ADAPTATION FOR PROTECTION FROM ENEMIES 85 



1. Why may a clam be called a bivalve ? 



2. What is the position of the clam in the mud ? What 

 is the position of the foot if the clam is undisturbed ? 

 Are the two valves tightly closed or slightly open at this 

 time? 



3. What changes take place in the shell as the clam 

 grows ? What markings on the surface of the shell indi- 

 cate this ? 



4. Where is the clam sensitive to touch or tactile stimu- 

 lus ? Why has the clam no eyes ? Zoologists have found 

 a structure in clams which they have supposed to be an 

 ear. Where do you think the structure is located ? Why 

 is the clam successful without eyes ? (There are many 

 bivalves which have them.) 



5. Examine several clams until you find some with en- 

 largements in the gills. Break off a small part of an 

 enlargement with your forceps and examine under the com- 

 pound microscope. Describe what you see. 



6. Drop some powdered chalk or carmine in the water 

 just above the siphon, watch the siphons for several min- 

 utes, and note what happens. What do you conclude to be 

 the use of the siphons .■■ Recalling what took place in 

 sponges, what would you suggest as the probable cause of 

 these currents .' What does the clam thus probably obtain ? 

 How do the two siphons differ .'' Why .-' 



7. Place a clam in water sufficient to cover it and heat 

 slowly to about 40 degrees Centigrade, until the valves 

 open slightly. Remove and proceed as follows : Raise one 

 valve, separate the mantle from it, and then cut through the 

 two large firm structures (adductor muscles) found at each 

 end. What does the valve do when the muscles are cut .■' 

 What is the cause of this .■' State your theory as to how a 

 clam opens and closes its shell. 



