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 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 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 cœlestin. 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 the vacuoles or "albumen globules" of Thalassicola nucleata and other Thalassicollida, as well as in the central capsule of Cœlographis 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 Brandt has given further accurate information regarding their occurrence, constitution, and physiological significance (L. N. [39], ii. Art., p. 235, figs. 62-73).