CHAPTER IV. 

 BRANCH ARTHROPODA. 



CLASS CRUSTACEA. 

 Example The Crayfish. 



Occurrence. Crayfishes are found fairly abundant in 

 streams and ponds in many parts of the United States. 

 They may be seen crawling about on the muddy bottom, 

 but, being nocturnal in their habits, they usually escape 

 observation during the daytime by hiding in holes, under 

 stones, and especially under ledges of rocks, overhanging 

 banks, or where the stream has washed out the soil about 

 the roots of trees standing on the banks. 



Crayfish Holes. Crayfishes are also found in holes 

 which they dig, usually in low ground. When the water 

 dries up from the ponds and creeks, crayfishes often dig 

 down to water, and, at this season, live in these holes. 

 The holes are frequently many feet deep. The soil and 

 clay are brought up in pellets, and with these a "chimney" 

 is built up around the mouth of the hole. 



How the Crayfish Walks. The crayfish walks by means 

 of the last four pairs of thoracic legs. Each of these legs 

 has seven segments ; and the successive joints admit of 

 motions in different planes, so that the whole appendage 

 has great freedom and variety of movement. The cray- 

 fish usually walks slowly, holding out the big pinchers in 

 front. It can, however, walk sideways or backward. When 

 taken out of water the crayfish walks with a heavy, awk- 



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