ICO THE MORPHOLOGY OF THE COMMON CRAYFISH. 



coalesced anterior thoracic and posterior cephalic ganglia 

 situated beneath them. Strong processes are given off 

 from their anterior and outer angles, which curve round 

 the tendons of the adductor muscles of the mandibles, and 

 give attachment to the abductors. 



In front of the mouth there is no such endophragmal 

 system as that which lies behind it. But the anterior gas- 

 tric muscles are attached to two flat calcified plates, which 

 appear to lie in the interior of the head (though they are 

 really situated in its upper and front wall) on each side 

 of the base of the rostrum, and are called the procephalic 

 'processes (figs. 40, 43, p.cp). Each of these plates con- 

 stitutes the posterior wall of a narrow cavity which opens 

 externall}^ into the roof of the orbit, and has been regarded 

 (though, as it appears to me, without sufficient reason) as 

 an olfactory organ. I am disposed to think, though I 

 have not been able to obtain complete evidence of the 

 fact, that the procephalic processes are the representa- 

 tives of the " procephalic lobes " which terminate the 

 anterior end of the body in the embryo crayfish. At 

 any rate, they occupy the same position relatively to the 

 eyes and to the carapace ; and the hidden position of 

 these processes, in the adult, appears to arise from the 

 extension of the carapace at the base of the rostrum 

 over the fore part of the originally free sternal sm-face of 

 the head. It has thus covered over the procephalic 

 processes, in which the sternal wall of the body termi- 

 nated; and the cavities which lie in front of them are 



