OTHER ANNULATA 



313 



however, a pair of parapodia on every segment of the body except 

 the peristomium and the terminal segment. The latter bears 

 the anus (Fig. 151) dorsally, and ventrally two anal cirri which 

 project posteriorly. 



Internally, the digestive tract is somewhat less differentiated than 

 in the earthworm, since it consists only of pharynx, esophagus 

 with a pair of digestive glands, and a stomach-intestine. The 

 circulatory and nervous systems resemble those of the earthworm. 

 The reproductive system, Uke that of all the polychsetes, is much 



A B 



Fig. 152. — Head and parapodium of Nereis virens. 



A, head in lateral view, showing sense organs. One of the eyes is seen as a small dot 

 on the dorsal side posterior to the tentacles. B, a single parapodium. 



ac, aciculum or specially developed seta; c, peristomial cirrus; m, mouth; p, palp. 

 pr, peristomium; s, sets; t, tentacle. 



simpler than in the earthworm, since the sexes are separate and 

 ovaries or testes are formed on the coelomic walls at the breeding 

 season as specialized regions of mesoderm in each somite except a 

 few at the anterior end. The ova and spermatozoa pass out by the 

 nephridia and not by special genital ducts. Fertihzation and 

 development occur in the open water, and there is a free-swimming 

 larva, the trochophore, which settles to the bottom as it changes 

 into the miniature adult. 



Other Polychoetes. — Nereis represents a free-Uving type of 

 polychaete to which the name Errantia, or " wanderers," was 

 apphed in one of the older classifications. This was in contrast 



