22 



General Part. 



of which are transparent, whilst they are strongly pigmented within, 

 as is the case also with the retinal cells, their outer rod-like ends 

 excepted. The cuticle covering the eye is thickened like a lens. The 

 next stage is represented by those eyes, in which the retinal cells have 

 lost all direct connection with the skin (Pig. 21, B, ocelli of Insects, 

 Insect-larvae and Spiders). Below the lens there is, therefore, a layer of 

 transparent cells, continuous with the adjacent epidermal cells, and 

 corresponding with those which in the simple type are pushed in 

 above the retina. The compound eye (Pig. 21, C,D), occurring 

 in most Insecta and Crustacea, consists of a large number (as many as 

 several thousands) of simple eyes, closely packed together, and each 



Fig. 21. Different kinds of Arthropod eyes, diagrammatic. A, B ocelli, C, D 

 ommatidia from a compound eye. n optic nerye, r retina, « rod of retinal cell, g ritreous 

 body, i: crystalline cone, I lens, c limit of the general cuticle, ep epidermal cells. — Orig. 



possessing a structure similar to that just described. The compound 

 eye is, however, peculiar, in that the eye-elements (ommatidia) of 

 which it consists are very narrow and elongate, and each one consists 

 of only a few retinal (6 — 8) and vitreous cells. As in the ocellus, 

 each retinal cell ends above or within in a rod-like transparent piece ; 

 there is frequently, also, in each vitreous cell, a peculiar refractile 

 body, which is closely united with its fellows of the neighbouring 

 cells to form the so-called crystalline cone (D, fc) . 



Essentially different from the forms of eye already described, 

 where the retina invariably represents a specially developed area of 

 the epidermis, is the visual organ of Vertebrata, whose retina is 

 not epidermic, but a specially modified portion of the brain. These 

 eyes will be described in the section Vertebrata. 



Besides the types of optic organ mentioned here, there are stiU others found 

 in certain animals, but they are simply specialised portions of the epidermis 

 (Platyhelminths, LameUihranchs, etc.). 



It should be noticed here, that not a few animals which have no 

 special visual organs, are, nevertheless, sensitive to light ; this is true. 



