Chapter II: The Central Capsule (1)

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.).