TRANSVERSE SECTIONS. 115 



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, pigmeDted, 

 and folded. They lie below the pericardium, 

 above the visceral mass, and opposite the bases 

 of the inner gills. Their inner walls are in con- 

 tact 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. 



