262 MOLLUSCA 



(2) Siphons. Notice the fringes of the posterior margins 



of the mantle. Place a tiny drop of ink above the 

 opening of the siphon and note the direction of the 

 current of water. 



(3) Sensitiveness. 



(a) Gently touch the margins of the siphons. Are they 



sensitive to touch? What do they do? 



(b) Find out, by experiment, if they are sensitive to light. 



(c) Find out if they are sensitive to jars or to currents of 



water. 



MORPHOPHYSIOLOGIC STUDY OF THE CLAM 



A. External Morphology 



1. The Shell externally. 



(1) Shape? Use of this shape to the clam? 



(2) Length? 



(3) Depth (dorsoventrally)? 



(4) Width (laterally)? 



(5) Weight of clam with shell? 



(6) Weight of clam without shell? 



(7) Shell is what per cent, of weight of whole clam? 



(8) Why so much shell? Advantage? 



(9) Why so little body? Advantage? 



(10) Disadvantage of so much shell? 



(11) Covering of the shell. 



(a) Color? 



(b) Use? 



(12) Hold the shell in your left hand with the two projec- 



tions (umbones) from you. The valve in your left 

 hand is the left valve, the one in your right hand is 

 the right valve. 



(a) How do the valves agree? 



(b) How do they differ from each other? 



(13) Concentric lines of growth about the umbones. 



(a) How many? 



(b) Age of your specimen? 



(c) Do you find other ridges in the shell? 



(d) Use? 



