98 BRANCH ARTHROPODA 



esophageal ganglion or " brain," from which a nerve cord 

 passes on each side, uniting below the esophagus in a double 

 (apparently single) ventral nerve cord (Fig. 69), which ex- 

 tends the whole length of the body and connects the ganglia. 

 We should expect to see a ganglion for each segment, but there 

 are but thirteen ganglia, some of these being formed from a 

 union of several. On each side of the esophagus is a large gang- 

 lion; there are five more ganglia in the thorax and six in the 

 abdomen. 



The stalked eyes are compound, being composed of many 

 facets. The sense of touch is well developed. The surface 

 of the body is sensitive and the antennae are especially adapted 

 for " feelers." The sense of smell is thought to be seated in the 

 hairs or setse on the antennules. 



Multiplication. — In the spring the little brown or black eggs 

 may be found attached to the swimmerets of the female. 

 For some time the young crayfishes, by means of hooks on 

 their claws, cling to the swimmerets of the mother for protec- 

 tion. 



Molting. — The young crayfish, which is of much the same ap- 

 pearance as the adult, grows rapidly. Since the shell is hard 

 the animal cannot enlarge except when it sheds its skin or molts, 

 which it does periodically. Even the hard lining of the stomach 

 is cast. Growth takes place while the new skin or shell is form- 

 ing. 



Restoring Lost Parts. — Crayfishes have the power of growing 

 a new leg to replace one broken off by accident or in a fight. 

 This accounts for the unequal size of the chelipeds in many 

 specimens. 



Habits. — Crayfishes inhabit fresh-water streams and ponds, 

 lurking under stones or ledges in daytime and feeding at night. 

 When the streams dry up, they dig holes in the ground until 

 they reach water. These are sometimes many feet deep. 

 The clay dug out around the hole is deposited in a " chimney." 

 In these holes they probably live till the next spring. Some 

 species do not live in the water, but burrow in the soft moist 

 earth, and one species has been found in the sea. Crayfishes 

 are omnivorous, eating anything they can get, but they prefer 

 worms, insect larvae, and snails. 



