152 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



of the large extensor muscles of the abdomen is attached 

 close to it. 



The sternum and the shield-shaped epimeral plates 

 constitute a solid, continuously^ calcified, ventral element 

 of the skeleton, to which the posterior pair of legs is 

 attached ; and as this structure is united with the 

 somites in front of and behind it only by soft cuticle, 

 except where the shield-shaped plate is connected, by 

 the intermediation of the triangular piece, with the 

 epimeron which lies in front of it, it is freely movable 

 backwards and forwards on the imperfect hinge thus 

 constituted. 



In the same way, the first somite of the abdomen, 

 and, consequently, the abdomen as a whole, moves upon 

 the hinges formed by the union of the L-shaped pieces 

 with the triangular pieces. 



In the rest of the thorax, the sternal and the epimeral 

 regions of the several somites are all firmly united 

 together. Nevertheless, shallow grooves answering to 

 folds of the cuticle, which run from the intervals 

 between the articular cavities for the limbs towards the 

 tergal end of the inner wall of the branchial chamber, 

 mark off the epimeral portions of as many somites as 

 there are sterna, from one another. 



A short distance above the articular cavities a trans- 

 verse groove separates a nearly square area of the lower 

 part of the epimeron from the rest. Towards the 

 anterior and upper angle of this area, in the two somites 



