The eggs and spermatozoa in the Polychaeta are discharged into the sea either by rupture of the body-wall or through the nephridia; the male and female elements unite, and the resulting fertilised eggs undergo development, either floating separately in the water, or embedded in jelly, or attached to the body or to the tube of the worm. The result of the segmentation of the egg is a free-swimming larva known as a "Trochosphere," similar to that of _Polygordius_. The larvae of different species present various more or less marked departures from this type, for instead of the two girdles of cilia there may be only the anterior girdle, or there may be several complete or incomplete girdles between the two typical ones, or there may be (Chaetopterids) only a single girdle of cilia about the middle of the body, the two typical girdles being absent.[328] The postoral region, after elongation, generally becomes marked out into three segments, and these segments develop chaetae, which are usually temporary and specially long. The little animal is thus equipped for an independent life: the provisional chaetae help in keeping it balanced; and in some cases (Spionidae) serve to protect the little soft creature, for when it is touched it curls up, and its chaetae stick out at the sides, so that it looks like a hairy caterpillar. But the larva is quite at the mercy of the sea, for it is carried hither and thither by currents, and in this way the species is disseminated. The larvae of the Polychaetes, like those of other animals, occur at certain periods of the year in large quantities at the surface of the sea, and serve as food for various larger animals. {275}[Illustration: FIG. 145.—A, Trochosphere of _Nephthys_. × 65. _a_, Anus; _b_, apical plate (brain); _c_, apical tuft of cilia; _c'_, girdle of cilia; _i_, intestine; _m_, mouth; _st_, stomach. B, Larva of _Spio_, with three segments, eight days old. × 100. _c_, Preoral girdle of cilia; _c'_, preanal girdle; _ch_, long provisional chaetae; _pr_, prostomium with eyes. (From Claparède and Metschnikoff.)]