In general, even the peculiarities of form already indicate this; thus the spirally coiled structure of the head, which is especially well developed in the spermatozoa of birds (Fig. 67, 5), in those of the skate (7), and of the freshwater snail (_Paludina_) (10), works like a corkscrew, and makes it possible for the spermatozoon to pierce through the resistant envelope of the ovum. Similarly, the sharply pointed head of the insect spermatozoon (Fig. 67, 8 & 9) seems adapted for slipping through the minute pre-formed micropyle in the hard egg-shell. Of the detailed and complicated structure of spermatozoa we have only recently been made aware through the increasing perfection of the microscope and of technical methods of investigation. Fig. 68 shows one after a diagrammatic figure by Wilson. We see the apical point (_sp_) for boring into the ovum, the nucleus (_n_) surrounded by a thin layer of protoplasm, which together form the head, then the middle portion (_m_) which contains the 'centrosome' (_c_), and the 'tail' or 'flagellum' which effects the movement of the whole and which itself possesses a complex structure with an 'axial filament' (_ax_) and an enveloping layer, the latter often drawn out into a spirally twisted, undulating membrane of the most extreme delicacy, as is most clearly seen in the newt (Fig. 67, 6). [Illustration: FIG. 68. Diagram of a spermatozoon, after Wilson. _sp_, apical point. _n_, nucleus. _c_, centrospere. _m_, middle piece. _ax_, axial filament. _e_, terminal filament.] Not in the Daphnids alone, but in other groups of Crustaceans as well, sperm-cells of quite peculiar form occur, as, for instance, in the crayfish and its marine relatives, the crabs and the long-tailed Decapods. In these cases the spermatozoa bear long and stiff thorn-like processes, which, as in the sperm-cells of _Moina_, make them adhesive, and, according to Brandes, render it possible for them to cling among the bristles on the abdomen of the female until one of the many eggs leaving the oviduct comes within reach. For among these Crustacea there is no true copulation, but the masses of sperm-cells are packed together into sperm-packets or 'spermatophores,' and are affixed by the male near the opening of the oviduct, where they burst and pour forth their contents between the appendages of the female.