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 chele still perform 
defensive movements, but the feeding movements no longer 
occur. Cutting the nerve cord further back interferes still 
more with the power of codrdinated 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 in 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 coérdinated; instead of a single adaptive reflex, we 
have a complex codperative response, such as occurs in 
walking, cleaning movements and mutual defense’ of ap- 
pendages which are seized. What a wonderful combination 
and coérdination of impulses! From the simple reflex of the 
isolated segmental 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- 
