II.] THE CRAYFISH AND LOBSTER. 1 85 



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 eomea, 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 relatively long and slender coimective rod, by which it is 

 united with a spindle-shaped transversely striated body — 

 the striated spiiidle. 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, coimective 7'ods and crystalline 

 co7ies, 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. Morpho- 

 logically, 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 



