314 Gilman A. D 



rew 



The siphons are the most exposed, and apparently the most 

 sensitive to stimuU of any portion of the mantle. They are sur- 

 rounded by sense tentacles and, in the expanded animal, protrude 

 a short distance beyond the posterior end of the shell. Tentacles 

 occur all over the branchial siphon and fringe its margin. The 

 cloacal siphon has tentacles around it and on its sides but its edge 

 is very thin and does not bear tentacles. When stimulated the 

 siphons contract and are withdrawn between the posterior bor- 

 ders of the shell valves. As has already been mentioned, the stimu- 

 lation of the siphons of a specimen that is embedded in the mud is 

 the signal for its disappearance. A very slight touch, such as 

 might be given by a drifting weed or a piece of dirt, will cause an 

 instant withdrawal of the siphons but may not cause the animal 

 to burrow. If the stimulation is repeated, burrowing is quite sure 

 to follow promptly. When the animal is removed from the mud, 

 stimulation of the siphons when not long continued simply cause 

 their complete withdrawal and the closing of the shell with the foot 

 retracted. Continued stimulation, especially when accompanied 

 with or preceded by the stimulation of the tenacles around the 

 ventral mantle opening, and with the animal held with the anterior 

 end pointing down, cause the foot to be protruded, swelled at 

 the end and withdrawn in a manner similar to the movements of 

 burrowing. If the stimulations are continued, these movements 

 are usually repeated until a dozen or more complete thrusts and 

 withdrawals have been made. 



The foot, which is also periodically exposed to external stimuli, 

 is likewise very sensitive. Stimulating its surface causes its with- 

 drawal but it is never thrown into burrowing activity as the result. 

 When the foot is withdrawn, the collar closes in over it, and if 

 stimulation has been more than slight the siphons are retracted 

 and the shell is closed. 



From the foregoing it will be seen that a reasonably strong stimu- 

 lation of any portion of the exposed animal affects it as a whole 

 and may cause either complete retraction into the shell and the 

 contraction of the muscles that close the shell, or may institute 

 movements that are intended for escape into the mud. The latter 

 movements seem never to be caused by the stimulation of either 



