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., Marphysa san- 

 guinea Mont., etc.) ; all the 

 rest are oviparous; many lay 

 their eggs in a jelly (Aricia, 



FIG. 372. A parapodium of Tomopteris with a 

 mass of ova Ov, and one free ovum (after 

 Gegenbaur). 



FIG. 373. Different forms of Nereis 

 dumerilii (after Claparede, from Per- 

 rier). 1, young form ; 2, heteronereid 

 female ; 8 heteronereid male. 



Ophelia, Phyllodoce), 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 

 (Spirorbis), 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 



