138 ELEMENTARY BIOLOGY. [CHAP. 



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 

 respective 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 per- 

 formed 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. Morphologi- 

 cally, the cones, connective rods and striated spindles, 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. These structures are properly 

 modifications of the epidermis ; inasmuch as the cerebral 

 vesicle, of which the retinal vesicles are outgrowths, are in- 

 volutions of the epidermis of the embryo, and, morphologi- 

 cally speaking, the free ends of the rods and cones of the 

 vertebrate eye are, as in the crustacean, turned outwards. 

 It seems probable, therefore, that the crustacean eye is to be 

 compared to the retina alone of the vertebrate eye, and that 

 vision is performed as it would be by the retina deprived of 

 its refractive adjuncts. 



The auditory organ of the Lobster and Crayfish is situated 

 in the basal joint of the antennule, on the dorsal surface of 

 which a small slit-like opening, protected by numerous 

 hairs, is to be seen. The chitinous layer of the integument 

 is invaginated at this opening, and thus gives rise to a small 



