_Occurrence._--The Coccidiidea in their mature condition usually live within the epithelial cells of various organs, and by choice inhabit those of the intestine and of its associated organs. They also occur frequently in the excretory organs of mammals, birds, amphibia, molluscs, arthropods, and may also be found in the testes and vas deferens, but the statement that they live in hen’s eggs, as well as in the oviducts of fowls, has not been confirmed.[171] Some species inhabit the nuclei of cells, others live in the connective tissue, but their presence in the latter situation is probably only secondary, that is, they have only reached it from the epithelium of the affected organs. [171] Notwithstanding the progress made during the last decades, the ova of helminthes and more particularly of trematodes, have been mistaken for Coccidia. Thus Poschinger (_Zool. Anz._, 1819, ix, p. 471) and Gebhard (_Virchow’s Arch._, 1897, No. 147, p. 536) mistook the ova of _Distoma turgidum_, Brds., for Coccidia. Podwyssotzki (_Centralbl. f. allg. Path._, 1890, i, p. 135) made a similar error with the ova (and vitelline sacs) of a species of _Prosthogonimus_ (_Distoma ovatum_ of the authors); von Willach (_Arch. f. wiss. u. prakt. Thierheilk._, 1892, xviii, p. 242) mistook the ova of a nematode for Coccidia. The size of the Coccidiidea, corresponding as a rule to the capacity of their habitat, is usually small, but there are said to be species that attain a diameter of 1 mm. Their form[172] is globular, oval, or elliptical. External appendages are lacking, at least during the trophic or vegetative period of their life, which is spent in epithelial cells, within which they increase in size. Frequently one only is present in each cell, but more can occur. The body substance is composed of a more or less finely granular or distinctly alveolar protoplasm which exhibits no differentiation into ecto- and endoplasm. All species possess a nucleus that enlarges with their growth; sometimes it only shows through the cytoplasm as a lighter spot, or may even be quite concealed. It is vesicular, and besides containing very delicate threads of chromatin in the clear nucleoplasm, it contains generally only one large karyosome. [172] The life-cycle given here is based on that of _Eimeria_ (_Coccidium_) _schubergi_, after Schaudinn (1900). See “Untersuchungen über den Generationswechsel bei Coccidien,” _Zool. Jahrb., Abt. f. Anat._, xiii, pp. 197–292, 4 plates.