ANNELIDA : THE EARTHWORM, NEREIS 231 



earthworm the whole body consists of similar divisions 

 arranged one after the other in a line or series. These 

 divisions are the segments. Each contains a ring of the 

 body-wall, with setae and openings, a separate portion of 

 the ccelom, a section of the gut, a ganglion, nephridia, 

 and blood vessels. A body so constructed is said to be 

 metamerically segmented. Most of the segments resemble 

 one another closely, but in the forepart of the body they 

 show considerable differences in the reproductive, alimentary, 



Fig. 147. — Nereis cultrifer. — From Thomson. 



, Anus ; a.c, anal cirri ; c, tentacular cirri ; e., eyes ; p., palp ; 

 pe. t peristomium ; t., tentacles. 



and other organs. The body of an earthworm is composed 

 of similar divisions, because all the organs are repeated 

 together at regular intervals. There are other animals 

 in which only some of the organs are thus repeated, as in 

 the frog, where the vertebra?, nerves, and to some extent the 

 muscles exhibit segmentation, so that the regions of which 

 the body might be regarded as composed are much less alike 

 than they are in the earthworm. In such cases segmentation 

 is said to be incomplete. The tapeworm presents an example 

 of a kind of segmentation which is very complete, but is of 

 quite a different nature from that of the earthworm, the 



