330 



LESS. XXVII 



FIG. 84. Diagram illustrating the action of the abdominal muscles 

 in the Crayfish. A shows the position in extension, B in flexion. 



Four abdominal segments are shown in sagittal section : tg, terga ; 

 j/, sterna ; art. m, tergal articular membranes ; art. m', sternal articular 

 membranes ; h. hinges. 



The muscles are represented as narrow bands (comp. Fig. 86 for their 

 actual dimensions), and their arrangement is greatly simplified, d. m, 

 dorsal muscles ; c.m, central muscle giving off extensor slips (ex] ; 

 env. m, enveloping muscles continued into anterior (ft] and posterior 

 (ft'} flexor slips. 



(From Parker and Haswell's Zoology.) 



abdomen upon the cephalothorax, causing the crayfish to 

 dart backwards with great rapidity. 



