370 THE CRAYFISH CHAP. 



extraordinarily 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 

 backwards 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 (ext) 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, oe) 

 into a capacious gizzard, often spoken of as the stomach, 



