These structures vary in length and number; they are radiant, but may anastomose, especially in Nassellaria and Phaeodaria, two groups in which the sarcoplegma and pseudopodia are chiefly derived from the protoplasm issuing from the single aperture of the one or principal aperture of the other. In Acantharia the pseudopodia are disposed in an orderly way, and certain of them, the axopodia, either placed one between each pair of spines, or surrounding in a circle the bases of the spines, are supported by stiff axial filaments penetrating the central capsule. Some Discoidea possess also a single thick striated sarcode-flagellum, of eminently contractile nature, but otherwise motionless, formed by the fusion of pseudopodia and traceable as far as the nucleus. Peculiar protoplasmic processes or myophrics, 5-30 or more, extend from the edges of the calymnal sheaths to the apices of the spines in Acantharia. They are contractile and expand the calymna. At death they are set free from the spines and remain projecting from the sheaths as the so-called 'ciliary coronas' or 'gelatinous cilia' The protoplasm of the sarcomatrix, etc, is more or less granular; some of the granules consist of fat; oil-globules occur in the large Collodaria, albumen globules in Thalassolampe primordialis and Collo-zoum. Vacuoles formed in the course of the sarcoplegmal threads and of small size are found in most Radiolaria. Large vacuoles are confined to the Collodaria and a few Sphaeroidea among Spumellaria, to Nassella, and some Plectoidea among Nassellaria, and to the Phaeocystina. They are frequently so numerous as to give the calymna a frothy appearance.
When the animal is irritated they disappear progressively from the periphery towards the centre, but are formed anew when it is left at rest. In some colonial forms, e. g. Collosphaera, the central part of the colony is occupied by a soft sphere of jelly2. Black or blue pigment occurs in some large Collodaria, brown in many Spumellarian Sphaeroidea and Discoidea, red in some Acantharia. The Phaeodaria are characterised by a dark mass, the phaeodium, of a greenish-brown or black hue surrounding the astropyle. It consists of rounded bodies, the phaeodellae, sometimes nucleated, ranging from .001 to .05 mm. in size, mixed with minute black particles. It is not certain whether or no they belong to the organism or are symbiotic algae.
1The sarcomatrix is said by Butschli (Protozoa, p. 431) to be wanting at the apical or aboral pole of the Nassellarian Cystidium and Plagiacantha. Haeckel states (op. cit. supra, p. lxvii.) that it is often so thin at this spot in Nassellaria that it can be demonstrated only by reagents. Brandt points out that the sarcomatrix, as well as spindle-shaped masses in the sarcoplegma of Collozoum inerme, C.fulvum, Sphaerozoum neapolitanum, Sph. acuferum stains strongly with osmic acid, and violet with iodine, while the rest of the protoplasm of the body does not do so. To the portions which stain he gives the name of 'Assimilation-plasma.' In Siphonosphaera tenera it is aggregated in masses, each of which has 2-5 central capsules round it. See his monograph cited, pp. 14-15,18, 92~4- The same authority describes (p. 21) globular or plate-like masses of protoplasm in the sarcoplegma of young Collozoa, especially C. pelagicum, and in Acrosphaera spinosa, which contain brownish granules, and are perhaps connected with vacuole-formation.
3 The distinction between true vacuoles and alveoles, mentioned note 2, p. 876, is applied by
The colonial Radiolarians, i. e. some Spumellarian Collodaria and Sphaeroidea, differ from the non-colonial in possessing numbers of central capsules, each with its own sarcomatrix, but with a common calymna and sarcoplegma; The individuals in these colonies, when adult are usually placed peripherally, whilst one or more vacuoles occupy the centre; they retreat on irritation from the surface. The colony may be spherical, elongated, rarely ring-shaped. Colonies of the same species, even if in different stages of growth, fuse when brought into contact, either naturally or artificially; and colonies of different species may adhere without fusion, especially if the water is somewhat foul.
Reproduction takes place by fission or spore-formation, rarely by gemmation. Binary fission of the young central capsule appears to occur in the colonial genera and in some tripylean Phaeodaria, i. e. those with three apertures to the central capsule. It is possible that it may be general. Young colonies of some species of Collozoum and Sphaerozoum contain in the sarcoplegma structures known as 'extra-capsular bodies,' produced by gemmation from the central capsules. They are strongly refractile, non-granular, provided with differentiated nuclei and groups of oil-drops. They give origin to anisospores in some instances, in others probably to central capsules. In the colonial Collosphaera and its allies the stage of growth in which a few individuals furnished with shells, and many without them, make up the colony, is perhaps comparable (Brandt). The colonies themselves are said also to multiply either by breaking at their nodes, e. g. the elongated beaded adult colony of Collozoum, or by simple constriction whilst young, e. g. spherical colony of Collosphaera. It may be noted that a young colony, at least in some instances, contains many more individuals than an adult.
Spores have been observed in the four main sub-divisions of the class, but the mode of their formation is most accurately known in the colonial Collodaria. It takes place in the central capsule, the extracapsular region having no share in it, unless a small portion of the sarcomatrix is retracted. There are two kinds of spores, iso- and aniso-spores, which differ from one another (1) in the mode of their formation, (2) in the character of their nuclei, and (3) in Collozoum and Sphaerozoum by their shape, a difference which probably does not obtain in Collosphaera and its allies. In the formation of isospores the nuclei, which are doubly refractile, and become more and more so, are scattered through the capsular protoplasm; they multiply by binary fission. Small crystalloids make their appearance, one for each spore, even before the required number of nuclei is attained. The oil of the oil-globule is dispersed as minute granules, some of which are destined for each spore. The protoplasm is resolved into as many portions as there are crystals. The ripe isospore is pointed anteriorly, rounded posteriorly. Two flagella spring from the point. The nucleus is large and anterior; the crystalloid with oil-granules posterior.