MOLLUSCA. 235 



experiments to test whether the animal shows sensitiveness 

 to stimuli of various sorts : jars, contacts, currents in the 

 water, light, warmth and cold. 



2. General Form. Renew your observations concerning 

 the symmetry of the clam by careful examination of the ani- 

 mal. Determine and show in a sketch all the points distin- 

 guishing the anterior and posterior ends. Are the right and 

 left halves symmetrical? Use a pair of empty shells for com- 

 pleter study. 



The shell : what is the relation between the valves ? How 

 are they held together? Are they normally open or closed? 

 Give your evidences? To what extent may the shell open 

 without violence? How does the shell vary in thickness at 

 various parts? Contrast the interior and exterior as to finish 

 and markings. Make note of everything found, with outline 

 drawings, showing position. Locate the following regions 

 and structures: hinge, umbo or beak (a prominence near the 

 hinge), hinge ligament, hinge teeth, pallial line (a slight de- 

 pression marking the attachment of the mantle muscle), 

 muscle impressions, lines of growth. Review after studying 

 soft parts. 



What is the oldest portion of the shell? Evidence. How 

 does the shell grow? How did the internal depressions come 

 to be? Evidence. 



What layers are discoverable in a broken shell? How do 

 the inner, outer, and middle layers differ in thickness and 

 appearance ? 



Do you find any differences worthy of note in different 

 individuals ? 



3. Soft Parts. Remove one valve (say the left) by cutting 

 the two muscles which hold the valves together. Leave all 

 the soft parts in the right valve as little disturbed as possible. 

 Make a sketch showing the relation of the body to the shell. If 

 there is any difficulty in cutting the muscles, the clam may be 

 made to open by immersing it in water heated to about 140 F. 

 Identify: 



