Nervous System of the Razor-shell Clam 321 



amount of stimulation of the surface of the foot, electrically, chem- 

 ically or mechanically results only in the contraction of muscle 

 fibers in the immediate vicinity of the point of stimulation. The 

 foot never makes a movement as a whole and will remain motion- 

 less for hours, probably until it dies. This seems to mean either that 

 there are no motor cells in the ganglia or that the sensory fibers 

 have no endings or collaterals in the pedal ganglia but are continued 

 directly through these ganglia to the cerebral ganglia. I am inclined 

 to beheve that motor cells are present in the pedal ganglia and that 

 the sensory fibers pass directly through them without endings or 

 collaterals, for the following reasons: (i) Microscopically the 

 ganglia show an abundance of ganglion cells and it seems more 

 reasonable to beheve that, in such a muscular organ, they are 

 not all sensory, especially as the action of sensory cells so placed, 

 if motor are not present, would have to be referred to the cerebral 

 gangha before movement could be effected. (2) When the cere- 

 bro-pedal connectives are cut the foot responds with contractions. 

 These have the character of tetanic contractions that would more 

 probably come from the action of disturbed nerve cells than from 

 the single stimulus caused by cutting motor fibers. If such move- 

 ments could be caused by the stimulation due to cutting fibers 

 only, then the cutting of the pedal nerves (below the pedal ganglia) 

 should cause them, but beside the single twitch caused at the 

 instant of cutting no movements follow this operation. (3) If 

 one of the cut cerebro-pedal connectives is stimulated, the foot as 

 a whole, both sides, responds, apparently with a complete, normal 

 contraction. The course of the fibers in the ganglia have not been 

 traced, but the effect is not what we would expect if the action is 

 the result of the stimulation of only half of the motor fibers that go 

 to the foot. It is much more easily explained by supposing that 

 impulses have been sent to association cells which cause the motor 

 cells of the foot, contained in the pedal ganglia, to act. Stimula- 

 tion of the nerves that leave one of the pedal ganglia, after the 

 pedal ganglia have been removed, does not cause complete contrac- 

 tion of the whole foot, as it should if the ganglia themselves have 

 had no effect. 



Whatever the arrangement, there can be no question that the 



