The spermatozoa of the great majority of animals have two characteristic features. Firstly, they are extraordinarily small, being usually the smallest cells in the body; and, secondly, they have, as a rule, a peculiarly lively motion, which is known as spermatozoic motion. The shape of the cell has a good deal to do with this motion. In most of the animals, and also in many of the lower plants (but not the higher) each of these spermatozoa has a very small, naked cell-body, enclosing an elongated nucleus, and a long thread hanging from it (Fig. 20). It was long before we could recognise that these structures are simple cells. They were formerly held to be special organisms, and were called “seed animals” (spermato-zoa, or spermato-zoidia); they are now scientifically known as _spermia_ or _spermidia,_ or as _spermatosomata_ (seed-bodies) or _spermatofila_ (seed threads). It took a good deal of comparative research to convince us that each of these spermatozoa is really a simple cell. They have the same shape as in many other vertebrates and most of the invertebrates. However, in many of the lower animals they have quite a different shape. Thus, for instance, in the craw fish they are large round cells, without any movement, equipped with stiff outgrowths like bristles (Fig. 21 _ f_ ). They have also a peculiar form in some of the worms, such as the thread-worms (_filaria_); in this case they are sometimes amœboid and like very small ova (Fig. 21 _ c_ to _e_). But in most of the lower animals (such as the sponges and polyps) they have the same pine-cone shape as in man and the other animals (Fig. 21 _a, h_). Fig.21 Spermatozoa or spermidia of various animals. Fig. 21—Spermatozoa or spermidia of various animals. (From _Lang_). _a_ of a fish, _b_ of a turbellaria worm (with two side-lashes), _c_ to _e_ of a nematode worm (amœboid spermatozoa), _f_ from a craw fish (star-shaped), _g_ from the salamander (with undulating membrane), _h_ of an annelid (_a_ and _h_ are the usual shape). When the Dutch naturalist Leeuwenhoek discovered these thread-like lively particles in 1677 in the male sperm, it was generally believed that they were special, independent, tiny animalcules, like the infusoria, and that the whole mature organism existed already, with all its parts, but very small and packed together, in each spermatozoon (see p.12). We now know that the mobile spermatozoa are nothing but simple and real cells, of the kind that we call “ciliated” (equipped with lashes, or _cilia_). In the previous illustrations we have distinguished in the spermatozoon a head, trunk, and tail. The “head” (Fig. 20 _k_) is merely the oval nucleus of the cell; the body or middle-part (_m_) is an accumulation of cell-matter; and the tail (_s_) is a thread-like prolongation of the same.