7 8 Laboratory Guide in Zoology 



dorsal surface find two prominences, the umbones, near 

 together. Notice that the umbones are nearer one 

 end than the other. The end to which they are nearer 

 is the anterior end, and the opposite extremity is the 

 posterior end. 

 IV. — We have noticed that the entire shell is divided, 

 equally into two parts, or valves. It is for this reason 

 that the term Bivalve was given to the class to which 

 the clam belongs. Find a strong, hard hinge or liga- 

 ment near the umbones that joins the valves. 



V. — With some sharp instrument carefully force the 



ventral edges of the shell apart, putting some object in 

 the space to hold them in position. Then pushing 

 aside the ruffled edge of a membrane, insert the knife 

 edge close under the valve and carefully push away the 

 soft membrane clinging to its inner surface. 



VI. — By forcing open the valves a little farther, you will 

 be able to see near each end a hard, smooth, white pil- 

 lar of muscle fastened to the inner side of the valve. 

 These pass directly through the body, and a.v6 attached 

 in similar positions to the opposite valve. Carefully cut 

 out these muscles close to the valve. The springing 

 open of the latter shows why the muscles are called the 

 adductors. Without disturbing any of the parts, force 

 the two valves together, and notice the spring ex- 

 erted by the ligament, or hinge. Remember to study 

 the mechanism causing this action after the soft parts 

 have been removed,- or on other shells. 



VII. — Study the concentric rings radiating from the 

 umbones. These are the growth lines of the shell as 

 it was gradually secreted by the soft membrane first 

 seen when the valves were opened. 



