In the majority of forms the tail of the living spermatozoon exhibits sinuous cilia-like movements. In two groups the movements are however of an amoeboid character. These groups are the Nematoda and the Crustacea; and the spermatozoa in both of them frequently present very abnormal forms. In Nematoda they are pear-shaped, cylindrical, spine-shaped, etc., and are mainly formed of protoplasm with a highly refracting nucleus. In the Crustacea the variations of form are still greater. In the Malacostraca they are sometimes simply spherical (Squilla), while in Astacus and a large number of Decapoda they are composed of a nucleated body with stellate rays. In Paludina amongst the Mollusca there are two _forms_ of completely developed spermatozoa existing side by side in the same individual. The spermatozoa are formed by the breaking up of the male germinal cells, or of cells secondarily derived from them by division. The cells which directly give rise by division to the spermatozoa may be called spermospores and are equivalent to the ova or oospores. Amongst the Sponges (Halisarca, Schultze, No. 141) a germinal cell, similar to that which in the female becomes an ovum, repeatedly divides and eventually gives rise to a ball of cells (a spermosphere or sperm-morula), each constituent cell of which becomes converted into a spermatozoon, and may be designated by the special term 'spermoblast.' In most Hydrozoa the subepithelial epiblastic cells become converted into germinal cells (spermospores), and then break up to form spermoblasts, each of which becomes a spermatozoon. In most higher Metazoa the spermospores usually form the epithelium of an ampulla or tube, though more rarely (many Chætopoda, Gephyrea, etc.) they may be derived from cells lining the body cavity, as in the case of ova. The spermatozoa are formed either by the direct division of the spermospores into a number of cells, spermoblasts, each of which grows into a spermatozoon; or by the nucleus of the spermospore becoming subdivided within the cell body, the latter differentiating itself into the tails of the spermatozoa while the segments of the nucleus give rise to the main part of the heads. In many instances interstitial cells which do not give rise to spermatozoa, are intermingled with the spermospores.