STUDY VII. MOLLUSCA. Soft-bodied Animals. 



Central idea : The effect of a protective skeleton. 



1. THE FRESH-TV ATBR CLAM. 



A. Habitat. (Dem.) 



1. Where do these animals live ? 



2. Do you find it necessary to search over a large area 



in order to find a large number of clams ? 



3. What marks in the mud show that clams are present ? 



B. General Observations. 



1. Show by sketches the outline of the clam as seen 



from above and from the side. 



2. Identify, — 



(a) the broad margin (dorsal margin) and the oppo- 

 site sharp edge (ventral margin) ; 



(6) the umbones, a pair of elevations on the dorsal 

 margin, usually nearer the anterior than the 

 posterior end ; 



(e) the hinge-ligament between and extending back 

 of the umbones. 



3. Draw the dorsal outline of the clam and indicate 



right, left, anterior, posterior, umbones and hinge- 

 ligament. (In this and all subsequent drawings 

 of the clam have the anterior end point toward 

 the top of the paper. ) 

 68 



