126 GENERAL BIOLOGY 



2. Interior Surface of the Shell. Wedge the shell open 

 and then cut the muscles which are attached to the valves. 

 When this is done what happens to the valves? What 

 causes this? What is the function of the muscles? (If the 

 animal is preserved and the valves are already open, cut 

 the muscles and determine the functions of muscle and 

 ligament.) Cut the ligament and remove the left valve, 

 being careful not to remove any of the body within the 

 shell; a fold of skin which sticks to the shell must be care- 

 fully separated. 



Compare the color and markings of the interior of the 

 shell with those on the outside. Are lines of growth present? 

 Are there any scars on the inside of the shell? What has 

 been the cause of these? Find the marks made by the 

 adductor, retractor, and protractor muscles. (In Venus there 

 is no protractor muscle.) How many of these muscles are 

 there? What is their function? When the clam was 

 smaller than it is now where were the muscles attached? 

 How do you know? Is there any evidence of growth over 

 any part of the muscle scars that now show? At the dorsal 

 side of the valve notice several teeth (not present in all 

 clams), the hinge teeth. WTiat is their function? 



Draw the interior of the shell. 



3. Structure of the Shell. Break the valve that has been 

 removed and examine the broken surface. Notice the 

 thickness of the shell in different regions. Look for layers 

 in the shell, an outer very thin layer, the periostracum, 

 greenish or brownish in color. (This layer is not always 

 seen since it may have been worn off.) Next comes the 

 prismatic layer, and inside the nacreous or mother-of-pearl 

 layer. The periostracum and the prismatic layers are 

 formed by the thickened edge of the mantle. Once formed 

 they cannot be added to except at the edge of the shell. 



