It is amazing how greatly the size of the sperms varies among the Daphnids, and how it stands in inverse proportion to their number, and how both are obviously regulated in relation to the difficulties which stand in the way of each sperm-cell before it can reach the ovum. In some species the sperm-cells are very large, in others extremely small. In the genera _Daphnia_, _Lynceus_, and others, copulation occurs as shown in Fig. 65; the sperm-cells (_sp_) are liberated by the male into the capacious brood-cavity of the female, which at the moment is closed to some extent by the abdomen of the male, in reality closed only partially at the posterior end and at the sides. It seems inevitable that a large proportion of the male elements should stream out again and be lost because of the violent movements of both animals. Accordingly, we find that the sperm-cells are only about the hundredth part of a millimetre in length and of round or rod-like form, and are voided in multitudes into the brood-cavity. Fig. 66, _f_, _g_, and _h_, show such cells in different species, as they occur in the testes to the number of many thousands. But in all the species in which the brood-cavity is _closed_, and in which therefore there is not such a serious loss of sperm-cells, the elements are much larger, and at the same time less numerous. They are largest and least numerous in species of genera like _Daphnella_, _Polyphemus_, and _Bythotrephes_, in which the males have a copulatory organ, so that the possibility of loss of the male cells is excluded. Thus the round, delicate, and viscid sperm-cells of _Bythotrephes_ (Fig. 66, _b_) are more than a tenth of a millimetre in length, but they are developed in proportionately smaller numbers, so that more than twenty are never found in the testis, and often only six or eight, while in copulation only from three to five are ejected. But as there are only two eggs to be fertilized at a time, and as the male cells are expressed into the brood-cavity directly upon the eggs, so that they immediately adhere to them, this small number is amply sufficient. [Illustration: FIG. 66. Spermatozoa of various Daphnids. _a, Sida._ _b, Bythotrephes._ _c, Daphnella._ _d, Moina paradoxa._ _e, Moina rectirostris._ _f, Eurycercus lamellatus._ _g, Alonella pygmæa._ _h, Peracantha truncata._ All magnified 300 times.]