NEREIS VIRENS. 73 



Circulatory System. — The dorsal blood-vessel lies along the 

 dorsal surface of the aUnicntar}^ 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 red coloring-matter in solution. 



Digestive System. — The mouth-cavity leads into a muscu- 

 lar pharynx, a portion of which is protrusible as the proboscis. 

 Examine carefully the muscles of the pharynx, protractors and 

 retractors, and ascertain their attachments. Posterior to the 

 pharynx find a narrow esophagus and a small dilation, the crop, 

 with a digestive gland at each side. Where does the duct of the 

 gland open? In the very long stomach-intestine, which follows the 

 crop, note the constrictions and their relations to the dissepi- 

 ments. Can you demonstrate dorsal or ventral mesenteries? 

 Cut open the pharynx and the anterior end of the stomach- 

 intestine and note the character of their walls. 



Make a draumig of the digestive system. 



Muscular System. — How many distinct bands of longitu- 

 dinal 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 7nuscles? Of what layers does 

 the body-wall consist? 



Excretory System. — The nephridia are not nearly as easily 

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

 beneath the lateral edges of the ventral muscle-bands find the 

 minute pear-shaped nephridia. Determine their distribution 

 in the body. Each nephridium consists of a tortuous canal in 

 a multi-nucleate mass of protoplasm. The external opening is 

 the nephridiopore above mentioned. The inner end perforates 

 the dissepiment anterior to the body of the nephridium and 

 opens into the coclomic cavity of the segment next in front, 

 by a ciliated funnel, the ncphrostome. With a hand-lens try to 

 find the nephrostome. Remove a nephridium by means of fine 

 forceps and examine it with a microscope. 



Reproductive System. — The sexes are separate, but no per- 

 manent gonads are present. At the breeding season the ova 



