PHYLUM ARTHROPODA 



505 



the sterna, and thus Ilexes 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 pc .verful, and 

 produce, when acting together, a sudden and violent bending of 



!>. Four segments of abdomen of Crayfish in sagittal section, with muscles (diagram- 

 matic). A, extension; B, flexion; ,-f. m., art. ;//., articular membranes; c. m. central 

 muscl' Mi-sal muscle: ex. extensor slip of central muscle; em: m. enveloping' 



muscle ; /?., jU, flexor slips ; h. hinge ; si. sternum ; iff. tergum. 



the abdomen upon the cephalothorax, causing the Crayfish to dart 

 backwards with great rapidity. 



It will be seen that the -^ocj^-muscles of the Crayfish cannot be 



said to form a layer of the boay-wall, as in ChaBtopods, the abdomen- 



of Apus, &c., but constitute an immense fleshy mass, filling up the 



i.ter part of the body-cavity, and . leaving a very small space 



around the enteric carol 



In the limbs (Fig, 398), each podomere is acted upon by two 

 muscles situated in the next proximal podornere. - These muscles 

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



