62 GENERAL BIOLOGY. 



tached to the ventricle and the broader 

 end to the dorsal border of the gills. 

 3. The vena cava running parallel to the heart 

 under the floor of the pericardial cavity ; it 

 will be seen later (C. II.) in section. 

 Note the rectum passing posteriorly from the 



posterior end of the heart. 

 Make a diagram of the heart. 



II. Excretory System. 



In the anterior angle of the pericardial cavity 

 on each side is a minute pore, the internal 

 opening of the excretory organ, the organ 

 of Bojanus, the dark mass on each side 

 beneath the floor of the pericardial cavity, 

 (For the external opening of the excretory 

 organ see A. II. 3.) 



III. Nervous System. 



Remove the mussel from the shell , place it dorsal 

 side down, and cut through the partition between 

 the 'branchial and the cloacal chambers. See : 

 1. Close to the ventral side of the posterior ad- 

 ductor, the visceral or parieto-splanchnic gan- 

 glion. 



Gently strip off the thin skin covering the 

 ganglion, and find the chief nerves given off 

 from it : 



(a) The posterior pallial nerve. 



(b) The lateral pallial nerve. 



(c) The branchial nerve. 



