Nervous Systetn of the Razor-shell Clam 325 



tions. If now the cerebral commissure is also cut and the left 

 side of the collar is then stimulated, or for that matter if the left 

 cerebral ganglion is stimulated directly, the foot responds with 

 convulsive contractions only after considerable delay. In some 

 cases no response could be obtained. In this experiment im- 

 pulses must be passed from the left cerebral around through the 

 visceral to the other cerebral and from this to the pedal before the 

 foot was stimulated. The path cannot be considered usual and 

 the action is both delayed and modified. It is interesting to find 

 that the centers are able to respond at all in this roundabout and 

 unusual way. 



SUMMARY 



1 This form is very satisfactory for experimental study of the 

 physiology of the nervous system because of its shape and activity, 

 and the ease with which its nervous system may be seen and oper- 

 ated upon. 



2 Continued stimulation of any portion of the body will in 

 time have its effect on all of the gangha. 



3 Certain organs hke the siphons, collar and foot, may be so 

 gently stimulated as to cause them to be withdrawn without dis- 

 turbing organs that receive their nerves from other gangha. 



4 The relation of ganglia of a pair is quite intimate. Stimu- 

 lating nerves connected with one causes organs connected with 

 both to respond promptly. 



5 Association fibers by which ganglia communicate with each 

 other are found only in commissures and connectives. Although 

 the anterior paUial nerves are united so that a connection is formed 

 between the cerebral gangha, and the circum-paUial nerves connect 

 the cerebral and visceral gangha of corresponding sides, there is no 

 evidence that the ganglia are able to communicate through them, 



6 Both cerebral and visceral ganglia are provided with sensory 

 and motor cells. The pedal gangha are apparently dependent 

 upon the cerebral for initiative. When the pedal ganglia are 

 isolated from the others, stimulation of the surface of the foot 

 causes only local responses due to the direct stimulation of muscle 

 fibers. It would seem that the sensory neurones have neither end- 



