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AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



while their slender apices abut against the optic ganglion. Each 

 ommatidium is surrounded by a pigmented sheath, and consists of 

 the following parts, commencing from the outside : (a) A corneal 

 lens, (b) modified epidermic corneal cells by which this is secreted, 

 (c) a group of four crystal cells, which secrete refracting structures 

 constituting a crystal cone, (d) a retinula, constituted by seven 

 slender cells which surround a transversely ridged rhabdom (striated 



Fig. 30. CRAYFISH EYE (after Carriere). Enlarged. A, Longitudinal 

 section; cu, cuticle, and ep, epidermis, of eyestalk; co, cornea; vt, 

 vitreous body, the dark lines in its outer part indicate pigment; 

 rt, retinulse imbedded in pigment, and bounded internally by a base- 

 ment membrane (indicated by a strong line) ; n, nerve-fibres running 

 from retinulse to the optic ganglion, g. B, An ommatidium ; co, corneal 

 lens; co.c, corneal cells; cr.c, crystal cells ; cr, crystal cone; rt, retinula, 

 enclosing rhabdom, rh ; 6, basement membrane ; n, nerve-fibres. 



spindle), secreted by them and continuous with the crystal cone. 

 Fibres of the optic nerve are probably continuous with the cells 

 of the retinula, which appear to be the end-organs for sight, while 

 the corneal lens, crystal cone, and rhabdom are refracting elements. 

 The crystal cones form collectively a " vitreous body." 



The action of the eye is best explained by the theory of "mosaic 

 vision," according to which each ommatidium is only affected by 

 rays which correspond with it in direction, so that if the animal 



