FEESH-WATEE MUSSEL 199 



the abdomen of the animal ? "What is the rela- 

 tion of its posterior end, the rectum, to the 

 hinge-ligament ? To the posterior adductor mus- 

 cle? Notice that the rectum runs through the 

 pericardial cavity. Into what does the anus open ? 

 Notice the prominent ridge or t3rph.losole on the 

 ventral side of the rectum. "Where is this ridge 

 largest ? How does this compare in position and 

 structure with the typhlosole of the earthworm? 

 Draw a diagrammatic longitudinal section of the 

 body, showing the course of the digestive system. 



B. — The Circulatory System. 



Remove from the shell a living mussel in water, 

 being very careful not to injure the soft parts, 

 and note the beating of the heart as seen through 

 the pericardial wall. Study the number of beats 

 per minute and the movements of the heart. 

 Open the pericardium, noting the thickness of 

 its wall and the shape and extent of the pericar- 

 dial cavity. If living specimens cannot be had, the 

 heart may be found by cutting the upper edge of 

 the upper siphon forward to the umbo. Notice 

 the heart, consisting of a median ventricle and 

 two lateral fan-shaped auricles. 



a. The ventricle (wrapped around the intestine). — 

 "What is its position in the pericardium ? "What 

 is its shape ? Size ? Color ? Are its walls firm 

 or flexible? Trace from the ventricle an ante- 

 rior aorta on the upper side, and a posterior 

 aorta on the lower side of the intestine. 



5. The auricles. — "What is their relation to the ven- 

 tricle? What is the structure of their walls? 



