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 nepbridia in each segment, but they are often reduced in 
number. They may open into the ccelome 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, 
Fic. 121.—Free-living Polycheete (Mereds cultrifera). 
Note, as compared with Avenicola, the absence of gills, and the 
well-developed parapodia, which are absent from the peris- 
tomium (e.), or first true segment. The prostomium bears 
eyes (¢.), the small tentacles (¢.), and the large palps (/.); 
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 /Vere7s, 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 
‘* Fleteronereis,” 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- 
