156 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



the eye-stalks and is united with adjacent parts only 

 by flexible cuticle, so that it is freely movable. This 

 represents the whole of the sternal region, and probably 

 more, of the ophthalmic somite. 



The sterna of fourteen somites are thus identifiable in 

 the cephalothorax. The corresponding epimera are 



Fig. 40. — Astacus fiuviatilu. — The ophthalmic and antennulary somites 

 ( x 3). /, ophthalmic, and II, antennulary sternum ; 1, articular 

 surface for eyestalk ; %, for antennule ; epm, epimeral plate; 

 pep, procephalic process ; r, base of rostrum ; t, tubercle. 



represented, in the thorax, by the thin inner walls of the 

 branchial chamber ; the pleura, by the branchiostegites ; 

 and the terga, by so much of the median region of the 

 carapace as lies behind the cervical groove. That part of 

 the carapace which is situated in front of this groove occu- 

 pies the place of the terga of the head ;' while the low 

 ridge, skirting the oral and prse-oral region, in which it 

 terminates laterally, represents the pleura of the cephalic 

 somites. 



The epimera of the head are, for the most part, very 

 narrow ; but those of the antennulary somite are broad 

 plates (fig. 40, epm.), which constitute the posterior 



