FRESH- WATER MUSSEL. 61 



II. External Openings. 



1. The labial palps are united at their bases to 



form ridge-like anterior and posterior lips. 

 Between these is the mouth. 



2. On either side of the visceral mass near the 



attachment of the gills is a small genital 

 opening. 



3. Close to 2, slightly more dorsal, is the ex- 



cretory opening. 



4. On the dorsal side of the cloacal chamber 



close to its external opening is the anus. 

 Make a diagram of the animal as it would be seen if 



the mantle-flap of one side were absent. 

 B. Anatomy. 

 I. Circulatory System. 



On the dorsal side of the body immediately 

 in front of the posterior adductor muscle 

 is the pericardial cavity with thin semi- 

 transparent walls. 



Gut with fine scissors through the thin wall a little 

 to the right of the median dorsal line, taking care 

 not to cut deeper, thus exposing : 



1. The pericardial cavity. It contains : 



2. The heart. 



(a) In the median line the opaque, muscular 



ventricle. 



(b) Into the ventricle opens on each side a 



thin-walled auricle. The auricles are 

 funnel-shaped with the narrow end at- 



