64 COMPARATIVE ANATOMY 



The excretory organs of Anodonta are often called the 

 organs of Bojanus. They lie under the floor of the pericardial 

 cavity and are paired, consisting on each side of a bent tube 

 opening by one end into the pericardial cavity, by the other 

 end into the branchial cavity. The pericardial opening of 

 each side is a semilunar slit in the floor of the anterior end of 

 the pericardial cavity, Jbelow and rather to the side of the 

 rectum, just behind the point where the latter passes first 

 through the pericardial wall. The slit leads into a wide 

 saccular tube whose walls are thrown into a number of folds 

 and are lined with a black glandular epithelium. This glan- 

 dular part of the excretory organ runs backwards below the 

 floor of the pericardium, and opens, a little way in front of its 

 posterior end, by a small aperture into a wide, non-glandular 

 tube or excretory duct, which runs forward, above and to the 

 outer side of the glandular part, as far as the anterior end of 

 the pericardium, where it turns sharply downwards to open by 

 a small excretory pore between the two laminae of the inner 

 gill-plate, about one-eighth of an inch in front of the place 

 where the inner lamina becomes free from the body-wall. 

 _The_genital ducts open by small pores just behind the 

 excretory pore of each side. The nervous system consists of 

 three pairs of ganglia united by nervous cords. The most 

 anterior pair is called the cerebral ganglion-pair, but it prob- 

 ably is a composite ganglion formed by the fusion of at least 

 two pairs of ganglia found in other molluscs. Each cerebral 

 ganglion lies at the hinder border of the mouth and below and 

 in front of the protractor muscle. It is of a deep orange colour, 

 and is connected with its fellow by a nerve running above the 

 mouth. Nerves connecting two ganglia of the same pair are 

 called commissures, and this is accordingly the cerebral com- 

 missure. Several nerves are given oft trom the cerebral ganglia 

 to the anterior part of the body and to the labial palps. Nerves 

 connecting ganglia of different pairs are called connectives, 

 and two such nerves are given off from each cerebral ganglion. 

 The cerebro-pedal connectives pass backwards and down- 

 wards to the pedal ganglia, which are situated close together 

 in the foot, at about one-third of the length of the foot from 

 the anterior edge, and just above the junction of the visceral 

 with the muscular part of the foot. Several nerves are given 

 off from the pedal ganglia, one of which passes back from each 



