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.
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.)
These larvae are at first very different from the adult animal, and the necessary changes to be passed through are more or less great according to the species. It is not our intention to describe these changes in detail.[[329]] The larva increases in size, the permanent chaetae make their appearance in regular order, and the body exhibits segmentation, the new segments always appearing just in front of the anal segment. The internal organs gradually develop, and the prostomial and parapodial appendages grow out in their turn. In the Sabelliformia the multifilamentous "gills" arise by the continued branching of an at first simple process (the palp) arising from the latero-ventral surface of each side of the preoral lobe.[[330]] These gradually encroach dorsally and ventrally till the prostomium is more or less encircled; meanwhile the peristomium grows forwards so as to conceal the prostomium, which no longer increases at the same rate as does the rest of the body.
Although most worms appear to discharge their ova directly into the sea and take no further care of them, some make provision for their offspring either by laying the eggs in a jelly, which will serve as food for the young larvae—Aricia, Ophelia, Protula, Phyllodoce—or by attaching them to their body. In certain Polynoids the eggs are attached by means of a secretion to the back, under the elytra, where they undergo development up to a certain stage. In Exogone and some other Syllids they are attached to the ventral cirri, or in Grubea limbata, all over the back. In the female Autolytus (Sacconereis) a ventrally-placed brood sac is formed by the hardening of a secretion; the eggs develop into embryos inside the brood sac, and then become free, with head appendages and three pairs of parapodia. Enormous numbers of such embryos may occur; for instance, some 300 were counted in a brood sac of Autolytus ebiensis. In the case of tubicolous worms, the eggs are frequently attached to the tube, either inside or outside. In Spirorbis and Salmacina the operculum serves as a brood pouch.
Only a very few species are known to be viviparous, viz. Syllis vivipara Kr., Cirratulus chrysoderma Clap., Marphysa sanguinea Mont., and Nereis diversicolor Müll.
In most genera there is no external difference between a mature worm filled with generative products and an immature one, except, it may be, in the colour; for the yolk of the eggs is frequently tinted yellow, or pink, or bluish, while the spermatozoa in mass are white; so that the normal colouring of the worm may be modified when filled with these elements. But in a few instances striking anatomical peculiarities are exhibited by the mature worm.[[331]] In many species of Nereis, for instance, those segments containing the generative products undergo more or less extensive changes, while the anterior ones remain unaltered. The body of the ripe Nereis is then distinguishable into an anterior non-sexual region and a posterior sexual region; and so great are these changes in certain species that the mature worms were for a long time believed to belong to a different genus, and received the name Heteronereis. But we now know their true relations, thanks to the work of Claparède and others. The males in the Heteronereid phase have fewer unaltered anterior segments than the females, so that there is a sexual dimorphism.
Fig. 146.—Male "Heteronereis" of N. pelagica L. × 1. A, Non-sexual region; B, sexual, modified region. (From Ehlers.)
The changes which Nereis undergoes in its transformation affect chiefly (a) the shape of the parapodia, and (b) the form of the chaetae of these parapodia. Other organs may also be affected, though less noticeably; thus the eyes become enlarged, the intestine may become so compressed by the generative products as to be functionless, and the tail develops special sensory papillae.[[332]]