II.] THE CRAYFISH AND LOBSTER. 185 



fringe the exopodite of the antennule, and are thought to 

 perform an olfactory function. 



The eyes are situated at the extremities of the eyestalks, 

 or ophthalmites, which represent the first pair of appendages 

 of the head. The rounded end of the eyestalk presents a 

 clear smooth area of somewhat crescentic form, divided into 

 a great number of small mostly four-sided facets. This area 

 corresponds with the cornea, which is simply the ordinary 

 chitinous layer of the integument become transparent. 

 The inner face of each facet of the cornea corresponds with 

 the outer end of an elongated transparent slightly conical 

 body the crystalline cone the inner end of which passes into 

 a spindle-shaped transversely striated body -the striated 

 spindle or rhabdome. The inner extremity of this again is 

 connected by a nerve fibre with the optic bulb, the dilated 

 gangliform termination of the optic nerve. The respective 

 striated spindles, and crystalline cones, thus radiate from the 

 outer surface of the terminal ganglion to the inner surface 

 of the cornea, and each is separated from its neighbour by 

 a nucleated sheath, parts of which are deeply pigmented. 

 Nothing is accurately known as to the manner in which the 

 function of vision is performed by the so-called compound 

 eye which has just been described. The inner and outer 

 faces of the corneal facets are flat and parallel. They 

 therefore cannot play the part of lenses ; and, if they could, 

 there is no trace of nerve endings so disposed as to be 

 affected by the points of light gathered together in the foci 

 of such lenses. Morphologically, the striated spindles and 

 their nerve fibres and probably the optic bulb itself wholly 

 or in part, are in many ways analogous to those elements of 

 the retina of the Vertebrata which make up the layers of 

 rods and cones and the granular layers. The aforemen- 

 tioned structures are, together with the optic nerve and 

 ganglion, derivatives of the epidermis ; in the vertebrate the 



