MOLLUSCA 



145 



particularly to the fresh water mussel. A horny or elastic 

 ligament unites the two valves on the dorsal aspect and 

 tends to open the shell along the ventral margin. The 

 concentric parallel ridges on the outside of the shell repre- 

 sent successive additions of lime and are called lines of 

 growth. The eleva- 

 tion near the hinge 

 is the wnbo. To 

 open the shell place 

 it in boiling water 

 a minute or two and 

 then slip the knife 

 between the two 

 slightly open valves 

 and loosen the ad- 

 ductor muscles from 

 one side of the shell. 

 The line within 

 the valve concentric 

 with the ventral 

 margin and connect- 

 ing the two muscle 

 scars is the pallial line along which a membrane, the 

 mantle, is attached. The two mantles covering the body of 

 the mussel are almost as large as the valves to which they 

 adhere closely. By removing the left mantle and immers- 

 ing the specimen in a pan of water the relations of the soft 

 parts may be made out. The firm conical mass projecting 

 ventral and anteriorly is the foot. The gills are the two 

 thin membranes on either side of the body just within the 

 mantle. They are covered with ciliated cells, whose pur- 



FIG. 170. Wood injured by the ship worm. 



