆODARIA or CANNOPYLEA which
have been examined, and closely resembles the germinal vesicle of an
amphibian ovum, being a large spherical or spheroidal vesicle with numerous
nucleoli. Its diameter usually amounts to half or two-thirds, sometimes
even three-quarters, that of the central capsule. The vertical main axis of
the latter is also that of the nucleus, which usually lies somewhat nearer
to the aboral pole. The nucleus is generally rather more strongly
compressed in the direction of the main axis than the capsule itself. The
membrane of the vesicular nucleus is thin, but firm, and encloses a clear
or finely granular mass of nuclein. The number and size of the contained
nucleoli are variable even in one and the same species, and stand in
inverse ratio to each other, an obvious result of the gradual process of
division. Commonly {xl}from twenty to fifty roundish or spherical, strongly
refracting nucleoli, are present; more rarely there are several hundred
very small ones. Sometimes the nucleus is penetrated by fine trabeculæ, in
whose meshes lie the nucleoli (Pl. 101, fig. 2). In certain nuclei, which
contained a few large nucleoli, these were of irregular form, probably the
result of amoeboid movements (Pl. 101, fig. 1). In the formation of spores
in the CANNOPYLEA, the nucleus apparently becomes dissolved, and its
numerous nucleoli develop directly into the nuclei or mother-nuclei, which
produce the nuclei of the flagellate spores. Furthermore, many PHÆODARIA
seem to multiply by simple cell-division, since very commonly (especially
in the #Phæocystina# and #Phæoconchia#) two large nuclei (right and left),
may be met with in one central capsule; sometimes also a single large
nucleus, in which a sagittal constriction marks the commencing division of
the capsule (Pl. 101, figs. 2, 36; Pl. 104, fig. 3; Pl. 124, fig. 6, &c.).

The large nucleus of the PHÆODARIA was first described in my Monograph in
1862, in the case of _Aulacantha_ (p. 263), _Aulosphæra_ (p. 359), and
_Coelodendrum_ (p. 361), as a "large, spherical, thin-walled endocyst,"
from 0.1 to 0.2 mm. in diameter. More detailed descriptions, especially
with respect to the behaviour of the nucleoli were given by R. Hertwig in
1879 (L. N. 33, p. 97).

71. _The Endoplasm or Intracapsular Protoplasm._--In all Radiolaria the
intracapsular protoplasm, which, for the sake of brevity, may be termed
"endoplasm," constitutes originally, and especially in the earliest stages,
the only important content of the central capsule, except the nucleus. In
certain SPUMELLARIA and NASSELLARIA, of simple structure and of small
dimensions, this condition persists for a long period, and the endoplasm
then appears as a homogeneous, colourless, turbid or finely granular,
mucous, semi-solid mass, which cannot be distinguished from the ordinary
undifferentiated protoplasm of young cells; no definite structure, and in
particular, no fibrillar network, can be discovered in it even by the use
of the customary reagents. In the great majority of the Radiolaria,
however, this primitive homogeneous condition of the endoplasm is very
transient, and it soon undergoes definite modifications, becoming
differentiated into separate parts or producing new constituent contents.
Such products of the internal protoplasm are in particular hyaline spheres
(vacuoles and alveoles), oil-globules, pigment-bodies, crystals, &c. The
most important of the differentiations which take place in the endoplasm is
that into an internal, granular, _medullary_ substance and an external,
fibrillar, _cortical_ substance; although the various legions behave
somewhat differently in this respect (§§ 77-80).

72. _Intracapsular Hyaline Spheres._--The central capsule of very many
Radiolaria contains in its endoplasm numerous spherical bodies of varying
size, which consist of watery or albuminous fluid, and have previously been
regarded as nuclei, or described as products of the internal protoplasm,
under various names, such as "spherical transparent {xli}vesicles" (see
note A, below), "albumen spheres" (see B), "gelatinous spheres" (see C),
"alveolar cells" (see D), &c. Some of these spheres are perfectly
transparent, structureless and of varying refractive power, producing the
impression of drops of fluid; others contain various formed constituents,
such as oil-globules, fat-granules, pigment-granules, concretions,
crystals, &c. From a morphological point of view they may all be divided
into two categories, membraneless vacuoles and vesicular alveoles. The
_vacuoles_ are simple spherical drops of fluid or of gelatinous material,
devoid of a special envelope, but immediately surrounded by the endoplasm.
The _alveoles_, on the other hand, are true vesicles with a thin spherical
envelope, enclosing a drop of fluid or jelly. This envelope is commonly
very thin, homogeneous, and often scarcely discernible, so that in practice
a sharp line of demarcation cannot be drawn between alveoles and vacuoles;
the former are usually somewhat larger than the latter. The fact is,
nevertheless, certain that the hyaline spheres, which may be isolated on
rupturing the central capsule of many Radiolaria, in certain cases,
particularly in large species, possess a clear, anatomically demonstrable
membrane, whilst in others no such appearance is presented. It may be
assumed that the vesicular alveoles are developed from the drop-like
vacuoles by increase in size, and by the precipitation of a delicate
envelope from the endoplasm. The character common to all these hyaline
spheres, whether vacuoles or alveoles, is found in their aqueous, not
adipose, constitution, and in their clear transparent appearance, which
allows of no structure (the above-mentioned contained bodies excepted)
being recognised. Their refractive power and consistency vary somewhat, and
probably their chemical constitution still more. Sometimes they are
strongly refractive and shining, and sometimes feebly refractive and pale;
their consistency shows all intermediate stages between a thin fluid, which
readily disappears in water, and a firm, insoluble jelly. As regards their
chemical composition (which is probably very variable), the hyaline spheres
may be best divided into two groups, the organic and inorganic. The
_inorganic hyaline spheres_ are simple drops of saline solution without any
carbonaceous constituent; the _organic_, on the other hand, contain a small
quantity of organic matter dissolved in the watery fluid, and may be either
albuminous or gelatinous spheres. The formed contents which are commonly
present are of very various natures, usually small fat-granules, more
rarely larger fat-granules or pigment-granules, sometimes concretions or
crystals. In many groups, especially among the large PHÆODARIA and
#Collodaria#, the numerous hyaline spheres are remarkable for their equal
size and even distribution throughout the endoplasm (Pl. 1, figs. 1, 4; Pl.
104, fig. 2, &c.). In some genera belonging to the Thalassicollida the
alveoles are of enormous size (Pl. 1, figs. 2, 3); they then become
flattened by mutual pressure into polyhedra and distend the central capsule
to unusual dimensions (in _Physematium_ and _Thalassolampe_ 8 to 12 mm.).

A. The "_spherical hyaline vesicles_," which I described in my Monograph
(1862, p. 71) as among the most important and constant contents of the
central capsule, are partly vacuoles, {xlii}partly homogeneous nuclei.
Most recent investigators, Bütschli in particular (1882, L. N. 41), have
pointed out and rightly criticised this confusion. The criticism might,
however, have been more justly expressed by stating that, in the
preparation of my Monograph (1859-1862), I did not make use of modern
methods of demonstrating the nucleus by staining fluids, which were quite
unknown at the time, and only discovered a decade later. In fact, without
the aid of such reagents, it is quite impossible to distinguish between
the various "spherical transparent vesicles," of which those found in the
central capsule of the PHÆODARIA and many monozootic #Collodaria# are
simple vacuoles lying in the endoplasm, whilst, on the other hand, those
of the Polycyttaria and many other Radiolaria are true homogeneous
nuclei. For not only are the general appearance of the small clear
spheres, their refractive power, and regular distribution in the
endoplasm quite similar, but they are also of much the same size, for the
diameter ranges from 0.005 to 0.015 mm., being generally between 0.008
and 0.012 mm. In addition to this there is generally in each hyaline
sphere a dark brightly shining granule, which, in the case of the
vacuole, is simply a fat-granule, whilst in the case of the nucleus, it
is a true nucleolus. The small hyaline spheres in the young uninucleate
capsules of the Polycyttaria are simple vacuoles (Pl. 3, fig. 12), whilst
in the ripe multinucleate capsules they are true nuclei (Pl. 3, figs. 3,
8, 9), and it is quite impossible to discriminate between these two
conditions without the use of reagents. This has been expressly
recognised by R. Hertwig, who has the merit of having been the first to
clearly distinguish, by the aid of staining fluids, between these two
different constituents (1879, L. N. 33, p. 108).

B. The "_albumen spheres_," which were first observed by A. Schneider in
1858 in the common cosmopolitan _Thalassicolla nucleata_ (L. N. 13, p.
40), and which appear to occur in only a few other Thalassicollida, are
distinguished from the ordinary hyaline spheres of about the same size by
their higher refractive power and by certain albuminoid reactions,
especially the coagulation of a membranous envelope under the influence
of certain reagents (see my Monograph, p. 250, and Hertwig, L. N. 26,
1876, p. 46). They often enclose various formed contents, and require
further investigation.

C. The _gelatinous spheres_ of various sizes, found in the endoplasm of
the Radiolaria, agree in their reactions (especially in staining by
certain reagents) with the common extracapsular jelly of the calymma, and
are hence distinguishable both from the true (coagulable) "albumen
sphere," and from the ordinary watery vacuoles.

D. The _alveoles_, which are only accurately known in the case of certain
large monozootic #Collodaria#, but which also seem to occur in the
central capsule of other remarkably large Radiolaria, were described in
my Monograph in the case of _Thalassolampe margarodes_ and _Physematium
mülleri_, under the name "intracapsular alveolar cells" (1862, pp. 77,
254, 257). They are not, however, true nucleated cells, and the body
described as a nucleus is not such in reality. Nevertheless these large
hyaline spheres do possess a special envelope, as I have recently
convinced myself by the examination of ruptured central capsules of
_Thalassolampe maxima_, _Thalassopila cladococcus_, and _Physematium
atlanticum_ (Pl. 1, figs. 2, 3). The central capsule of these
#Collodaria# becomes distended to most unusual dimensions (2 to 12 mm. in
diameter) by the great development of these large hyaline vesicles, each
of which measure from 0.1 to 0.5 mm. in diameter.

73. _The Intracapsular Fat-Globules._--Fat is present in the central
capsule of all Radiolaria in larger or smaller quantities, and generally
appears in the form of very {xliii}numerous, small, spherical granules,
which are either distributed evenly in the endoplasm (as an emulsion) or
enclosed in the vacuoles; the latter, in particular, is the case in most
PHÆODARIA, perhaps generally. In this group each vacuole contains as a rule
a single dark, shining fat-granule, and sometimes also an irregular bunch
composed of from two to five or more granules. In addition to these small
fat-granules (_granula adiposa_) which are always present, the central
capsule of many Radiolaria contains also larger fat-globules (_globuli
adiposi_). These appear to be generally wanting in the PHÆODARIA, and are
on the whole rare in the ACANTHARIA; whilst, on the contrary, they are very
common in the NASSELLARIA and SPUMELLARIA. The Polycyttaria or social
Radiolaria are as a rule distinguished by the possession of a single large
central oil-globule, which lies in the centre of the central capsule, and
is on an average about one-third of it in diameter (Pl. 3, figs. 4, 5).
This is absent, however, in those young capsules of the Polycyttaria in
which the primary nucleus is centrally situated (Pl. 3, fig. 12). Those
species of Polycyttaria whose central capsule reaches a considerable size,
often enclose numerous oil-globules, and in _Collophidium_ (species of
_Collozoum_ with an elongated cylindrical capsule, Pl. 3, figs. 1, 3) the
axis of each capsule is occupied by a row of numerous oil-globules. In the
monozootic SPUMELLARIA, in which the nucleus is always centrally situated,
the large oil-globules are, of course, excentric, being in apposition to
the inner surface of the capsule-membrane (Pl. 1, fig. 3; Pl. 2, figs. 2,
5). In the #Discoidea# the oil-globules, which are often present in large
numbers, form elegant concentric rings around the central nucleus, and in
those species with segmented arms, there are one or more transverse rows in
each segment (Pl. 43, fig. 15). In the NASSELLARIA the number and
distribution of the oil-globules are dependent upon the form of the central
capsule. When this is simple, without lobes, and ovoid or conical, they
generally lie in its aboral half above the podoconus (Pl. 51, figs. 5, 13;
Pl. 97, fig. 1). When, on the contrary, the basal portion of the capsule
sends out three or four dependent processes (as in the majority of the
#Cyrtoidea#), a large globule may generally be seen in the swollen distal
part of each conical or ovoid lobe (Pl. 53, fig. 19; Pl. 60, figs. 4-7). In
many #Stephoidea# and #Spyroidea#, whose central capsule is separated into
two lateral portions by the constriction corresponding to the sagittal
ring, each of these contains either a single large globule or a group of
small ones (Pl. 90, figs. 7, 10). These oil-globules are usually colourless
and highly refractive; rarely they are yellow or brown, sometimes
rose-coloured, or an intense blood-red (_e.g._, in _Thalassophysa
sanguinolenta_) or even orange (in _Physematium mülleri_). In many
SPUMELLARIA, and particularly in the Polycyttaria, an albuminous substratum
may be recognised in them, which is sometimes disposed in layers, and after
extraction of the fat presents the appearance of a laminated sphere. The
physiological significance of the oil-globules is twofold; in the first
place they tend to diminish the specific gravity of the organism; in the
second they may be utilised as a reserve store {xliv}of nutriment. In the
latter respect they are of special importance in the process of
spore-formation, each flagellate spore usually containing a fat-granule.

74. _The Intracapsular Pigment-Bodies._--In the majority of Radiolaria when
observed alive, the central capsule is coloured, only in the minority is it
colourless. The colour is never diffuse, but always due to the formation
of definite pigment granules or vesicles, which are sometimes distributed
evenly throughout the endoplasm, sometimes aggregated in the central or
peripheral regions. Their form may be either spherical, irregularly
rounded, or polyhedral. They vary much in dimensions, but in most cases
are immeasurably small, and appear under a high magnifying power as fine
dust; occasionally, however, their diameter may amount to from 0.001 to
0.005 or more. The chemical constitution of the intracapsular pigment is
unknown in most Radiolaria, and is probably very various. In many instances
the pigment-granules consist of fat, in others not. The commonest colours
are yellow, red, and brown; violet and blue are rare, and green still
rarer. Sometimes a definite tone of colour prevails throughout a whole
group, and may then be attributed to inheritance, _e.g._, red is found in
most #Sphæroidea#, and blue in the Polycyttaria (see note A). One colour is
almost always constant in the members of the same species. True
pigment-cells, belonging to the Radiolarian organism, do not occur within
the central capsule. The peculiar yellow cells which are found in the
central capsule of many ACANTHARIA are symbiotic xanthellæ (see § 76).

A. The number of Radiolaria whose pigment has been examined in the living
state, is too small to allow of any general conclusions being drawn.
Regarding the different colours known, see my Monograph, L. N. 16, p. 76.

75. _The Intracapsular Crystals._--The crystals found in the central
capsule of many Radiolaria may be divided into two groups, of very
different significance; small crystals, which are very widely distributed,
and large crystals, which occur in only a few genera. The _small crystals_
may also be termed "spore-crystals," since each swarm-spore often contains
such a crystal. They are rod-like or spindle-shaped, and consist of an
organic substance which probably serves as a reserve of nutriment for the
developing spores. Such spore-crystals have been observed in numerous
SPUMELLARIA and ACANTHARIA belonging to various families, and are probably
present throughout the two legions which make up the Porulosa. On the other
hand, they have not been noticed in the Osculosa (NASSELLARIA and
PHÆODARIA), the few swarm-spores belonging to these groups which have been
observed not exhibiting any crystals. The _large crystals_, which occur in
small numbers in the endoplasm, have hitherto only been observed in a few
species of SPUMELLARIA, belonging to the Polycyttaria. They were first
noticed in the common _Collosphæra huxleyi_, and regarded as coelestin.
They are also found in the central capsule of many other Collosphærida,
_e.g._, _Buccinosphæra_ (Pl. 5, figs. 11, 12). Crystal-masses,
crystal-sheaves, or spherical masses of radiating acicular crystals are
enclosed in {xlv}the vacuoles or "albumen globules" of _Thalassicola
nucleata_ and other Thalassicollida, as well as in the central capsule of
_Coelographis_ and some other PHÆODARIA (Pl. 127, figs. 4-7). All these
large crystals are probably to be regarded as excretory products.

75A. _The Intracapsular Concrements._--Concretions, either mineral or
organic, of varying form and constitution, are to be found in the endoplasm
of Radiolaria belonging to very different families. They are most abundant
and multiform in _Thalassicolla nucleata_, being usually circular or
elliptical discs, which are concentrically laminated and highly refractive,
resembling starch-grains. Among them twin forms may frequently be observed,
as though the concrements were in process of division (see note A). Similar
amyloid concretions are to be seen in the central capsule of different
SPUMELLARIA and NASSELLARIA, _e.g._, in _Cephalospyris triangulata_ (Pl.
96, fig. 28). Violin-shaped, highly refractive concrements have been
observed in the central capsule of numerous SPUMELLARIA, NASSELLARIA, and
ACANTHARIA, _e.g._, _Thalassosphæra_, _Spongosphæra_, _Plegmosphæra_,
_Cyrtocalpis_, _Peripyramis_, _Botryocella_, &c. (see note B). The chemical
constitution of these concrements is insufficiently known.

A. The amyloid concretions of _Thalassicolla nucleata_ have been
described in detail in my Monograph (pp. 80, 250, Taf. iii. figs. 2, 3),
and by R. Hertwig in the Histologie der Radiolarien (1876, p. 47, Taf.
iii. figs. 9-13).

B. The violin-shaped concretions of _Thalassosphæra bifurca_ have been
figured in my Monograph (pp. 80, 261, Taf. xii. fig. 1).

76. _The Intracapsular Xanthellæ._--The xanthellæ, zooxanthellæ, or
symbiotic "yellow cells" are found within the central capsule only in the
ACANTHARIA, whilst in other Radiolaria they only occur in the
extracapsulum. They are most frequent in the #Acanthometra#, rarer in the
#Acanthophracta#, but even in the former they are often wanting. Their
number is very variable, but usually small, from ten to thirty in one
capsule. They lie for the most part immediately below the capsule membrane,
in the cortical layer of the endoplasm. The form of the yellow cells is
either spherical or ellipsoidal, often also spheroidal or even lentiform.
The diameter varies from 0.01 to 0.03 mm. They possess a distinct membrane
and an excentric nucleus, and contain numerous yellow pigment-granules in
the endoplasm. This yellow pigment dissolves in mineral acids to form a
green fluid, and in other respects also behaves somewhat differently from
the yellow pigment in the extracapsular yellow cells of the SPUMELLARIA and
NASSELLARIA. In both cases, however, the xanthellæ are not integral
portions of the organism, but unicellular algae, living as parasites or
symbiontes in the body.

A. The yellow cells in the central capsule of the ACANTHARIA were first
observed by Joh. Müller (L. N. 12, pp. 14, 47). In my Monograph I
described them at greater length, and indicated their differences from
the extracapsular yellow cells of other Radiolaria (L. N. 16, pp. 77,
86). Since then, R. Hertwig has demonstrated their cellular nature (L. N.
33, pp. 12, 113), and still more recently {xlvi}Brandt has given further
accurate information regarding their occurrence, constitution, and
physiological significance (L. N. 39, ii. Art., p. 235, figs. 62-73).

77. _The Endoplasm of the Peripylea._--The intracapsular protoplasm of the
SPUMELLARIA or PERIPYLEA is usually distinguished by a more or less
complete radial arrangement, which does not occur in the same form in other
Radiolaria; it may be regarded as characteristic of this legion, for it
probably occurs in all the species at some period of life or other, and
stands in a direct causal relationship with the typical structure of the
capsule-membrane in all the "PERIPYLEA" (see note A). For as this is
commonly perforated by very numerous pores distributed at equal intervals
over the whole surface of the capsule, and since a communication between
the intra- and extracapsular sarcode takes place through these, the radiate
structure of the endoplasm may be readily explained as due to the influence
of radial currents which take place continuously or intermittently in the
endoplasm. This radiate structure is most obvious when the endoplasm
contains no secondary products or only an insignificant amount of these,
and thus appears colourless and almost homogeneous, or only finely
granular. Under these circumstances, an optical section of the central
capsule usually reveals a distinct radial striation; numerous narrow,
straight, dark streaks alternating regularly with still narrower clear
ones; the latter consist of homogeneous, the former of more or less
granular protoplasm (Pl. 20, fig. 1_a_). Often there may be distinguished
in each darker streak a single straight row of strongly refracting (fat?)
granules, sometimes several such rows. Occasionally the whole endoplasm
becomes divided up into a number of large "radial wedges," club-shaped,
conical or pyramidal masses of granular protoplasm, separated by clear
divisions of hyaline plasma (_e.g._, in _Actissa radiata_, p. 14, where in
the optical section of the central capsule, between the membrane and the
nucleus, twenty-five dark radial wedges of equal size were separated by
thick clear partitions of hyaline protoplasm). In the majority of the
SPUMELLARIA this radial striation is partially or entirely concealed by the
formation of pigment or of other products. Very often it is only visible in
the cortical layer, which lies immediately below the capsule-membrane (Pl.
1, figs. 1, 3). The remarkable "centripetal cones" which characterise the
Thalassicollid genus _Physematium_, and were formerly described as
"centripetal cell-groups," are probably a special development of these
cortical radial wedges; they are conical cortical bodies, regularly
distributed on the inner surface of the membrane of the central capsule,
and disposed with the apex turned towards the centre (see note B). More
rarely than in the cortical layer, a similar radial structure is to be
found in the innermost medullary layer immediately surrounding the nucleus.
Here the endoplasm sometimes breaks up into fine radial threads, which are
anatomically separable and hang down from the free nucleus as thin
processes (see note C). In some cases it is also possible to isolate radial
rods from the cortical layer of teased out central capsules.

{xlvii}A. The radial structure of the endoplasm was first described in my
Monograph (1862, p. 74), though R. Hertwig (1879, p. 112) was the first
to indicate its typical significance in the case of the PERIPYLEA, and to
demonstrate its causal relation with the radial currents in the central
capsule of this legion. More recent investigations have led me to the
conviction that this phenomenon is more widespread, and often more
strongly developed, than was formerly imagined, and that it is probably
one of the typical characters of all SPUMELLARIA (at least of the
Monozoa).

B. The centripetal cones of _Physematium_, which have hitherto been known
only in these colossal Thalassosphærida, were fully described in my
Monograph under the name "conical centripetal cell-groups"; by their
first discoverer, A. Schneider (L. N. 13), they were termed "nests," and
compared with the "nests" (central capsules) of the Polycyttaria. In the
_Physematium mülleri_ of the Mediterranean (hitherto only observed by
Schneider and myself at Messina) it appeared as though each centripetal
cone were composed of a group of from three to nine (usually four or
five) slender wedge-shaped cells, whose common centripetal apex was
produced into a radial thread of sarcode (L. N. 16, p. 258, Taf. iii.
fig. 7). Since then (1866) I have observed at Lanzerote, in the Canary
Islands, a nearly related form, which I take to be _Physematium
atlanticum_, Meyen. In this, however, the "centripetal cell-groups" were
wanting, and the whole cortical layer of the endoplasm was cleft into
numerous radial portions, each enclosing a nucleus (probably the
mother-cells of flagellate spores, see p. 35).

C. The radial fibres of the medullary endoplasm which cling to an
extracted nucleus have been observed by Hertwig in certain #Sphæroidea#
(_Diplosphæra_, _Arachnosphæra_) (L. N. 33, p. 40).

78. _The Endoplasm of the Actipylea._--The intracapsular protoplasm of the
ACANTHARIA or ACTIPYLEA is often distinguished by a partial or complete
radial arrangement like that of the PERIPYLEA, but differing in the number,
size, form, and distribution of the radial portions into which the
endoplasm is differentiated. For since the pores of the capsule membrane
are distributed at equal distances all over the surface in the SPUMELLARIA,
whilst in the ACANTHARIA they are arranged in definite groups, and since
the number and arrangement of the pores has a direct influence upon the
internal currents of the endoplasm, it follows that the radial structure in
the latter legion must be very different from that in the former. In
addition to this there must not be forgotten the important influence which
the early centrogenous formation of the skeletal rods exercises upon the
disposition and growth of the intracapsular structures. Hence the endoplasm
of the ACANTHARIA does not separate into innumerable thin, closely packed
radial wedges or cortical radial rods, but into a small number of large
pyramidal portions between which run the radially disposed heterogeneous
portions of the contents of the capsule, viz., the radial bars of acanthin
and the peculiar intracapsular "axial threads." As a direct consequence of
the regular disposition of these heterogeneous radial portions, which is
often characteristic of the various families of the ACANTHARIA, a
corresponding differentiation of the endoplasm is brought about; it divides
into a number of conical or pyramidal portions (radial pyramids), whose
bases rest upon the capsule-membrane and whose apices are directed towards
the centre of {xlviii}the capsule (the central star of the skeleton).
These radial pyramids are, however, but rarely visible, being usually more
or less concealed by a dark pigment.

The differentiations of the endoplasm in the central capsule of the
ACTIPYLEA have been but little investigated, but they appear to vary
somewhat in the different groups of this legion. In all ACANTHARIA in
which the twenty radial bars are regularly arranged according to the
Müllerian law (see p. 717) and in which axial threads constant in number
and disposition run between them from the central star to the
capsule-membrane, it obviously follows that the endoplasm must be divided
into more or less distinct radial pyramids, and this must the case
whether these take the form of continuous tracts or of actually separable
portions. The regular polygonal figures, often seen on the surface of the
central capsule (with special distinctness in _Acanthometron elasticum_
and _Acanthometron pellucidum_) separated by a network of granular
threads, are the bases of such radial pyramids (see Hertwig, L. N. 43, p.
12, Taf. i. figs. 1-7).

79. _The Endoplasm of the Monopylea._--The intracapsular protoplasm of the
NASSELLARIA or MONOPYLEA is distinguished from that of any of the other
three legions by the development of a quite peculiar fibrillar structure,
the axial "pseudopodial cone," which may shortly be termed the "podoconus"
(foot-cone). Since this is in direct correlation with the peculiar
structure of the capsular opening, the large "porochora," which is situated
at the basal pole of the main axis, it is quite as characteristic of the
legion as the latter itself (see note A). The podoconus is primitively a
vertical regular cone whose circular base occupies the horizontal porochora
or "basal porous area" of the central capsule, while its vertical axis
coincides with that of the latter. The apex of the cone, usually somewhat
rounded off, is therefore directed towards the aboral or apical pole of the
central capsule and separated from it by a larger or smaller interval. In
this interval the nucleus originally lies (as in Pl. 51, fig. 13; Pl. 98,
fig. 13); but it is usually displaced subsequently and lies excentrically.
The cone is of very variable height; on an average its vertical height is
about equal to the diameter of its horizontal base; these dimensions are,
however, dependent upon the form of the central capsule; the height being
greater in slender ovoid or conical capsules, and less in depressed
sphæroidal or discoidal ones, than the diameter of the base. The podoconus
consists of differentiated endoplasm, which becomes more deeply stained by
carmine and offers greater resistance to solvents than the surrounding
finely granular protoplasm. The apex, especially, becomes very intensely
stained. It always exhibits a very characteristic fine but distinct
striation, numerous straight radial lines diverging from the apex of the
cone towards the base. The number of these striæ appears to correspond with
that of the vertical rods in the porochora, and each of these latter stands
apparently in direct communication with the basal end of an apical stria (§
59). These threads are probably differentiated constant contractile threads
of endoplasm, or even myophanes, comparable with the contractile cortical
threads of the CANNOPYLEA and the permanent axial threads of the ACTIPYLEA.
The numerous modifications, {xlix}undergone by the form and contents of
the central capsule in the different groups of MONOPYLEA, especially those
due to the formation of the skeleton, are not without influence upon the
podoconus. The most important divergencies from the above described primary
form are the following:--(1) The vertical axial cone becomes oblique, its
axis inclining in the sagittal plane and approaching either the dorsal or
the ventral wall of the capsule; the cause of this appears to be usually
the excentric development of the growing nucleus or the formation of a
large oil-globule. (2) The smooth mantle of the podoconus becomes divided
by three longitudinal furrows into three equal prominent ridges, which
correspond to three circular lobes in the porochora; the cause of this
basal triradial lobular formation lies probably in the triradial
development of the skeleton in many NASSELLARIA or in the cortinar
structure of the collar septum. (3) The simple podoconus splits into three
or four elongated lobes, which eventually become almost completely
separated and correspond to the lobes of the central capsule, in the axial
wall of which they lie as longitudinally striated bands. The behaviour of
these bands justifies the hypothesis that the podoconus is a muscular
differentiated portion of the endoplasm and is composed of myophane
fibrillæ, whose contraction determines the opening of the central capsule.

A. The podoconus of the MONOPYLEA was first described by R. Hertwig in
1879, and recognised as a characteristic component of the central capsule
in the most various groups of this legion (in #Plectoidea#, #Stephoidea#,
#Spyroidea#, and #Cyrtoidea#; see his figures, _loc. cit._, Taf. vii.,
viii., and the description, pp. 71, 73, 83, 106). Hertwig called it the
"pseudopodial cone," and regarded it as a conical process of the
capsule-membrane, which is developed from this latter and projects from
the porous area into the interior of the central capsule; "it is
penetrated by fine canals which arise at the apex of the cone, diverge
towards the base, and terminate there in the rods of the pseudopodial
area. The intracapsular protoplasm penetrates at the apex of the
pseudopodial cone into its fine canals, runs along them and emerges from
the rods of the porous area in the form of slender threads" (_loc. cit._,
p. 19). I cannot agree with this view of Hertwig, although I have been
able to confirm the accuracy of his description by my own observations
upon numerous excellently stained and preserved preparations in the
Challenger collection. As I have proved by numerous teased out
preparations, and as Hertwig himself correctly states, "the cone is more
readily detached from the membrane than from the protoplasm, when the
capsule is teased" (_loc. cit._, p. 73). Hence I regard the podoconus not
as a differentiated portion of the capsule-membrane but as endoplasm, and
believe that it is composed of myophanes or "contractile muscular
fibrils" in the same manner as the cortical layer of the CANNOPYLEA.
Probably the contraction of these fibrils serves to raise the opercular
rods and hence to allow the exit of the endoplasm through the pores which
lie between these opercular rhabdillae (compare § 59).

80. _The Endoplasm of the Cannopylea._--The intracapsular protoplasm of the
PHÆODARIA or CANNOPYLEA is distinguished from that of the other three
legions by several characteristic peculiarities, which are very important,
since they stand in causal relation to the typical structure of the
capsule-membrane and in particular of its {l}remarkable aperture. In the
case of many and perhaps of all PHÆODARIA the endoplasm is differentiated
into a granular medullary and a thin fibrillar cortical layer, the former
of which usually encloses numerous small vacuoles, while the latter
contains muscular fibrillæ. In the voluminous central capsule of large
PHÆODARIA the whole cortical layer of the endoplasm, which lies immediately
below the delicate inner capsule-membrane, sometimes appears delicately and
regularly striated, and most distinctly so under the apertures, towards the
centre of each of which the dark striæ are radially directed (see note A,
below). These striæ are probably contractile muscular fibrillæ; or
"myophanes," by whose contraction the openings are voluntarily widened. In
the Tripylea this fibrillar star is much more strongly developed under the
astropyle (the main opening) than under the parapylæ (or accessory
openings); and probably the peculiar radial structure of the operculum of
the former is due to the stronger development of these radial fibrils
(being their impression). In many PHÆODARIA, indeed, the fine myophane
fibrils are only visible under the apertures, whilst in others they form a
continuous fibrillar cortical layer on the whole inner surface of the inner
capsule-membrane; the fine fibrillæ run meridionally from one pole of the
main axis to the other; perhaps the whole central capsule may change its
form in consequence of their contractions. The medullary portion of the
endoplasm, which lies below this thin cortical layer, is usually finely
granular in the PHÆODARIA, and permeated by numerous spherical vacuoles,
which are noteworthy from their equal size and regular distribution. Each
clear vacuole usually contains a dark shining fat-granule, more rarely a
group of such granules (see note B). Compare § 60, and Pl. 101, figs. 1-3;
Pl. 104, figs. 1, 2; Pl. 111, fig. 2; Pl. 128, fig. 2, &c.

A. The fine fibrillæ in the cortical layer of the endoplasm were first
described by Hertwig in 1879 (L. N. 33, p. 98, Taf x. figs. 6-10). He
found them, however, only below the three openings in the capsule of the
Tripylea, where they form three stellate groups of fibrils. I find them
very clearly shown, and with especial distinctness, under the astropyle
in most PHÆODARIA of which I have had the opportunity of examining
well-stained and preserved central capsules. In many cases, also, the
striation is not confined to the apertures, but spreads over the whole
cortical layer. Perhaps this constitutes in all PHÆODARIA a thin
myophane-sheet, whose contractile fibrils run from one pole of the main
axis to the other and cause by their contraction changes in the form of
the spheroidal central capsule.

B. The granular medullary portion of the endoplasm of the PHÆODARIA, with
its numerous clear spherical vacuoles, was first described in my
Monograph (1862), in the case of _Aulacantha_ (p. 263), _Aulosphæra_ (p.
359), and _Coelodendrum_ (p. 361) as a "finely granular, mucous substance
(intracapsular sarcode), packed more or less closely with clear spherical
vesicles from 0.005 to 0.015 mm. in diameter, each of which contains one
or two, rarely three, dark shining granules." That these clear spheres
are true vacuoles was first clearly proved by Hertwig (L. N. 33, p. 98).
As a rule all the vacuoles of the same central capsule are of equal size
(generally from 0.008 to 0.012 mm. in diameter), and are distributed at
equal intervals throughout the finely granular endoplasm.

{li}CHAPTER III.--THE EXTRACAPSULUM.

(§§ 81-100).

81. _The Components of the Extracapsulum._--The extracapsulum or
extracapsular malacoma, under which name are included all those parts of
the soft body which lie outside the central capsule, consists of the
following constant, and important constituents:--(1) The _calymma_ or
extracapsular jelly-veil; (2) the _sarcomatrix_ or layer of exoplasm
immediately surrounding the membrane of the central capsule; (3) the
_sarcodictyum_ or network of exoplasm, covering the surface of the calymma;
(4) the _pseudopodia_ or radial fibres of exoplasm, which may again be
subdivided into intracalymmar pseudopodia, uniting the sarcomatrix and
sarcodictyum, and extracalymmar pseudopodia, radiating freely into the
water outside the calymma.

82. _The Calymma._--The calymma or extracapsular jelly-veil of the
Radiolaria is always the most voluminous portion of the extracapsulum, and
in spite of its simple structureless constitution is of great morphological
and physiological importance. In all Radiolaria this gelatinous mantle
completely surrounds the central capsule, but is separated from its outer
surface by a continuous, though thin, layer of exoplasm, the sarcomatrix.
The pseudopodia radiating from the latter pierce the calymma, form the
sarcodictyum at its surface, and radiate from its nodal points freely into
the surrounding water. The calymma is rarely visible in living freshly
captured Radiolaria, examined in sea-water, for its gelatinous substance is
perfectly hyaline, colourless and pellucid, and possesses the same
refractive index as sea-water; but when the object is removed from this
fluid and transferred to carmine solution or some other colouring matter,
the extent and figure of the calymma become apparent, for the staining
fluid does not at first penetrate into the gelatinous material. When this
has taken place, however (after a longer or shorter time), and the
gelatinous material has become coloured, its form and size may be observed
by the converse experiment; the object is transferred once more to water
and the outlines of the calymma become as clear as those of the central
capsule. The same is the case with dead specimens in which the sticky
surface of the calymma has become covered with dust.

The jelly-veil of the Radiolaria was recognised even by the earliest
observers of the group, Meyen (1834), and Huxley (1851), and compared
with that of the Palmellaria; the former noticed it in _Physematium_ and
_Sphærozoum_ (L. N. 1, p. 283), and the latter in _Thalassicolla_ and
_Collosphæra_ (L. N. 5, p. 433). In all these SPUMELLARIA, both in the
monozootic _Thalassicolla_ and in the polyzootic _Sphærozoum_ and
_Collosphæra_, the calymma is very voluminous and filled with large
alveoli. Meyen called them "muco-gelatinous masses, in the interior of
which are contained small equal-sized vesicles"; Huxley likewise found
clear vesicles in the jelly and compared them with Dujardin's vacuoles.
Johannes Müller observed the jelly-veil in many different Radiolaria, in
particular in the #Acanthometra#, first discovered by him, but
erroneously believed that it only originated {lii}after death by
liquefaction of the sarcode (L. N. 12, p. 6). This mistake is, however,
easy to understand, since in living Radiolaria the calymma is usually
invisible on account of its perfect transparency, whilst in dead
specimens it is usually quite distinct on account of the dust clinging to
its adhesive surface. I myself believed that the formation of the
voluminous hyaline jelly-veil was only partially due to liquefaction
after death, but that it was to some extent present in the living
organism and that it might vanish and subsequently reappear by means of
imbibition (L. N. 16, pp. 109, 110). R. Hertwig was the first to
demonstrate, in 1879, that the jelly-veil is constantly present in living
Radiolaria, that it forms the basis of the extracapsular malacoma and
surrounds the central capsule as a second protective sheath (L. N. 33, p.
114).

83. _The Structure of the Calymma._--The extracapsular jelly-veil appears
structureless in most Radiolaria, inasmuch as it represents a homogeneous
pellucid excretion of the exoplasm and contains neither fibres nor other
formed structures. In some groups, however, definite structural characters
become secondarily developed. The most common and striking of these is the
formation of alveoles, which takes place in the extracapsulum (see § 86).
In consequence of this the calymma assumes a remarkable frothy consistency
and appears to be composed of large, clear, thin-walled vesicles; this is
especially the case in the #Collodaria# (#Colloidea#, Pls. 1, 3, and
#Beloidea#, Pls. 2, 4), and in many large PHÆODARIA, especially among the
#Phæocystina# (Phæodinida and Cannorrhaphida, Pl. 101, and Aulacanthida,
Pls. 102-104). More rarely the calymma is not permeated by vacuoles, but
there appear in it fine striæ parallel to the surface as though it were
composed of thin concentric laminæ like an onion; perhaps these are the
expressions of a different quantity of water in the various layers. In the
calymma of many Radiolaria thin, straight, radial lines are to be seen,
which are probably pseudopodia, and not to be attributed to any structural
modification, or they may be slender canals which serve for the exit of the
pseudopodia. On the outer surface of the calymma of different Radiolaria,
and especially in the ACANTHARIA, a peculiar network of fibres is to be
found, composed of polygonal meshes, like elastic fibres, probably due to a
local thickening of the jelly. These polygonal meshes are often very
regularly distributed between the radial spines of the #Acanthometra#, and
stand in a definite relation to them. The fibres which form the meshes are
often rather strong, resembling elastic fibres, as above-mentioned, and
either simple or composed of bundles of very fine fibrillæ (L. N. 33, p.
15, Taf. i. fig. 1, Taf. ii. fig. 4).

84. _The Consistency of the Calymma._--The gelatinous material of which the
calymma of the Radiolaria consists is a pellucid mass, rich in water and
usually quite hyaline and structureless; its consistency is very variable.
In the majority of the Radiolaria it may perhaps be about equal to that of
the jelly which composes the umbrella of most Medusæ; but as in these
latter it may vary between very wide extremes, constituting on the one hand
a very soft jelly-mantle, offering but little {liii}resistance to
mechanical influences and almost disintegrating under the eyes of the
observer, and on the other hand forming a firm gelatinous shell, comparable
to cartilage in hardness, elasticity, and power of mechanical resistance.
In many Radiolaria of large dimensions with an alveolar calymma (especially
in numerous #Collodaria# and PHÆODARIA) this may be split by means of
dissecting needles and the central capsule extracted like the stone from a
cherry, and then it is easy to ascertain that the firmness and elasticity
of this jelly-veil are not less than those of a cherry. The different
degrees of consistency in the various Radiolaria may be dependent either
upon the relative amount of water which they contain, or upon qualitative
or quantitative variations in the organic substance of which the jelly
consists. Great importance is to be attached to the considerable
consistency of the calymma, because it furnishes the indispensable
groundwork for the deposition of many parts of the skeleton and
particularly of the lattice-shells.

85. _The Primary and Secondary Calymma._--In most Radiolaria the external
form and volume of the calymma are different at different stages of growth,
and this difference is mainly dependent upon the development of the
skeleton. Hence it is advisable to distinguish in general the primary from
the secondary calymma. The _primary calymma_ is in the great majority of
Radiolaria a perfect sphere, in the middle of which lies the concentric
central capsule; on the surface of this gelatinous plate the primary
spherical lattice-shell is secreted in most SPUMELLARIA and
#Acanthophracta#, as well as in those PHÆODARIA which possess a spherical
shell; in the remaining PHÆODARIA also and in the NASSELLARIA, where the
lattice-shell is not spherical but monaxon, it is secreted on the surface
of the primary calymma. This takes place at a definite time, very important
in the development of the Radiolarian, which for the sake of brevity we
shall term the "_lorication-period_." Since the firm surface of the primary
calymma furnishes the necessary foundation for the deposition of the
primary lattice-shell, it is of the greatest mechanical significance in all
shell-bearing Radiolaria. The _secondary calymma_ arises only after the
lorication-period by further growth of the primitive jelly-mantle and in
the fully developed Radiolarian usually encloses wholly or partially the
external parts of the skeleton, in consequence of which it assumes the most
various forms. Very often the secondary calymma is polyhedral, being
stretched between the radial spines of the skeleton, the distal ends of the
latter then forming the fixed points of the gelatinous polyhedron.

86. _The Extracapsular Vacuoles and Alveoles._--The calymma of the
Radiolaria usually appears completely homogeneous and hyaline without any
structure; sometimes it encloses numerous clear vesicles, vacuoles or
alveoles, and then assumes a frothy appearance, the expression of a more or
less distinct alveolar structure. {liv}The clear vesicles to which this is
due are either spherical, or polyhedral from mutual pressure, and like the
similar ones in the central capsule may be divided into membraneless
vacuoles and vesicular alveoles. The _vacuoles_ are simple drops of fluid,
without a special envelope, and immediately surrounded by the gelatinous
substance of the calymma, in which they appear as simple cavities. The
_alveoles_ on the contrary are true vesicles, with a thin envelope, which
encloses a drop of fluid or a globule of jelly; in the latter case its
contents are different in refracting power and amount of contained water
from the substance of the surrounding calymma. A sharp boundary between the
membraneless vacuoles and the vesicular alveoles cannot be drawn in the
case of the extracapsular hyaline spheres any more than in the
intracapsular; the envelope of the alveoles is sometimes very distinct and
even anatomically separable, whilst at other times it is very thin and
scarcely recognisable; it may occasionally arise and disappear within a
very short time (see note A). There is no doubt that in the calymma as in
the central capsule the vesicular alveoles are secondary products, which
have arisen from the vacuoles by the secretion of an enveloping membrane.
This membrane is either a delicate sheath of exoplasm, or a firmer and more
resistant skin, distinct from the exoplasm, and probably an excretion from
it (_e.g._, Pl. 4, figs. 2, 3). In many cases the outer surface even of the
vacuoles is covered by a network of pseudopodia, which form a sarcoplegma
similar to a fenestrated alveolar membrane. The colourless pellucid fluid
in the vacuoles and alveoles is usually simple sea-water, more rarely it
contains a small quantity of albumen ("albumen-spheres") or jelly
("gelatinous spheres"). The size of these spheres is very variable. Quite
small vacuoles may be found in the calymma of many Radiolaria. Large
vacuoles, on the other hand, producing the appearance of an alveolar
structure, are confined to but few groups, to a part of the SPUMELLARIA
(#Colloidea#, #Beloidea#, and a few #Sphæroidea#), and to the #Phæocystina#
(PHÆODARIA with incomplete skeleton); besides they occur only rarely in
individual genera, _e.g._, _Nassella_ among the skeletonless NASSELLARIA.
Since the volume of the calymma is much increased by the development of
vacuoles, and the power of mechanical resistance is at the same time much
increased, the fact is explained that the vacuoles occur mainly in
Radiolaria which have no skeleton or only an incomplete one (see note B).
Among the monozootic #Collodaria# the alveolar structure is especially well
developed in the following genera; _Thalassicolla_ (Pl. 1, figs. 4, 5),
_Thalassophysa_, _Thalassoplancta_, _Lampoxanthium_ (Pl. 2, figs. 1, 2);
among the PHÆODARIA in most genera of the Phæodinida, Cannorrhaphida and
Aulacanthida (Pls. 101-104), and probably also in other voluminous
PHÆODARIA (_e.g._, #Phæosphæria#). The alveoles or vacuoles in the calymma
of these large Radiolaria lie usually in several layers, one above another,
and increase in size from within outwards. The Polycyttaria or social
Radiolaria (the three families Collozoida, Sphærozoida and Collosphærida)
without exception have an alveolar structure, and the special form of
{lv}their colonies or coenobia is to a great extent determined by the
development, number, size and arrangement of the alveoles in their calymma
(compare Pls. 3-8). In these cases there is not unfrequently developed a
large central alveole (see note C) whose thickened wall encloses a globe of
jelly and serves as the central support of the whole colony (Pl. 5, fig.
1). Still more striking, however, is the arrangement of certain
Polycyttaria, where each individual of the colony (or each central capsule
with its calymma) is enclosed in a large alveole, whose firm wall often
attains considerable thickness (Pl. 4, figs. 2, 3). The whole colony then
appears as an aggregate of numerous cells, each of which possesses two
envelopes, the inner central capsule and the outer alveolar membrane;
between these lies in the Collosphærida the siliceous lattice-shell (Pl. 6,
fig. 2). These pericapsular alveoles may be regarded as

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