POLYCHAETA. 



467 



body-cavity, where they ma- 

 ture ; they eventually escape 

 either by rupture of the body- 

 wall, or through the nephridia. 

 There are no special generative 

 ducts. Only a few Polychaets 

 are viviparous (e.g., Syllis 

 vivipara Kr., Marjrfiysa san- 

 guinea Mont., etc.) ; all the 

 rest are oviparous ; many lay 

 their eggs in a jelly (Aricia, 



FIG. 373. Different forms of Nereis 

 dumerilii (after Claparede, from Per- 

 rier). 1, young form ; 2, heteronereid 

 female ; 3 heteronereid male. 



FIG. 372. A parapodium of Tomopteris with a 

 mass of ova Ov, and one free ovum (after 

 Gegenbaur). 



Ophelia, Pliyllodoce), others attach 

 them to their own body, e.g., to the 

 back beneath the elytra (Polynoe 

 cirrata), to the dorsal or ventral 

 cirri (Exogone), in a ventral brood- 

 sac (Autolytus), in the operculum 

 (SpirorMs), or in various tubicolous 

 forms to the tube. 



Polymorphism. The forms for- 

 merly placed in the genus Hetero- 

 nereis have been shown by Malmgren 

 to be merely the sexually mature in- 

 dividuals of certain species of Nereis. 

 The genus has consequently been 

 given up, but the name has been 

 retained to denote the sexual phase 

 in the life-history of these Nereids. 



The changes which the worms 

 undergo in passing from the imma- 

 ture condition to the mature Hetero- 

 nereid condition chiefly affect the 

 posterior part of the body in which 

 the generative organs are contained; 

 in this part (Fig. 373) the parapodia 

 become larger and acquire flattened 

 foliaceous outgrowths, while the setae 

 are thrown off and replaced by new 

 setae of a flattened form and a fan- 

 like arrangement. Moreover, the eyes 

 become enlarged, the dorsal cirri 



