396 



ARTHROPODA. 



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 tJte 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 sim- 

 plified 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 

 retinula? (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. 



The crystalline cones are formed from the coalescence of cuticular deposits 



in several cells, the nuclei of which usually 

 remain as Semper'.s nuclei. These cells 

 are probably simple hypodermis cells, but 

 in some forms, e. g. Phronima, thei'e may 

 be a continuous layer of hypodermis cells 

 between them and the cuticle. In various 

 Insect eyes the cells which usually give 

 rise to a crystalline cone may remain 

 distinct, ami such eyes have been called 

 by Greiiacher aconous eyes, while eyes 

 with incompletely formed crystalline 

 cones are called by him pseudoconous 

 ey-s. 



The rhabdom of the retinulae is, like 

 the crystalline cone, developed by the 

 coalescence of a series of parts, which 

 are primitively separate rods placed each 

 in its own cell : this condition of the 

 retinulse is permanently retained in the 

 eyes of the TipuliJpe. 



The development of the compound eye has so far only been satis- 

 factorily studied in some Crustacea by Bobretzky (No. 367); by whom 

 it has been worked out in Palamion and Astacus, but more fully in 

 the latter, to which the following account refers: 



FIG. 282. SECTION THROUGH THE 

 SIMPLE EYE OF A YOUNG DYTISCUS 

 LARVA. (From Gegenbaur; after 

 Grenadier. ) 



1. corueal lens ; g. vitreous hu- 

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

 hypodermis. 



