90 ANNELIDA. 



A preserved specimen can easily be sectioned transversely 

 with a sharp razor at the somatic boundaries. These sections 

 are valuable for comparison during dissection. 



Find the septa which divide the coelom, or hody-cavity, into 

 metameric chambers. Cut through the septa with scissors and 

 pin the edges of the body-wall apart, progressing toward the 

 head. 



Circulatory System.— The dorsal blood-vessel lies akmg tlio 

 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 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 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, 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 draiving 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 muscles? 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 



