150 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
men (figs. 15 and 36), it will be obvious that the tergal 
and the sternal regions of the two answer to one another ; 
while the branchiostegites correspond with greatly de- 
veloped pleura; and the inner wall of the branchial 
chamber, which extends from the bases of the appendages 
to the attachment of the branchiostegite, represents an 
immensely enlarged epimeral region. 
On examination of the sternal aspect of the cephalo- 
thorax the signs of division into somites become plain 
(figs. 3 and 89, A). Between the last two ambulatory 
limbs. there is an easily recognisable sternum (XIV.), 
though it is considerably narrower than any of the 
sterna of the abdominal somites, and differs from them 
in shape. 
The deep transverse fold which separates this hinder- 
most thoracic sternum from the rest of the sternal wall 
of the cephalothorax, is continued upwards on the inner 
or epimeral wall of the branchial cavity ; and thus the 
sternal and the epimeral portions of the posterior thoracic 
somite are naturally marked off from those of the more 
anterior somites. 
The epimeral region of this somite presents a very 
curious structure (fig. 88). Immediately above the ar- 
ticular cavities for the appendages there is a shield- 
shaped plate, the posterior, convex edge of which is 
sharp, prominent, and setose. Close to its upper 
boundary the plate exhibits a round perforation (plb.), 
to the margins of which the stem of the hindermost 
