It is remarkable how different the sperm-cells sometimes are in quite nearly related species of Daphnids, as a glance at Fig. 66 will show; and, on the other hand, how similar they may be in two species which belong to different families, like _Bythotrephes longimanus_ (_b_), and _Daphnella hyalina_ (_c_). The last fact may be explained as an adaptation to similar conditions of fertilization. Both species have effective copulatory organs, and their large delicate sperm-cells must immediately adhere when they come into contact with the shell-less ovum, and penetrate into it by means of amœboid processes. Conversely, the difference between sperm-cells of allied species like _Sida crystallina_ (_a_), _Moina rectirostris_ (_e_) and _M. paradoxa_ (_d_) is related to different adaptations to nearly the same conditions of fertilization. In _Sida_ (Fig. 66 _a_) the large flat sperm-cells, with their fringed ends and their large soft surface, adhere easily to the ova, and the same end is attained in _Moina rectirostris_ by means of stiff radiating processes, while in the nearly related species, _Moina paradoxa_, the male cell (_d_) resembles an Australian boomerang and presses in like a wedge between the ova and the wall of the brood-sac. [Illustration: FIG. 67. Spermatozoa of various animals, after Ballowitz, Kölliker, and vom Rath. 1, man. 2, bat (_Vesperugo_). 3, pig. 4, rat. 5, bullfinch. 6, newt. 7, skate (_Raja_). 8, beetle. 9, mole-cricket (_Gryllotalpa_). 10, freshwater snail (_Paludina_). 11, sea-urchin. Much magnified.] In Fig. 67 a small selection of animal male cells is figured, all of which take the form of sperm-threads or spermatozoa, and yet they differ very much from one another in detail. It would undoubtedly be of great interest to follow out these minute adaptations of the sperm-cells to the conditions of fertilization, and to demonstrate that their size, and especially their form, in the different species of animals are adjusted to the special constitution of the ovum, its envelope, and its micropyles, and to the ease or difficulty with which it can be reached; but much information must be forthcoming before we can even suggest, for instance, why the sperm-cell of the salamander is so enormously long, large, and pointed at the head, while that of Man (Fig. 67, 1) is comparatively short, with broad, flat head and a recently discovered minute apex; or why those of Man and many fishes (such as _Cobitis_) should be so much alike, and so on. From many sides, however, we are led to conclude that even down to the minutest details nothing is in vain, and that everything depends on adaptation.