THE CRAYFISH AND ARTHROPODS IN GENERAL 



209 



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Muscular System. The principal muscles in the body of the 

 crayfish are situated in the abdomen, and are used to bend that 

 part of the animal forward upon 

 the ventral surface of the thorax, 

 thus producing backward loco- 

 motion in swimming. Other 

 muscles extend the abdomen in 

 preparation for another stroke. 

 Figure 100 shows a cross section 

 of an abdominal segment. The 

 powerful flexor muscles (FM) 

 occupy almost the entire abdomi- 

 nal space. In the dorsal region 

 are the less powerful extensor 

 muscles (EM). Other muscles of 

 considerable size are situated 

 within the tubular appendages, 

 especially the chelipeds. Figure 

 105 shows how the muscles of a 

 walking leg are arranged. A 

 comparison of the skeleton and 

 muscles of the crayfish with those 

 of man is interesting. The skele- 

 ton of the crayfish is external 

 and tubular, except in the ven- 

 tral part of the thorax (Fig. 101, 

 39). The muscles are internal, 

 and attached to the inner surface 

 of the skeleton. In man, on the 

 other hand, the skeleton is in- 

 ternal and the muscles external. 



Reproductive System. Cray- 

 fishes are normally dioecious, 

 there being only a few cases on 

 record where both male and 

 p 



FIG. 105. Part of the leg of a 

 crayfish showing muscles. 

 (From Parker and Haswell.) 



