Certain of the more important of the Coccidiidea may now be considered. Genus. *Eimeria*, Aimé Schneider, 1875. Syn.: _Psorospermium_, Rivolta, 1878; _Cytospermium_, Rivolta, 1878; _Coccidium_, R. Leuckart, 1879; _Pfeifferia_, Labbé, 1894; _Pfeifferella_, Labbé, 1899. The Eimeria belong to Léger’s old family, the Tetrasporocystidæ, which comprises forms producing oöcysts with four sporocysts, each containing two sporozoites. The cysts are spherical or oval, as are also usually the schizonts. The members of the genus are confined chiefly to vertebrate hosts, the more important economically occurring in mammals and birds. From the mammalian hosts very rarely the parasites may reach man. _Eimeria_ (_Coccidium_) _avium_ of wild birds and poultry, and _Eimeria stiedæ_ parasitic in rabbits, may be considered. There is a general similarity in their life-cycles and each is of great practical importance. *Eimeria avium*, Silvestrini and Rivolta. _Eimeria avium_ is responsible for fatal epizoötics among game birds such as grouse, pheasants and partridges, and domestic poultry such as fowls, ducks, pigeons and turkeys, and can pass from any one of these hosts to any of the others with the same effect. The organism is parasitic in the alimentary tract of the host, affecting more especially the small intestine (duodenum) and the cæca, but in some cases penetrating to the liver and multiplying there (as in turkeys), producing necrotic cheesy patches, that ultimately become full of oöcysts. The gut is rendered very frail by the action of the parasites, its mucous membrane is greatly injured, and is often reduced to an almost structureless pulp, riddled with parasites (fig. 68). Infection is conveyed from host to host by the ingestion of food or drink contaminated with the oöcysts voided in the fæces of infected birds. Oval oöcysts from 24 µ to 35 µ long and from 14 µ to 20 µ broad are the means of infection. The oöcysts develop internally four sporocysts or spores, from each of which two sporozoites are produced. The life-history[174] presents two phases: (1) The asexual multiplicative phase, schizogony, for the increase in numbers of the parasites within the same host; (2) the reproductive phase, following the formation of gametes (gametogony), leading to the production of resistant oöcysts, destined for the transference of the parasite to new hosts (sporogony). [174] Fantham, H. B. (1910), “The Morphology and Life History of _Eimeria_ (_Coccidium_) _avium_, a Sporozoön causing a fatal disease among young Grouse,” _Proc. Zool. Soc. Lond._, 1910, pp. 672–691, 4 plates. Also Fantham, H. B. (1911), “Coccidiosis in British Game Birds and Poultry,” _Journ. Econ. Biol._, vi, pp. 75–96.