106 W. J. DAKIN. 



general epithelium of the body-wall. The inner wall of this 

 optic vesicle is, however, also made up of a layer of cells, 

 which though forming a complete hollow sphere, are differen- 

 tiated in three regions, in structure and function. Those 

 cells immediately under the cornea just spoken of are low and 

 form a second and inner cornea. The cells lining the proximal 

 lialf of the optic vesicle are the retinal cells, and between this 

 area and the inner cornea the cells are again different and 

 contain pigment. 



There is only one series of recipient cells in the retina, and 

 they bear rods which resemble those of the Pecten eye and 

 contain a very distinct axial neurofibril. They are, howevei*, 

 directed towards the lens, that is, not inverted. The lens is 

 spherical and non-cellular, and another difference from the 

 eye of Pecten is produced by the presence of a vitreous body 

 between lens and retina. 



There are several interesting arthropod eyes that may be 

 briefly referred to. The ocelli of Cloeon (one of the May- 

 flies) are distinctly peculiar and are superficially rather like 

 the Pecten eye, but this resemblance is due to the dioptric 

 part of the eye, and not to the retina. We have again a 

 closed vesicle. The cuticle extends over the cornea, but 

 remains thin and does not form a corneal lens. The hypo- 

 dermis forms a cornea similar to that of Pecten. Under this 

 cornea and lying in the optic vesicle is a cellular lens 

 strikingly like that of Pecten and altogether unlike other 

 ai'thropod eyes. The retina is made up of two layers of cells, 

 but the distal ones are not visual and the proximal ones 

 forming the retina proper are not inverted. 



Another interesting arthropod eye is the ocellus of Agrion. 

 This bears some resemblance to the Pecten eye in the fact 

 that there are two series of recipient cells in the retina. 

 They are, howevei", not inverted. The distal part of the 

 optic vesicle is quite different, and the chitinous exoskeleton 

 or cuticle is thickened over the free surface, forming a 

 corneal lens. This is a monomeniscous arthropod eye there- 

 foi'e, and the arrangement of the retinal cells is interesting. 



