of the projecting spines is often differentiated into a bundle of fibres converging on to the spines some way up (distally); these, comparable to the myonemes of Infusoria (q.v.), &c., and termed “myophrisks”, possibly serve to drag outwards the surface and so extend it, with concurrent dilatation of the alveoli, and lower the specific gravity of the animal. In this group also a thick temporary flagellum “sarcoflagellum” may be formed, apparently by the coalescence of a number of pseudopodia. The pigmented mass or “phaeodium” in the ectoplasm of Phaeodaria appears to be an excretory product, formed within the central capsule and passing immediately outwards; a similar uniform deposit of pigmented granules occurs in the Colloid species, Thalassicolla nucleata. The wall of the central capsule is simple in the Spumellaria, but formed of two layers in the Nassellaria and Phaeodaria. In the Nassellaria the oscule is simply a perforated area, and a cone of differentiated fibres in the intracapsular cytoplasm has its base on it: it is termed the “porocone,” and the fibres may possibly be muscular (myonemes). In Phaeodaria, the inner membrane at each oscule is prolonged through the outer into a tube (“proboscis”): the outer membrane of the principal oscule forms a large radially striated circular plate, the “astropyle,” or “operculum.” The innermost shell of some with concentric shells may lie within the central capsule, or even within the nucleus; this is due to the growth of these organs after the initial shell is formed, so that they pass out by lobes through the latticed openings of the embryonic shell, which lobes ultimately coalesce outside the embryonic chamber, and so come finally to invest it (fig. III. 17). In some, a symbiosis occurs with Zooxanthella, Brandt, a Flagellate of the group Chrysomadineae, which in the resting state inhabits the extracapsular cytoplasm growing and dividing freely therein, and only (under study) becoming free and flagellate on the death of the host (fig. III. 4, 6-13). The Silicoflagellata or Dictyochidae, also possessing a vegetable-colouring matter, but with a skeleton of impure silica (like that of Phaeodaria), may pass some of their lives in symbiosis with Radiolaria.
Fig. I.—Thalassicolla pelagica, Haeckel; ×25. CK, central capsule; EP, extracapsular protoplasm; al, alveoli, liquid-holding vacuoles in the protoplasm similar to those of Heliozoa, Hastigerina, &c.; ps, pseudopodia. The minute unlettered dots are the “yellow cells.”
Living Radiolaria were first observed and partially described by W. I. Tilesius in 1803-6 and 1814, by W. Baird in 1830, and by C. G. Ehrenberg in 1831, as luminous organisms in the sea; F. J. F. Meyen in 1834 recognized their animal character and the siliceous nature of their spicules. Ehrenberg a little later described a large number of Nassellarian skeletons under the name of Polycystina (1838), but without more than a very slight knowledge of a few living forms. T. H. Huxley in 1851 made the first adequate study of the living animal, and was followed by Joh. Müller in the same decade. E. Haeckel began his publications in 1862, and in two enormous, abundantly illustrated, systematic works, besides minor publications, has dealt exhaustively with the cytology, classification, and distribution of the class. Next in value come the contributions of Richard Hertwig (largely developmental), besides those of L. Cienkowsky, Karl Brandt and A. Borgert, while to F. Dreyer and V. Häcker we owe valuable studies on the physical relations of the skeleton.
Fig. II.—Eucyrtidium cranioides, Haeck.; ×150; one of the Nassellaria. Entire animal as seen in the living condition. The central capsule is hidden by the beehive-shaped siliceous shell within which it is lodged.
Our classification is taken from Haeckel.
A. Spumellaria, Haeck. (Peripylaea, Hertwig). Central capsule perforated with numerous evenly distributed pores. Skeleton siliceous, latticed or of detached spicules, or absent. Form homaxonic or with at least three planes of symmetry intersecting at right angles, rarely irregular or spiral, sometimes forming colonies, i.e. with several central capsules in a common external cytoplasm.
I. Skeleton of detached spicules, or absent.
Fam. 1. Colloidea. Skeleton absent. Thalassicolla, Huxl. (figs. I. and III. 1); Thalassophysa, Haeck.; Collozoum, Haeck. (fig. III. 2-5, 15, 16); Actissa, Haeck.
Fam. 2. Beloidea. Skeleton spicular. Sphaerozoum, Haeck.; Raphidozoum, Haeck.
II. Skeleton latticed or spongy-reticulated.
Fam. 3. Sphaeroidea. Skeleton homaxial, sometimes colonial. Collosphaera, Mull.; Haliomma, Ehrb.; Actinomma, Haeck. (fig. III. 17), showing concentric latticed shells, the smallest intranuclear, all connected by radial spines; Spongosphaera, Haeck. (fig. IV. 8); Heliosphaera, Haeck. (fig. III. 14).
Fam. 4. Prunoidea. Skeleton a prolate spheroid or cylinder of circular section, sometimes constricted like a dice-box.
Fam. 5. Discoidea. Shell flattened, of circular plan, rarely becoming spiral.
Fam. 6. Larcoidea. Shell with three unequal axes, elliptical in the plane of any two, more rarely becoming irregular or spiral.
B. Acantharia, Haeck. (Actipylaea, Hertw.). Skeleton of spicules of acanthin radiating from a centre, and usually twenty