THE CLAM: LABORATORY WORK. 



For this purpose the student can use either the fresh- 

 water clam or the long clam of the Northern sea-shore. 

 For the study of the nervous system clams which have 

 been a few days in alcohol are better than fresh specimens. 



EXTERNAL. 



Notice the shell ; of how many parts or valves is it com- 

 posed ? Are the valves equal in size ? They are joined by 

 a hinge, dorsal in position, and each valve has a promi- 

 nence (umbo) near the hinge. On each valve see the lines 

 of growth running parallel with the free margin of the 

 shell. Draw a line from the umbo to the free margin of 

 the shell, perpendicular to the latter. This divides the 

 valve into unequal parts, and of these the smaller is the 

 anterior. Now with these facts tell which is the right 

 and which the left valve of the shell. Draw one of the 

 valves, inserting all points made out. 



INTERNAL. 



Remove the left valve from the clam by inserting a knife 

 at either end close to the shell, and cutting the muscles 

 which lie near the hinge-line. Then carefully remove the 

 valve, seeing that all fleshy portions are left in the right 

 valve. If properly done, this will leave the animal covered 

 with a thin membrane, the mantle. Projecting through 

 this, near the dorsal line, are the adductor muscles, which 

 keep the shell closed, and which were cut in removing the 

 valve. According to their position, these are known as 

 the anterior and posterior adductors. Are the edges of the 



248 



