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 

 (iigs. 3 and 39, A). Between the last two ambulatory 

 limbs there is an easily recognisable sternum (Z2F.), 

 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. 38). 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 



