722 Gil man A. Drew 



pedal ganglia are deficient in originating power, and that when the 

 pedal are separated from the cerebral ganglia the foot will not by 

 itself, or as the result of surface stimulation, execute movements 

 more than are to be accounted for by the local direct stimulation 

 of muscle fibers. To make sure that the movements did not come 

 from stimulating the gangha, they were entirely removed and still 

 stimulation of the surface of the foot gave exactly similar contrac- 

 tions. 



From the foregoing experiments it would seem that both cerebral 

 and visceral ganglia are able to receive impulses and to direct the 

 movements of certain organs with which they are connected, when 

 they are entirely separated from the other gangha, and that the 

 pedal cannot act by themselves. This is somewhat surprising 

 but possibly the habits of the animal may account for it. Appar- 

 ently the cerebral ganglia are central for the nervous system. 

 This is indicated by their connections with the other ganglia as well 

 as by experiments. They would then have charge of the special 

 activities of the whole animal, as well as of the special organs in 

 their immediate vicinity which they supply with nerves. The 

 visceral ganglia govern over organs that are in constant activity, 

 organs that must give warning of the approach of enemies. They 

 must give warning to the cerebral ganglia and then be ready to 

 cover the retreat by closing and withdrawing the siphons and con- 

 tracting the posterior margins of the mantle and posterior adduc- 

 tor muscle. The cerebral ganglia may now take charge of the 

 advance with the aid of the efficient accessory pedal ganglia. 

 They have little more to do during periods of burrowing. Dur- 

 ing the life of the animal the foot is not in a position that it will 

 be called upon to give such an alarm very often if ever, and dur- 

 ing burrowing the cerebral ganglia can devote nearly their whole 

 attention to the process. It is desirable in directing a retreat of 

 this kind to have a general in charge that is in constant communica- 

 tion with outposts that may give information regarding the enemy. 

 The cerebral ganglia have such communications; the pedal gan- 

 glia only indirectly. 



The cerebral ganglia are the only pair that are far enough apart 

 to allow the cutting of the connecting commissure without injuring 



