ARTHROPODA 



277 



corneagen cells comes a bundle of two to five vitrellae or crystal cells, 

 grouped around a refractive body, the crystalline cone, which they 

 have secreted. The vitrellae taper inwards and their apex is clasped 

 by a second bundle of cells, four to eight in number, which together 

 form the retiniila. Like the vitrellae the retinular cells secrete in the 

 axis of the ommatidium a refractive body. This is the rhabdom, and 

 is made up of rhabdomeres, one for each of the cells. Each retinular 

 cell passes at its base into a nerve fibre which pierces the basement 

 membrane of the eye and enters the optic ganglia. Around each 

 ommatidium, separating it from its neighbours, there are usually 



^opt.ga. 



opt.n. 



Fig. 201. The eye of Astacus. A, The left eye. B, A portion of the cornea 

 removed, to show the facets. C, A longitudinal section of the eye. m. muscles 

 which move the eye; n.fi. nerve fibres; omm. ommatidia; opt.ga. optic 

 ganglion; opt.n. optic nerve. 



pigmented cells, known as iris cells. The eyes of arachnids, other than 

 the lateral eyes of Limulus, simulate the ocelli of insects, but are 

 thought, from details of their structure, to have been formed by the 

 degeneration of compound eyes resembling the lateral eyes of 

 Limuliis. The median eye of the Crustacea (Fig. 215) is composed of 

 three cups, which may (some copepods) separate widely. The paired 

 eyes probably do not, as has been suggested, represent a pair of 

 appendages. The foremost, or preantennal, somite, to which they 

 would in that case belong, possesses, in Peripatus and as a rudiment 



