XI 



PHYLUM ARTHROPODA 



547 



muscle must obviously tend to approximate the terga, and so aid 

 the dorsal muscles in extending the abdomen. Around this central 

 muscle is wrapped, in each segment, a band of muscle (env. ra.) in 

 the form of a loop, the outer limb of which turns forwards and is 

 inserted into a sternum, while the inner limb turns backwards and 

 is inserted into another and more posterior sternum. The con- 

 traction of this enveloping muscle produces an approximation of 

 the sterna, and thus flexes the abdomen, the central muscle always 



dm. 



FIG. 434. Fourisegments of abdomen of Crayfish in sagittal section, with muscles (diagram- 

 matic). A, extension ; B, flexion; art. m., art. m'., articular membranes; r. HI. central 

 muscles ; d. m. dorsal muscle ; ex. extensor slip of central muscle ; env. m. enveloping 

 muscle ; fl.,fl. 1 , flexor slips ; h. hinge ; st. sternum ; tg. tergum. 



keeping the middle of the loop .in place. The ventral muscles 

 are, like the dorsal, traceable into the thorax, where they arise 

 from the endophragmal system (p. 542) : their various parts are 

 connected by a complex system of fibres extending between the 

 central and enveloping muscles, and connecting both with their 

 fellows of the opposite side. The flexor muscles are immensely 

 powerful, and produce, when acting together, a sudden and violent 

 bending of the abdomen upon the cephalothorax, causing the 

 Crayfish to dart backwards with great rapidity. 



N N 2 



