" WORM-FEET " OR PARAPODIA 



267 



pair of nephridia in each segment, but they are often reduced in 

 number. They may open into the coelom by a ciliated funnel or 

 nephrostomy or end in a group of solenocytes, which are comparable 

 to the flame-cells of Flatworms (see Fig. 114, A). With the nephridia 

 there are often associated ciliated " coelomoducts," which typically 

 open to the exterior and into the coelom. They often combine with 

 the nephridia. Though the sexes are usually separate, there are a few 

 hermaphrodite forms. There is a metamorphosis in development, 

 and some interesting peculiarities occur in regard to reproduction. 

 Thus several species of the common genus Nereis, when sexually 

 mature, have the body divided into two regions — a posterior region 



Fig. 144. — Parapodium of " Heteronereis 

 Nereis pelagica. — After Ehlers. 



of 



I, 2, 3, 4, The leaf-like outgrowths ; ci., notopodial cirrus ; c~., 

 neuropodial cirrus ; fli., a'-., acicula or supporting bristles 

 of notopodium and neuropodium ; s., setce. 



containing the ova or sperms, and an anterior unmodified asexual 

 region. The posterior region is distinguished by the structure of its 

 parapodia, which become converted into broad, flattened swimming 

 organs, and there is sexual dimorphism. Worms of this peculiar 

 type were long described as a distinct genus under the name of 

 " Heteronereis," and even yet the subject is imperfectly understood, 

 for there is from unknown causes much variation as regards the 

 extent of the modification. A complete change of habit at the spawn- 

 ing season is probably common here as elsewhere in marine Inverte- 

 brates. In the Syllidce a phenomenon occurs similar to the formation of 

 a " Heteronereis," but a process of fission may result in the division of 

 the modified form into an anterior asexual zooid and a posterior sexual 



