THE RIVER MUSSEL. 147 



swells up with limpid transparent blood from the gills, 

 and then it contracts and fills the ventricle, which then 

 contracts in turn, sending the blood out through the pedal 

 artery. 



II. Digestive System. On either side of the front 

 part of the abdomen observe two soft triangular flaps 

 narrowing toward the front. These are the labial palps. 

 Between them, directly in front, is the mouth, which 

 leads through a short esophagus into the stomach. The 

 stomach is surrounded by a dark brown mass called 

 liver, easily seen when the mantle is removed. The course 

 of the digestive tract is difficult to trace in a fresh speci- 

 men, but may be followed more readily in one that has 

 been hardened in alcohol. Posterior to the stomach there 

 are several turns of the intestine l before it emerges from 

 the abdomen to pass into and through the pericardial 

 cavity, where it has already been noticed running through 

 the ventricle. It terminates in the cloacal chamber. If 

 the heart and the part of the intestine posterior to the 

 abdomen be removed, two dark-colored renal organs will 

 be seen below them. These organs open below into the 

 cloacal chamber, and above into the pericardial cavity, 

 thus connecting the body cavity with the exterior, as do 

 the renal (segmental) organs in the earthworm. 



III. Nervous System. This will be studied with ex- 

 treme difficulty in a fresh specimen. It will require 

 less time and patience if a specimen which has been 

 hardened in alcohol or by boiling be used. It may best 

 be studied in a specimen which has been soaked for a few 

 days in 10-per-cent nitric acid. 



Remove the mussel entirely from its shell, and pin it 

 in a dissecting pan, with its dorsal surface down. Sepa- 



1 It may be demonstrated by injecting the alimentary canal through 

 the mouth with starch mass (see Appendix, p. 284) colored with lamp- 

 black, and dissecting away the sheet of overlying muscle of one side. 



