II. Organs. 5. Sense Organs, 



21 



the optic organ may come to be a closed capsule lying beneath it. 

 The side of the capsule away from the surface is thickened and 

 pigmented^ and forms an organ for the perception of light, the 

 r e t i n a,* whilst the opposite side is thin and transparent, as is also 

 the skin above it (the cornea) ; the cavity of the visual organ then 

 contains a jelly-like mass, the vitreous-body (Fig. 2U, 5, most 

 Gastropods, Chsetopods). In others again, the lens, secreted by the 



Different forms of optic organ, diagrammatic, n optic nerve, r retina, 

 ej) epidermis, g vitreous body, I lens. — Orig. 



front part of the capsule, floats in the vitreous humour, and may be 

 regarded as a specially modified part of that body (Fig. 20, 6, some 

 Ohsetopods and Grastropods). With the development of this lens the 

 optic capsule has become a true eye ; and as in the human eye a real 

 image may be formed upon the retina, whilst the simpler forms can at 

 most merely differentiate between light and darkness. 



In such eyes as are described above, the retina consists partly of pigmented 

 cells, which appear to be the true perceptive cells, and partly of colour- 

 less supporting cells, which apparently secrete the vitreous body. The 

 former frequently bear, on the ends towards the vitreous body, unpigmented, 

 transparent rods. 



The optic organs of Arthropods exhibit a peculiar structure. 

 In its simplest form (Fig. 21, A), peculiar to certain insect-larvae, the 

 arthropod eye appears as an invaginate area of the skin, passing 

 directly into the general epidermis : so far, therefore, it re- 

 sembles that represented in Fig. 20, 2. The retinal or perceptive 

 cells are, however, always distinguished by being removed from the 

 surface and covered by neighbouring epidermal cells, the outer portions 



* This name is vised to denote that part of the optic organ which is actually 

 sensitive to light. 



