53 i 



ZOOLOGY 



SECT. 



arb.m, 



dm. 



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

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

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



from the endophrag- 

 mal system (p. 529) : 

 their various parts 

 are connected by a 

 complex system of 

 fibres extending be- 

 tween 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 abdo- 

 men upon the cepha- 

 lothorax, causing the 

 Crayfish to dart back- 

 wards with great 

 rapidity. 



It will be seen that 

 the body-muscles of 

 the Crayfish cannot 

 be said to form a 

 layer of the body- 

 wall, as in Chseto- 

 pods, the abdomen 

 of Apus, &c., but con- 

 stitute an immense fleshy mass, filling up the greater part of the 

 body-cavity, and leaving a very small space around the enteric canal. 

 In the limbs (Fig. 440) each podomere is acted upon by two 

 muscles situated in the next proximal podomere. These muscles 

 are inserted, by chitinous and often calcified tendons, into the 

 proximal edge of the segment to be moved, the smaller on the 

 extensor (ext.), the larger on the flexor (fl.) side, in each case half- 

 way between the two hinges, so that a line joining the two muscular 

 insertions is at right angles to the axis of articulation. 



-d.m, 



c.r 



X 



FIG. 442. Four segments of abdomen of Crayfish in 

 sagittal section with muscles (diagrammatic'). A, 

 extension ; B, flexion ; art. m,, art. m'. articular mem- 

 branes ; c. m. central muscles ; d. m. dorsal muscle ; 

 ex. extensor slip of central muscle ; env. m. enveloping 

 muscle ; fl., fli, flexor slips ; ft. hinge ; st. sternum ; tg. 

 tergum. 



