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 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:—
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, Amœboid or Irregular Central Capsule.
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:—
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. 1a; 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 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. 5b) 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 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.
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 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, 2a). 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 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 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 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; 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. Amœboid 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).