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 fluviatilis—The ophthalmic and antennulary somites 
(x 8). J, ophthalmic, and Z/,antennulary sternum; 1, articular 
surface for eyestalk; 2, for antennule; epm, epimeral plate ; 
pep, procephalic process ; 7, base of rostrum ; ¢, 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 pre-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 
