2 7 6 



POLYCHAETA 



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 Spirorlis 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 

 sanyuinea Mont., and Nereis diversicolor Mull. 



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 sperma- 

 tozoa 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 peculiari- 

 ties are exhibited by the mature worm. 1 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 pos- 

 terior 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 

 Jfeteronereis. But we now know their true 

 relations, thanks to the work of Claparede 

 xi. A, NOII- and others. The males in the Heteronereid 

 P h ase have fewer unaltered anterior segments 

 (From Ehiers.) than the females, so that there is a sexual 



dimorphism. 



The changes which Nereis undergoes in its transformation 

 affect chiefly (a) the shape of the parapodia, and (&) 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 pro- 



1 Many of the Polynoids are sexually dimorphic. 



