110 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



tion and movements are largely determined b}' the in- 

 fluences received through the feelers and the ej^es. These 

 receive their nerves from the cerebral ganglia ; and, as 

 might be expected, when these ganglia are extirpated, 

 the crayfish exhibits no tendenc}' to get away from the 

 light, and the feelers maj^ not only be touched, but 

 sharply pinched, without efi^ect. Clearly, therefore, the 

 cerebral ganglia serve as a ganglionic centre, by which 

 the afferent impulses derived from the feelers and the 

 eyes are transmuted into efferent impulses. Another 

 very curious result follows upon the extirpation of the 

 cerebral ganglia. If an uninjured crayfish is placed upon 

 its back, it makes unceasing and well-directed efforts to 

 turn over ; and if everything else fails, it will give a 

 powerful flap with the abdomen, and trust to the cliai^ter 

 of accidents to turn over as it darts back. But the 

 brainless crayfish belaaves in a very different way. Its 

 limbs are in incessant motion, hut they are " all abroad ; " 

 and if it turns over on one side, it does not seem able 

 to steady itself, but rolls on to its back again. 



If anything is put between the chelfe of an unmjured 

 crayfish, while on its back, it either rejects the object at 

 once, or tries to make use of it for leverage to turn over. 

 In the brainless crayfish a similar operation gives rise to 

 a very curious si^ectacle.* If the object, whatever it be 



* My attention was first drawn to these phenomena by my friend 

 Dr. M. Foster, F.R.S., to whom I had suggested the desirableness of 

 an experimental study of the nerve physiology of the crayfish. 



