A Polych&tous Annelid. 45 



length? Do the constrictions invariably correspond to the 

 surrounding body-rings? Are there any local changes in 

 the texture or color of the stomach-intestine that might 

 argue changes in function? Do you find a ventral mesen- 

 tery attaching the tube to the mid-ventral line ? (The dor- 

 sal mesentery has probably been destroyed in opening the 

 animal.) 



Make a drawing of the dorsal view of the alimentary 

 tract, naming the parts. 



With a pair of sharp scissors open pharynx, crop and in- 

 testine, along the mid-dorsal line. The interior of the 

 pharynx will be found to present an anterior, tooth-bearing 

 and a posterior, undifferentiated portion. Note the internal 

 lining of the crop, and the small opening leading from it to 

 the much thinner-walled stomach-intestine. Is the lining 

 membrane of the latter different from that of the crop ? 



The Circulatory System in Nereis consists of a closed 

 system of tubes in which the blood, holding in suspension 

 numerous red blood-cells, circulates. The median ventral 

 blood-vessel extends nearly the entire length of the animal. 

 It lies ventrally to the alimentary tract and sends out lateral 

 branches to each segment of the body. These lateral 

 blood-vessels ramify through the gills and over the organs, 

 uniting again, above, in a median dorsal blood-vessel. The 

 latter rests upon the intestine. In the anterior portion 

 of the animal, due to the presence of the complicated 

 pharyngeal apparatus, the circulatory system is somewhat 

 less regularly arranged. The dorsal vessel is contractile, 

 and forces the blood from the posterior towards the head. 



