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. [Illustration: 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 {277}products as to be functionless, and the tail develops special sensory papillae.[332] In the parapodia an increase in size and a sharper delineation of the various parts take place; then flattened foliaceous outgrowths (Fig. 147, _x_, _y_) arise from certain lobes of the feet, in which, too, the blood supply becomes greatly increased. The old chaetae are pushed out by the development of new ones of quite a different shape; these are jointed like the old ones, but the appendix is, in many species at least, flattened and oar-shaped (Fig. 123, C, p. 246); and the chaetae are arranged in a fan-like manner. Both these modifications are in evident relation to the free-swimming habit which the Heteronereid now adopts. The new foot serves as a swimming organ, the old one was a walking appendage. [Illustration: FIG. 147.—Parapodium of male "Heteronereis" of _N. pelagica_ L. × 10. (From Ehlers.) _a_, Notopodial cirrus; _b_, notopodium; _c_, neuropodium with new chaetae; _c'_, foliaceous outgrowth; _d_, neuropodial cirrus; _x_, _y_, foliaceous outgrowths.]