Schaudinn admits 24 genera (and 7 doubtful) and 41 species (and 18 doubtful). None are known fossil. Their geographical distribution is cosmopolitan, as is the case with most of the minute fresh-water Protista; 8 genera are exclusively marine, and _Orbulinella_ has only been found in a salt-pond; _Actinophrys sol_ is both fresh-water and marine, and _Actinolophus_ has 1 species fresh-water, the other marine. One of the 14 species of _Acanthocystis_ is marine; the remaining genera and species are all inhabitants of fresh water.[85] 4. RADIOLARIA _Sarcodina with the protoplasm divided by a perforated chitinous central capsule into a central mass surrounding the nucleus, and an outer layer; the pseudopodia radiate, never anastomosing enough to form a marked network; skeleton either siliceous, of spicules, or perforated; or of definitely arranged spicules of proteid matter (acanthin), sometimes also coalescing into a latticed shell; reproduction by fission and by zoospores formed in the central capsule. Habitat marine, suspended at the surface (plankton), at varying depths (zonarial), or near the bottom (abyssal)._ {76}[Illustration: FIG. 22.—_Collozoum inerme._ A, B, C, three forms of colony; D, small colony with central capsules (_c.caps_), containing nuclei, and alveoli (_vac_) in ectoplasm; E, isospores, with crystals (_c_); F, anisospores; _nu_, nucleus. (From Parker and Haswell.)] The following is Haeckel's classification of the Radiolaria:— I. PORULOSA (HOLOTRYPASTA).—Homaxonic, or nearly so. Central capsule spherical in the first instance; pores numerous, minute, scattered; mostly pelagic. A. SPUMELLARIA (PERIPYLAEA).—Pores evenly scattered; skeleton of solid siliceous spicules, or continuous, and reticulate or latticed, rarely absent; nucleus dividing late, as an antecedent to reproduction. B. ACANTHARIA (ACTIPYLAEA).—Pores aggregated into distinct areas; skeleton of usually 20 centrogenous, regularly radiating spines of acanthin, whose branches may coalesce into a latticed shell; nucleus dividing early. II. OSCULOSA (MONOTRYPASTA).—Monaxonic; pores of central capsule limited to the basal area (osculum), sometimes accompanied by two (or more) smaller oscula at apical pole, mostly zonarial or abyssal. C. NASSELLARIA (MONOPYLAEA).—Central capsule ovoid, of a single layer; pores numerous on the operculum or basal field; skeleton siliceous, usually with a principal tripod or calthrop-shaped spicule passing, by branching, into a complex ring or a latticed bell-shaped shell; nucleus eccentric, near apical pole.