
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 chele for 
food, seizing food in the chelz 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 antennz, 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 
