MOLLUSCA 241 



sensitiveness to stimuli of varibus 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 more complete 

 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 with- 

 out doing violence to the animal ? 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), mus- 

 cle impressions, 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 Pari5.— Remove the left valve 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 

 16 



