VENUS MERCENARIA. 105 



lies, is a somewhat triangular space that appears clear, through 

 the mantle. It lies just anterior to the posterior adductor 

 muscle. Open the pericardium, and notice the beating of the 

 heart. The heart consists of three parts: 



1. A central portion, the ventricle, that surrounds the intes- 

 tine and gives rise to a blood-vessel at each end. 



2. Two triangular portions, the auricles, that receive blood 

 from the gills and open into the sides of the ventricle. 



Notice the sequence and power of the contractions. 



Just posterior to the pericardium is an enlarged portion of 

 the alimentary canal. This has no relation to the heart, for 

 which it is sometimes mistaken. 



Excretory and Genital Systems. The excretory system con- 

 sists of a pair of dark colored glandular organs that lie beneath 

 the pericardium. Each communicates with the pericardium 

 by a small opening that is not easy to demonstrate in dissections, 

 and with the cloacal chamber by another small opening. 



By turning the two gills (of Venus) dorsally a very small 

 papilla may be seen, just beneath the free border of the inner 

 gill, lying in the cloacal chamber. On the tip of this papilla 

 are two openings. The inner one is the opening of the excretory 

 organ. The outer one is the opening of the genital duct. 



The genital glands are light colored organs that, during the 

 breeding season, extend through the principal part of the vis- 

 ceral mass. Neither the genital nor the excretory systems can 

 be profitably studied in a general dissection of this form. In 

 Unio the excretory organs are more satisfactory for study. Do 

 you understand the supposed significance of their connection 

 with the pericardium? 



Nervous System. 1. Carefully remove the body-wall by 

 the side of the esophagus and notice the cerebral ganglion of the 

 corresponding side. This is a rounded, slightly yellow organ, 

 about the size of a pin-head, lying just posterior to the dorsal 

 border of the anterior adductor muscle. (In Unio it is more 

 ventral in position.) The cerebral ganglia of the two sides are 

 united by a commissure that passes anterior to the esophagus. 

 Two connectives leave each cerebral ganglion. One passes 



