One of these Coccidia lives in the intestinal cells of a small centipede, _Lithobius_; in Fig. 121 the parasite is shown as a so-called 'Sporozoite,' that is, as a minute sickle-shaped cell, which at first moves freely about the intestine of the host (1), but then soon penetrates into an epithelial cell (2). There it grows to a spherical shape (3), and then, after having devoured the cell, it gives rise, by a peculiar process of division (Schizogony), to a number of very minute nucleated pieces, again sickle-shaped, the Schizonts, each of which bores its way into an epithelial cell as in 2, and follows the same path of development, so that a large number of cells in the intestine of the same host are attacked in this manner. But there is still another mode of reproduction, with which amphimixis is associated, which leads directly to the formation of 'lasting' germs which are enclosed in a capsule or cyst, reach the exterior with the excrement of the host, and thus spread the infection to other centipedes. The Schizonts which take this course develop into so-called macro-gametes and microgametes, the former being the female, the latter the male germ-cells. Then follows the penetration of a male gamete, actively motile because of its two flagella, into the female gamete (8). Amphimixis is accomplished, and the product of the fusion of the two sex-cells (9) surrounds itself with a thinner cyst, within which it multiplies by twofold division into four cells (10). These are the 'lasting' spores, which may dry up within the voided excrement of the centipede (11), and if they be eaten by another animal of the species, they infect it, for the sporozoites which have been formed by the previous divisions creep out, and in form 1 begin the life-history anew. We have thus an alternation of four generations which are all unicellular, and of which one series (1-5) shows multiplication by fission, while the other (6-11) includes, besides multiplication by fission and as a condition of this, the process of amphimixis. Amphimixis _must_ occur in order that the formation of 'lasting' spores and new sporozoites may result. We have thus a regular alternation of 'asexual' and 'sexual' reproduction, and the latter shows great resemblance to that of multicellular organisms. The macrogamete corresponds to the ovum, the microgametes to the spermatozoa, and they resemble these also in their greater numbers and in their structure.