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 
