MOLLUSCA. 217 



237. The Fresh -water Clam (Any Species): a Practical Study. 

 The mollusks are not a good group for dissection by an elementary 

 class; and no single specimen which might be dissected will give a 

 fair idea of the branch as a whole. It will be better therefore to 

 study only the external appearance and general form of several 

 types. 



Get both parts of a clam shell. Make a complete study of the 

 outside, noting all the markings and differentiations of parts that 

 can be seen, Make an outline diagram. Study the interior of the 

 shells in the same way. Compare internal and external finish and 

 markings. Are the shells strictly symmetrical? Interpret the 

 meaning of all the internal markings. Compare with texts and see 

 if your interpretation is correct. Is a shell, from which the animal 

 has recently decayed, normally open or shut? What causes it to be 

 so? After studying a living specimen and one that has been cut 

 open, see if you can answer the following questions: Does the shell 

 grow? If so, how? What is the oldest point? Evidences. Have 

 you any way of determining which is the right and which the left 

 shell ? What other facts must you determine first ? See if you can de- 

 termine them from the animal within. Have the muscles which 

 close the shells always been where they are now? Evidences. 



Examine briefly the soft parts of the animal within. Identify 

 the mantle; the sheet-like gills (from which the name lamellibranch) ; 

 the thick, muscular foot; the closing muscles; the anterior end; the 

 ventral surface. What organ produces the shell ? Can you justly 

 say that the animal has a head? 



Compare a snail-shell, point by point, with the clam. Enumerate 

 the items of similarity; those of difference. Compare your notes 

 on shells in Chapter IV. 



238. Library and Museum Exercise: a Comparison of 

 the Clam, Snail, and Squid, in General Habits and Organi- 

 zation. Make rulings for a parallel-column study such 

 as was indicated in section 225, and record the results 

 of your study of these three animals in the following 

 particulars : the symmetry and general shape of the body ; 

 head and its organs ; the kind of food used and the method 

 of getting it; the mantle and its different uses; the shell, 

 its degree of development and the method of using; the 

 contents of the mantle cavity; the foot, its parts and the 



