240 ZOOLOGY 



ness 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 animal. 

 Determine and show in a sketch all the points distinguishing 

 the anterior and posterior ends. Are the right and left halves 

 symmetrical ? Use a pair of empty shells for completer 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 depression 

 marking the attachment of the mantle muscle), muscle im- 

 pressions, lines of growth. Review after studying soft parts. 



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 : 



Left mantle flap. How related to the right ? to the shell ? 



Siphons : modifications of the posterior margins of the mantle. 

 (These will be conspicuous or rudimentary in accordance with 

 the species studied.) Number? 



Adductor muscles of the valves ; number and position. 



Mantle cavity. Separate the right and left mantle lobes 

 along the ventral margin, except in the region of the siphon, 

 and fold back the left. Where is it attached to the body? 



