Order ii.— Eimeriinea
This order also consists of intracellular parasites. A number of spores are produced within a cyst, itself the development of a single zygote. There is an alteration of generations, non-sexual schizogony alternating with a sexual cycle or propagative schizogony.
The Eimeriinea are divided into a number of suborders, amongst which are the Eimeriidea (or true Coccidia), the Hæmogregarinidæ, and the Adeleidæ.
Suborder Eimeriidea.— The typical life-cycle of a coccidium is that of C. schubergi in the centipede Lithobius forficatus. The young parasites or sporozoites are liberated from a cyst in the intestinal tract and penetrate epithelial cells, where they grow into large schizonts, characterized by a large vesicular nucleus and a karyosome (nucleolus) . When full grown the nucleus divides by repeated fission till a variable number, generally about thirty, are produced. The schizont now divides into as many merozoites as there are nuclei, and is then full grown. The cells burst, the merozoites are set free, and, entering other cells, may develop in two ways, either into schizonts again or into sporonts. The sexes of the sporonts can be distinguished; in the male the protoplasm is clear, but in the female it is crowded with reserve food material.
The male sporont develops further; the nucleus gives off a number of chromidial bodies which become aggregated together to form secondary nuclei; the nuclei develop into microgametes— small, slender bodies provided with two flagella. The host cell then bursts, liberating the microgametes, which endeavour to enter the female cell or macrogamete. When one such has effected its entrance the macrogamete secretes a tough membrane and becomes an oöcyst, thus effectually preventing the entrance of any other microgametes. The penetrating microgamete fuses with the female nucleus, forming a synkaryon; fertilization is then complete, and in this condition the oöcyst is passed from the original host and falls to the ground, where further development takes place. The zygote breaks up into a number of sporoblasts; each sporoblast secretes a tough envelope, the sporocyst. Therefore, when sporogony is complete, the original oöcyst contains four sporocysts, each containing two sporozoites. In order to develop further the oöcyst must be swallowed by a new host, whereupon the tough membranes dissolve, liberating the sporozoites.
Coccidia are common parasites of vertebrates (rabbits and fowls), but are also found in invertebrates.
Suborder Hæmogregarinidæ.— Although, strictly speaking, parasites of the blood of vertebrates, these parasites have been relegated, apparently on adequate grounds, to this suborder. Their life cycle in many particulars resembles that of the Adeleidæ in that the male and female gametocytes are associated, but differs in that the sexual cycle is passed in an invertebrate host instead of in an oöcyst on the ground. The
