PHYLUM ANNELIDA 



197 



the parapodial muscles. A dorsal and a ventral cirrus are usually- 

 present. The notopodium and the neuropodium each have a large 

 group of setae. The parapodia are used principally as locomotor 

 and respiratory organs. 



The head of Nereis seems to have resulted from the fusion and 

 specialization of the anterior segments. It is composed of a prnsto- 

 mium, which bears a pair of tentacles, a pair of palps, and two pairs 

 of eyes. The peristomium constitutes the first segment and bears four 

 paii*s of cirri or tentacles. The pharynx is equipped with muscles by 

 which it can be everted, and a pair of chitinous jaws which protrude 



Fig. 100. — External anatomy of Nereis virens and parapodium. A, anterior end 

 and posterior end; B^ parapodium (enlarged); 1, palp; 2, terminal tentacle; 5, 

 prostomium ; 4, eye ; 5, lateral tentacles ; 6, peristomium ; 7, segment ; 8, para- 

 podium ; 9, anus; 10, anal cirrus; XI, dorsal cirrus; 12, gill plate; 13, setae 

 (chaetae) ; Ui, notopodium; 15, neuropodium; 16, ventral cirrus; 17, aciculum. 

 (Courtesy of General Biological Supply House.) 



when the pharynx is extended. The jaws serve in capturing small 

 organisms and crushing anything which is to be swallowed. The suc- 

 ceeding segments are all alike except the posterior one which bears 

 a pair of ventral ciVrt extending posteriorly. 



The digestive tract consists of an essentially straight tube. The 

 mouth opens directly into the muscular protrusible pharynx, which 

 may be everted by use of protractor muscles to form a sort of pro- 

 hoscis. The pharynx leads into the relatively narrow esophagus 



