A FRESH- WATER MUSSEL. 8 1 



Exercise 3. Remove the left mantle lobe, except a small portion imme- 

 diately surrounding (he siphonal openings, and make a sketch of the 

 body of the animal lying thus in the right half of the shell. 



INTERNAL ANATOMY. Very carefully remove the thin wall of the 

 pericardium and make out the following: 



a) The heart, consisting of a single thick-walled ventricle and two 



auricles of thin filmy tissue. The latter are triangular pockets 

 attached by the narrow end to the sides of the ventricle and by 

 the broader end to the lateral part of the floor of the pericardium. 



b) The pericardium is traversed by a portion of the alimentary tract, 



the rectum, which emerges from the visceral mass at a point 

 beneath the umbo and passes posteriorly over the adductor 

 muscle. It ends hi the anus just within the upper siphonal open- 

 ing. Trace it out. The ventricle of the heart is strung on the 

 rectum like a bead on a string, but has no communication with it. 



c) From the ventricle an anterior aorta passes forward above, and 



a posterior aorta passes back beneath the rectum. These blood- 

 vessels have such thin walls that the student will probably not be 

 able to make them out unless the specimen has been injected. 



Exercise 4. Begin another sketch, fitting within the outline oj the right 

 valve of the shell only the organs contained in the pericardial chamber. 



d) Probe the outer part of the suprabranchial cavity and note that 



farther back it divides into four suprabranchial passages, one 

 just above the dorsal border of each gill. Understand precisely 

 the attachment of the gills to each other and to the other organs. 



e) Remove the heart and rectum. Immediately beneath the floor 



of the pericardium are two dark-colored excretory organs. 

 Each extends from the anterior part of the pericardium to the 

 posterior adductor muscle, where it turns and runs forward again. 

 It is thus narrowly U-shaped, one arm, the ureter, lying upon 

 the other kidney. Make an opening into the bend of the organ 

 and probe both ureter and kidney. The external openings of the 

 excretory organs are too difficult of demonstration for this 

 exercise. 



