232 



PHYLUM ANNELIDA. 



communicating tube (Nebendarm), which is of interest, because it has 

 been compared to the notochord of Vertebrates. There is typically a 

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

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

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

 to the flame-cells of Flatworms (see Fig. 254A). With the nephridia 

 there are often associated ciliated " ccelomoducts," which typically 

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

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

 hermaphrodite forms. There is a metamorphosis in development, 



FIG. 121. Free-living Polychaete (Nereis cultrifera). 



Note, as compared with Arenicola, the absence of gills, and the 

 well developed parapodia, which are absent from the peris- 

 tomium (pe.\ or first true segment. The prostomium bears 

 eyes (e.\ the small tentacles (/.), and ^ the large palps (/.)J 

 c., the four paired cirri, borne by the peristomium ; a., the anus, 

 with two long cirri. 



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 

 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 unknpwn causes much variation as regards the 

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



