37o THE CRAYFISH CHAP. 



arily complex and cannot be described in detail here. They 

 partly aid the dorsal muscles in extending the abdomen, but 

 are chiefly important in producing an approximation of the 

 sterna, and thus in flexing the abdomen. The ventral 

 muscles are, like the dorsal, traceable into the thorax, where 

 they arise from the endophragmal system (p. 363). 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 back- 

 wards with great rapidity. There is also a paired rotator 

 of the abdomen. 



It will thus be seen that the body-muscles of the crayfish 

 cannot be said to form a layer of the body-wall, as in the 

 earthworm (Fig. 81), but constitute an immense fleshy mass, 

 filling up the greater part of the body-cavity (see p. 373), 

 and leaving a very small space around the enteric canal. 



In the limbs the essential arrangement of the muscles in 

 relation with the joints in Arthropods is more easily seen 

 (Fig. 92) : 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 (ex/) 

 on the extensor, the larger (ft) on the flexor 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. 



The digestive organs are constructed on the same general 

 plan as those of the earthworm/* but present many striking 

 differences. The mouth lies in the middle ventral line of 

 the head and is bounded in front by a shield-shaped process, 

 the labrum, at the sides by the mandibles, and behind by a 

 pair of delicate lobes, the paragnatha. It leads by a short 

 wide gullet (Fig. 93, <K) into a capacious gizzard, usually 



