160 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, 48, p.cp). Each of these plates con- 
stitutes the posterior wall of a narrow cavity which opens 
externally into the roof of the orbit, and has been regarded 
(though, as it appears to me, without sufficient reason) as 
an olfactory organ. JI 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 surface 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 
