The fresh-water sponge (_Spongilla_) exhibits an analogous rhythm. The ova are fertilized by sperms from a different short-lived individual. They develop into sponges which have no power of producing ova or sperms. But on the approach of winter in Europe, and of the dry season in India, a number of cells collect and group themselves into a so-called gemmule. Round this is formed a sort of crust beset with spicules, which, in some cases, have the form of two toothed discs united by an axial shaft. When these gemmules have thus been formed, the sponge dies; but the gemmules live on in a quiescent state during the winter or the dry season, and with the advent of spring develop into sponges, male or female. These have the power of producing sperms or ova, but no power of producing gemmules. The power of producing ova, and that of producing gemmules, thus alternates in rhythmic fashion. [Illustration: Fig. 10.--Liver-fluke: Embryonic stages. (After A. P. Thomas.) A. ovum: em., embryo; op., operculum. B. _Limnæus truncatulus_ (natural size). C. Free embryo: e.s., eye-spot; ex., excretory vessel; g.c., germinal cells; h.p., head-papilla. D. Embryo preparing to become a sporocyst: g.c., germinal cells. E. Sporocyst: g., gastrula; m., morula; re., redia. F. Redia: b.o., birth-opening; ce., cercaria; col., collar; di., digestive sac; ph., pharynx; p.pr., posterior processes; re., daughter redia. G. Cercaria: cys., cystogenous organ; di., digestive sac; o.s., oral sucker; p.s., posterior sucker; ph., pharynx.]