NEREIS 93 



and one through the posterior part. With scissors cut through 

 the body wall, longitudinally, near the mid-dorsal line. 



A preserved specimen can easily be segmented trans- 

 versely with a sharp razor at the somatic boundaries. These 

 sections are valuable for comparison during dissection. 



Find the septa which divide the coelom, or body cavity, 

 into metameric chambers. Cut through the septa with scis- 

 sors and pin the edges of the body wall apart, progressing 

 toward the head. 



Circulatory System. — The dorsal blood vessel lies along 

 the dorsal surface of the alimentary canal and gives off 

 branches in each segment, which ramify through the body 

 wall and viscera and connect with the longitudinal, ventral 

 blood vessel. The blood plasma contains hemoglobin in solu- 

 tion. 



Digestive System. — The buccal cavity leads into a mus- 

 cular pharynx. The eversible buccal region and the protru- 

 sible pharynx form the proboscis. Examine carefully the 

 muscles of the pharynx, protractors and retractors, and as- 

 certain their attachments. Posterior to the pharynx find a 

 small dilation and a narrow esophagus with a digestive gland 

 at each side. Where does the duct of the gland open? In 

 the very long stomach-intestine note the constrictions and 

 their relations to the dissepiments. Can you demonstrate 

 dorsal or ventral mesenteries? Cut open the pharynx and 

 the anterior end of the stomach-intestine and note the char- 

 acter of their walls. 



Make a drawing of the digestive system. 



Muscular System. — How many distinct bands of longi- 

 tudinal muscles can be distinguished? Examine with a hand 

 lens the parapodial muscles attached to the base of the 

 acicula. Can you make out a layer of circular muscles? Of 

 what layers does the body wall consist? 



Excretory System. — The nephridia are not nearly so 

 easily found or studied as they are in the earthworm. Near 

 or just beneath the lateral edges of the ventral muscle bands 



