266 COlklPAEATIVE ANATOMY. 



an elevation of the cepHalotliorax. In tlie Pycnogonida 4 eyes occupy 

 a similar position. On tlie other hand they are reduced to 2 in many 

 Mites, and so also in the Tardigrada ; in many parasitic Mites they 

 have completely disappeared. The principal point in their structure 

 is the presence of a cornea-lens, which is ordinarily very large in each, 

 eye ; behind this is a layer of cells, which represents the vitreous 

 body, and to this the retina is attached. In the Aranea the retina is 

 formed in two ways, the eyes directed anteriorly differing in structure 

 from those which are turned upwards. That is to say, the retinal 

 cells of the former surround a small longitudinally bisected rod at 

 their anterior end (Epeira). 



§ 206. 



The compound eye is characterised by the above-mentioned 

 fusion (§ 204) of a number (7-4) of retinal cells into a structure 

 which surrounds the rhabdom — the "retinula" (Fig. 135, C r). 

 The eye is made up of these retinulje, each of which, is enveloped 

 in pigment. The multifid crystalline cone lies in front of the 

 retinula. Two of these structures are represented in Fig. C. The 

 crystalline cones may be made out in front of the retinula, and be- 

 hind the cornea-lenses (c). The whole arrangement is easy to under- 

 stand, when we derive it from the simple eye. A reduction of the 

 retinal elements of the simple eye gives rise to the retinula, and a 

 compound eye is formed by the gradual concrescence of a number 

 of simple eyes. Most of the Crustacea have eyes of this kind. In 

 the Cladocera the movable eye (Fig. 136, oc) lies in a cavity roofed 

 over by the integument. In the Lfemodipoda also the cuticular 

 layer of the integument seems to take no part in forming the eye. 



On the other hand in the Phyl- 

 lopoda we meet with a faceting 

 of the inner surface of the cuticle 

 covering the eye, the facets corre- 

 sponding to the crystalline cones. 

 In the Isopoda the compound eye 

 still consists of a number of simple 

 eyes, distinctly separated from one 

 another. When a number of these 

 structures, which form the end- 

 Fig. 135. A Diagrammatic section Organs of an optic nerve, are closely 

 through a compound Arthropod. Connected together, a process, con- 

 eye, n Optic nerve. </ Its ganglionic vextowards the outerface,is formed, 



swelhngs. r Eetinnlse. c Faceted .r^ ■ f „],,-„l, ,•„ rlp^p^rlp^f on 



cuticular laver, each facet of ^vhich '^f® Size 01 WJllcn IS Uepenaent on 

 forms a cornea-lens. B A few cuticular the number 01 the "retmulfe" (Fig. 



facets, seen from the surface. C Two 135). The chitinous Covering of the 

 rctinul8e,r, with their cornea-lenses, c. ^.j^oig ^^^ -g g-^j^g^. smooth on its 



surface, and only forms on its inner 

 face curves, which correspond to each of the crystalline cones, or it 

 forms convexities for each of the separate crystalline cones, or even 



