INSTINCT 103 



are soon rejected. The legs are almost incessantly engaged 

 in cleaning movements, picking at one another, and at the 

 abdomen and its appendages. 



With the nerve cord cut between the first and second 

 thoracic ganglia the responses of the parts in front of the cut 

 are little affected, but those of the last four pairs of thoracic 

 legs are much reduced in vigor. The chelae still perform 

 defensive movements, but the feeding movements no longer 

 occur. Cutting the nerve cord further back interferes still 

 more with the power of coordinated locomotion, although 

 the withdrawing and defensive movements still persist. In 

 fact, any pair of legs will perform these movements if the 

 cord is cut both in front of and behind the ganglion supply- 

 ing these legs with nerves. 



Each ganglion is a reflex center regulating the movements 

 of the appendages of the segment hi which it lies. Into it 

 impulses pass from the appendages and are sent out to 

 muscles which effect the withdrawing or the defensive acts 

 in response to the outer stimulus. When several ganglia 

 are joined together, the impulses from the various append- 

 ages are coordinated; instead of a single adaptive reflex, we 

 have a complex cooperative response, such as occurs in 

 walking, cleaning movements and mutual defense~of ap- 

 pendages which are seized. What a wonderful combination 

 and coordination of impulses ! From the simple reflex of the 

 isolated segments] ganglion to the complex behavior of the 

 brainless crayfish, and from this to the still more complex 

 behavior of the normal animal there is a regular gradation. 

 Nowhere can we draw a sharp line between reflex behavior 

 on the one hand and instinct on the other. Both are based 

 on a wonderfully complex and beautifully organized nervous 

 mechanism. 



Among the most remarkable of the instincts of crusta- 



