132 ELEMENTARY BIOLOGY. [XII. 



three pair of prseoral ganglia having coalesced into the brain ; 

 and the five which follow the mouth having united into the 

 suboesophageal mass. 



The only organs of special sense which are recognizable 

 in the Lobster and Crayfish are eyes and auditory organs. 



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 four-sided facets. This area cor- 

 responds 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 relatively long and slender connective rod, by which it is 

 united with a spindle-shaped transversely striated body 

 the striated spindle. The inner extremity of this again is 

 connected with the convex surface of the dilated cushion- 

 shaped ganglionic termination of the optic nerve. The respec- 

 tive striated spindles, connective rods 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 cones, connective rods 

 and striated spindles, are in many ways analogous to those 



