ORGANS OF VISION. 



481 



(Lowne) in the larvae of some Diptera (Eristalis), and also in 

 some Chilognatha. 



(b} A type of simple eye found in some Chilopoda, and in 

 some Insect larvae (Dytiscus, etc.) (fig. 282), the parts of which 

 are entirely derived from the epidermis. There is present a 

 lens (/) formed as a thickening of the cuticle, a so-called vitreous 

 humour (gl) formed of modified hypodermis cells, and a retina 

 (r) derived from the same source. 

 The outer ends of the retinal cells 

 terminate in rods, and their inner 

 ends are continuous with nerve- 

 fibres. 



(c) A type of simple eye found 

 in the Arachnida, and apparently 

 some Chilopoda, and forming the 

 simple eyes of most Insects, which 

 differs from type (a) in the cells 

 of the retina forming a distinct 

 layer beneath the hypodermis ; the 

 latter only obviously giving rise to 

 the vitreous humour. 



The development of the simple eyes has not yet been 

 studied. 



The simple eyes so far described are always placed on the 

 head, and are usually rather numerous. 



(2) Compound eyes. Compound eyes are almost always 

 present in the Crustacea, and are usually found in adult Insects. 

 In both groups they are paired, though in the Crustacea a median 

 much simplified compound eye may either take the place of the 

 paired eyes in the Nauplius larva and lower forms, or be present 

 together with them during a period in the development of higher 

 forms. 



The typical compound eye is formed (fig. 283) of a series of 

 corneal lenses (c) developed from the cuticle; below which 

 are placed bodies known as the crystalline cones, one to each 

 corneal lens ; and below the crystalline cones are placed bodies 

 known as the retinulae (r) constituting the percipient elements 

 of the eye, each of them being formed of an axial rod, the 

 rhabdom, and a number of cells surrounding it. 



B. in. 31 



FIG. 282. SECTION THROUGH 



THE SIMPLE EYE OF A YOUNG DYTIS- 

 CUS LARVA. (From Gegenbaur ; after 

 Grenacher.) 



/. corneal lens ; g. vitreous hu 

 mour ; r. retina ; o. optic nerve ; h. 

 hypodermis. 



