The COCCIDIACEAE never attain so high a degree of cellular differentiation as the Gregarines, which may be due to their habitat; for in the growing state they are intracellular parasites. Their life-history shows a double cycle, which has been most thoroughly worked out in COCCIDIIDAE by Schaudinn and Siedlecki in parasites of our common Centipedes. We take that of _Coccidium schubergi_ (in _Lithobius forficatus_[107]), beginning with the sporozoite, which is liberated from the spores taken in with the food, in the gut of the Centipede. This active sickle-shaped cell (Fig. 34, _l_) enters an epithelial cell of the mid-gut, and grows therein till it attains its full size (_a_), when it is termed a "schizont"; for it segments (Gk. σχίζω, "I split") superficially into a large number of sickle-shaped zoospores, the "merozoites" (_c_), resembling the sporozoites. The segmentation is superficial, so that there may remain a large mass of residual epiplasm. The merozoites are set free by the destruction of the epithelium-cell in which they were formed, and which becomes disorganised, like the residual epiplasm. Each merozoite may repeat the {100}behaviour of the sporozoite, so that the disease spreads freely, and becomes acute after several reinfections. After a time the adult parasites, instead of becoming schizonts and simply forming merozoites by division, differentiate into cells that undergo a binary sexual differentiation. Some cells, the "oocytes" (_d_, _e_), escape into the gut, and the nucleus undergoes changes by which some of its substance (or an abortive daughter-nucleus) is expelled to the exterior (_f_), such a cell is now an "oogamete" or oosphere. Others, again, are spermatogones (_h_): each when full grown on escaping into the gut commences a division (_i_, _j_), like that of the schizonts. The products of this division or segment-cells are the flagellate sperms (_s_): they are more numerous and more minute than the merozoites produced by the schizonts, and are attracted to the oosphere by chemiotaxy (p. 23), and one enters it and fuses with it (_g_). The oosperm, zygote or fertilised egg, thus formed invests itself with a dense cyst-wall, as a "oospore" (_k_), its contents form one or more (2, 4, 8, etc.) spores; and each spore forms again one, two, or four sickle-shaped zoospores ("sporozoites"), destined to be liberated for a fresh cycle of parasitic life when the spores are swallowed by another host. [Illustration: FIG. 33.—Bisexual pairing of _Stylorhynchus_. _a_, Spermatozoon; _b-e_, fusion of cytoplasm of spermatozoon and oosphere; _f_, _g_, fusion of nuclei; _h-j_, development of wall to zygote; _k_, _l_, formation of four sporoblasts; _l_, side view of spore; _m_, mature sporozoites in spore. (After Léger.)]