142 MOLLUSKS. 



7. That there are two round fringed openings at the 

 posterior end of the dorsal margin. These are the siphon 

 openings. If the water be shalloAv enough, there may be 

 seen near these a play of wavelets on the surface, indicat- 

 ing currents. Place a drop of india ink or other colored 

 fluid in the water near these openings, and discover the di- 

 rection of the currents in each of them. Record the result. 



8. That there is an exserted membrane fringing the 

 free border of each valve all around. This is the edge of 

 the mantle. Touch it in various places, and note its sensi- 

 tiveness. 



9. That there is a white, flexible, muscular foot pro- 

 truded downward and forward between the mantle margins 

 into the sand. By quietly placing a finger horizontally 

 in the sand in front of the mussel, and directly in its 

 course, and waiting for the animal to travel over it, you 

 may discover how the foot is used. 



Pick up an active mussel quickly out of the water, and 

 see how quickly the foot is retracted. 



Place a sheet of tin or a pane of glass on the sand, and 

 lay a mussel on it at its center. Note how the animal 

 protrudes and uses its foot, in its efforts to rise. 



Explain its inability to rise on edge and move away. 



Discover by experiment whether the fringes of the 

 siphons are sensitive to light. 



Structure. In dissecting a river mussel, the first thing 

 to be done is to get the shell open so as to get at the 

 animal so securely locked inside. The valves are held 

 together by two stout, transverse muscles. Select a live 

 specimen of large size for the first dissection. Place it 

 for a few moments in water as warm as the hand can 

 bear. This will relax the muscles. The valves may then 

 be opened slightly, and a block inserted between them 

 to keep them so. 



