TEANSVEESE SECTIONS. 107 



b. The ventricle surrounds the rectum : its outer wall 



is thick and muscular, its inner wall thin. 



c. The auricles are a pair of very thin-walled sacs lying 



at the sides of the pericardial cavity. Each is 

 attached along its outer side to the wall of the 

 cavity, and by its inner border to the ventricle, 

 into which it opens by a slit-like valved aperture. 



d. The vena cava is a median thin-walled tube lying 



in the floor of the pericardium. 



e. The afferent branchial vessels, right and left, lie 



just above the attachment of the outer lamella of 

 the inner gill and inner lamella of the outer gill. 



6. The excretory organs. 



a. The kidneys are two wide tubes with thick spongy 



walls, whose epithelium is glandular, pigmented, 

 and folded. They lie below the pericardium, above 

 the visceral mass, and opposite the bases of the 

 inner gills. Their inner walls are in contact with 

 each other below ; and are separated dorsally by 

 the vena cava, from which large sinuses enter 

 them. 



b. The ureters are a pair of thin-walled tubes lying 



along the dorsal surface and outer sides of the 

 kidneys, immediately below the pericardium. 

 They are separated from each other in the 

 median plane by the vena cava. 



7. The nervous system. 



The connectives between the cerebral and visceral 

 ganglia lie side by side, between the two kidneys and 

 immediately below the vena cava. 



C. Transverse Section through the Middle of the Posterior 

 Adductor Muscle. 



1. The posterior adductor is a large mass of transverse 



muscular bands running across the dorsal part of the 



section. 



