42 Invertebrate Zoology. 



A POLYCH^ETOUS ANNELID 



(Nereis virens). 



External Anatomy. The elongated body is terminated 

 anteriorly by a triangular piece, the prostomium, which 

 bears certain sense-organs, and forms the most anterior part 

 of the " head" while the posterior end is terminated by a 

 pair of flexible caudal cirri. Of how many segments or 

 metameres does the body consist? What is the arrange- 

 ment of the lateral appendages ? Note, through the trans- 

 parent skin, the median dorsal blood-vessel, and on the lower 

 side the median ventral nerve-chain. The latter appears 

 as a whitish line, extending almost the entire length of the 

 animal, and presents enlargements, ganglia, for each body- 

 segment. 



The prostomium is borne on an enlarged ring, the peri- 

 s to miu m or buccal somite. The tentacles are small and fusi- 

 form ; and take their origin near the terminal point of the 

 prostomium, from which they abruptly diverge. Immedi- 

 ately lateral to and below the tentacles are the much larger 

 and clearly segmented palps. In the furrow between the 

 pro- and peristomium the cirri, four on each side, take 

 their origin. Are they segmented ? The eyes or ocelli are 

 somewhat difficult to find in alcoholic specimens. They are 

 four in number, and are arranged in pairs on the dorsal sur- 

 face of the prostomium. 



