INSTINCT 101 



as locomotion by walking and swimming, darting back upon 

 the approach of danger, seeking dark and protected situ- 

 ations, rearing up when threatened and holding the claws 

 in a position for defence, withdrawing movements, moving 

 toward certain odors and feeling about with the chelae for 

 food, seizing food in the chelaB and passing it to the mouth, 

 chewing and swallowing, rejecting objects from the mouth, 

 and a number of others. The crayfish may form associa- 

 tions to a limited degree, but if it had to rely entirely on its 

 congenital endowment of instincts it would probably get 

 through the world almost as successfully as it does with its 

 modicum of intelligence. 



The relation of instincts and reflexes in the crayfish has 

 been studied with considerable thoroughness by means of 

 operations on the nervous system. The nervous mechanism 

 of the crayfish consists of a brain which gives branches to 

 the eyes, first and second antennae, and anterior part of the 

 thorax; a ventral nerve cord consisting of a double chain of 

 ganglia which is connected with the brain by commissures 

 passing around the esophagus; and a small visceral system. 

 The ganglia of the ventral nerve cord are connected by 

 cross commissures as well as the longitudinal ones which 

 form the larger part of the nerve chain, and they give off 

 nerves which are distributed to the segments in which 

 they lie. Typically there is a pair of ganglia in each seg- 

 ment of the body, but anteriorly the ganglia belonging to 

 the segments bearing the mouth parts have become fused 

 into a single sub-esophageal ganglion which supplies these 

 appendages. The brain may be regarded as a nerve center 

 homologous with the ganglia of the ventral nerve cord, but 

 like the subesophageal ganglion, it is formed of more than 

 one pair of ganglia which have been fused together. 



Experiments on the crayfish show very clearly that the 



