I Family Acanthometrina. The Acanthometrae (fig, 18, a) are all minute, and are found floating near the surface in the open ocean, sometimes in great numbers. They consist of sarcode-bodies, which are supported by a framework of radiating siliceous, or horny spines, the extremities of which usually project considerably beyond the body. The substance of the body admits of division into an outer membranous layer, or "ectosarc," and an internal granular layer, or "endosarc." The siliceous spines are hollow, being grooved at the base by a gutter, which is continued further up the spine by a canal terminating at the apex of the spine by a distinct aperture. The spines, in consequence of this structure, are able to serve for the transmission of the pseudopodia, which gain the exterior by running through the canals and escaping at their apices. Many of the pseudopodia, however, do not occupy the canals of the spines.
II. Fam. Polycystina. - The members of this family are closely related to the Foraminifera, differing from them chiefly in the fact that their shells are composed of flint instead of carbonate of lime, as in most of the latter. They possess a body of sarcode, which is enclosed in a foraminated siliceous shell, which is often furnished with spine-like processes, and is usually of great beauty (figs. 17 and 18, b). The sarcodic substance of the body is olive-brown in colour, with yellow globules, and often does not entirely fill the shell. The pseudopodia are emitted through the foramina in the test, and are long, ray-like filaments, which display a slow movement of granules along their borders.
The Polycystina are all microscopic, and are all inhabitants of the sea, having a very wide distribution. They likewise extend to great depths; and one of the numerous facts of interest brought to light by the researches of the Challenger Expedition, under Sir Wyville Thomson, has been that large areas of the sea-bottom, up to the enormous depth of 4500 fathoms, are formed by an "ooze" composed of the siliceous cases of Polycystina and other Radiolarians. Similar deposits of Tertiary age are known as occurring in the crust of the earth in various regions. One of the best known of these is the "Barbadoes Earth" (fig. 19), which is almost wholly composed of the delicate flinty shells of the Polycystina. The remains of Polycystina have also now been detected in rocks as old as the Jurassic formation.
III. Fam. Collozoa. - In this family the organism is usually compound, though occasionally simple. A skeleton may be wholly wanting (as in the composite Collozoum), or may exist in the form of spicules or of a foraminated shell. The simple types always possess a mere spicular skeleton, and the same is true in such forms as Sphaerozoum (fig. 20, b). On the other hand, in such forms as Collosphaera (fig. 20, a) there is a spheroidal fenestrated test, the skeleton thus approximating in character to that of the Polycystina. The members of this family sometimes attain a considerable size, and are found floating near the surface in most seas.
Fig. 19. - Shells of Polycystina from the "Barbadoes Earth ; " greatly magnified. (Original.)
IV. Fam. Thalassicollida. - This family, as now restricted, comprises floating marine organisms, which are in many respects closely allied to the preceding, but in which the intracapsular sarcode contains a complex nucleus. The skeleton may be wanting (as in Thalassicolla and Thalassolampe), or it may be present in the form of spicules or spines developed in the extra-capsular sarcode.