In the Coccidia, which are extremely simple unicellular organisms, equipped, however, with a nucleus, the adaptations relating to amphimixis are more extensive and more complex than in the Rhizopods. For while in the latter the two conjugating cells are absolutely alike in external appearance, in the former the male cell is distinct from the female, and indeed the differences are as marked as those that usually occur in multicellular animals. [Illustration: FIG. 121. Life-cycle of _Coccidium lithobii_, a cell-parasite of the centipede _Lithobius_; after Schaudinn. 1, a 'sporozoite'; 2, the same penetrating into an intestinal epithelial cell; 3, the same growing into a 'schizont' capable of division; 4, the same dividing, and 5, breaking up into numerous pieces which separate from the 'residual body' in the centre, and either, as in 1, migrate into epithelial cells and repeat the history, or pass on to the phase of sexual reproduction. In the latter case, after eliminating a portion of the nucleus (reduction) in 6 and 6 _a_, they form the 'macrogamete' (the ovum); or within the mother-cell they produce microgametes (or sperm-cells), 7 and 7 _a_. The penetration of a sperm-cell into an egg-cell (amphimixis) is shown in 8, the fertilized egg-cell (9) becomes the so-called oocyst or permanent spore, from which by repeated division (10 and 11), new sporozoites, as in 1, arise, and begin the cycle afresh.] We owe our present knowledge of these processes especially to Schuberg, Schaudinn, and Siedlecki, and, because of their theoretical importance, I should like to summarize the essential points.