THE CLAM 131 



edges of the mantle are extended. Use a bristle or a delicate 

 glass rod to test the sensitiveness of the various parts ex- 

 posed. Determine whether foot or mantle, and which 

 portions of the mantle, are most sensitive; test especially 

 the edges of the siphons. What response is made to delicate 

 tactile stimuli? To stronger stimuli? 



Sensitiveness to chemical substances may be tested by 

 directing gentle currents from a pipette against 'various 

 portions of the body. 



Place a specimen in a very dim light, and when the shell 

 opens direct a beam of light against the body. Is there a 

 response to this stimulus? Place the animal in sunlight 

 or bright light from some artificial source, and interpose a 

 screen between the light and the animal, thus casting a 

 shadow upon the body. Note the character of such re- 

 sponses as occur. 



2. Circulation of Water. Introduce a few drops of a 

 colored fluid into the water near the posterior end of a clam 

 whose shell is open. If the water is drawn into the shell 

 observe the place where it enters. Does the colored fluid 

 pass out of the shell again? Where is the point of exit? 

 What service to the clam would there be in such incurrent 

 and excurrent movements of the water? Further light on 

 this advantage will be gained by observing a clam partly 

 buried in the sand in the normal manner. 



3. Feeding. Carefully insert a knife between the valves 

 of a living clam and cut the two adductor muscles where 

 they are attached to the shell. Loosen the mantle and 

 remove one valve entirely, thus exposing the body. Lay 

 back the mantle, or cut it loose, to expose gills and palps. 

 Drop some powdered carmine, chalk, or other small solid 

 particles upon the surface of the gills and note any movement 

 of these particles. In what direction is the movement? 



