XI 



PHYLUM ARTHROPOD A 



505 



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 endophragmal system : 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 



art m, 



: 



\ 



envm. 



B 



X 



!'!<;. 400. Four segments of abdomen of Crayfish in sagittal section, with muscles (diagram- 

 matic). A, extension; B, flexion; art. //<., nrf. m'., articular membranes; c. //<. central 

 muscles; </. m. dorsal muscle; ex. extensor slip of central muscle; c/ir. m. enveloping 

 muscle; fl.,fl.l, flexor slips; It. hinge; st. sternum ; tg. tergum. 



the abdomen upon the cephalothorax, causing the Crayfish ^to dart 

 backwards 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 Chsetopods, the abdomen 

 of Apus, &c., but constitute 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. 398), each podomere is acted upon by two 

 muscles situated in the next proximal podomere. These muscles 

 inserted, by chitinous and often calcified tendons, into the 



