The recognition of this unsuspected complicated process was bound to effect reforms in the classification of the coccidia; and all the forms that had been regarded as developmental stages (Eimeria, etc.) had to be reconsidered.
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.
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.
The infected epithelial cells degenerate sooner or later as the parasite feeds on them (fig. [67], II-IV). After their form has been changed by the action of the growing parasite, they finally perish. The cell membrane then alone surrounds the coccidia, which, at least in the species sufficiently known, begin to propagate by an asexual process (schizogony), the parasites themselves becoming schizonts, as the initial stage is usually called. They differ from later stages (sporonts or gametocytes), which resemble them in form, by the absence of granulations in the cytoplasm, as well as by the vesicular nucleus. The form is not always the same, for in some cases, at least, many schizonts assume a globular form.
Schizogony (fig. 67, V-VII) commences with a division of the nucleus, which takes place in different ways in the various species. This finally leads to the formation of numerous daughter nuclei which become smaller and smaller, and which collect beneath the surface of the schizonts. In some species the daughter nuclei collect only in one half of the schizont. A part of the protoplasm of the periphery now divides around each daughter nucleus, the remaining part (residual body) being left in the centre or on one side. The segments of the divided cytoplasm, each containing a nucleus, assume a fusiform shape and become merozoites, which very soon become free (fig. 67, VIII) and leave the residual body. They are distinguishable from the very similar sporozoites (fig. 67, I), as the merozoites possess a karyosome.
Fig. 67.—Life-cycle of Eimeria (Coccidium) schubergi, Schaud., from the intestine of Lithobius. (After Schaudinn.) The infection is caused by a cyst (XX), containing spores, which reaches the intestine of a Lithobius, where it discharges the sporozoites (I). II, A sporozoite invading an intestinal epithelial cell; III, intestinal epithelial cell with young trophozoite; IV, intestinal epithelial cell with a globular schizont; V, nuclear segmentation within the schizont; VI, the daughter nuclei arranging themselves superficially; VII, formation of the merozoites; VIII, merozoites that have become free, and which, penetrating into other epithelial cells of the same intestine, repeat the schizogony (II-VIII); IX and X, merozoites which, likewise invading the epithelial cells of the same intestine, become sexually differentiated; xia, young macrogametocyte; XIb, older macrogametocyte; XIc, mature macrogametocyte (discharging particles of chromatin); XIIa, young microgametocyte; XIIb, older microgametocyte; XIIc, increase of nuclei in the microgametocyte; XIId, the globular residual body around which numerous microgametes have formed; XIIe, an isolated microgamete; XIII, the mature macrogamete surrounded by numerous microgametes and forming a cone of reception or fertilization prominence; XIV, shows the nucleus of a microgamete that has penetrated and fused with the nucleus of the macrogamete (fertilization)—the latter forms a membrane and becomes an oöcyst; XV, XVI, XVII, nuclear segmentation in the oöcyst; XVIII, oöcyst with four sporoblasts; XIX, the sporoblasts transformed into spores, each containing two sporozoites; XX, the cyst introduced into the intestine and liberating the sporozoites by bursting.