VI ARTHROPODA: SENSE-ORGANS 



237 



plasm highly specialized for carrying out the primary function of the 

 eye, namely the conversion of light waves of the ether into living impulses. 

 These specially modified portions of cytoplasm, characterized by their 

 glassy transparency, are known as rods — from their shape in some of the 

 better-known types of eye. 



In the Arthropoda there exists great variety in details of eye structure, 

 but amongst this variety there stand out two main types of structure 

 (i) the simple or camera type of eye and (2) the " compound " or, better, 

 radiate type. 



The simple eye (Fig. 106, A) consists primarily of a biconvex thickening 

 of the cuticle — the lens (Z) — rclear and transparent and serving to condense 

 the light upon an underlying clump of sensory cells — the retina (r). 

 The latter is usually displaced inwards, so as to be in the neighbourhood 

 of the focus of the lens, by a mass of transparent cells constituting the 

 vitreous body (v). When the eye has its retina displaced inwards in this 

 fashion it no longer serves merely for the detection of light, and thus 

 distinguishing between light and dark, but provides the means for 

 producing a definite sensory picture of the external world. 



The radiate eye is of a fundamentally different type — characterized 

 above all by the fact that the retina is subdivided up into a number — 

 it may be a vast number, many thousands — of retinulae, commonly 

 arranged in radiating fashion owing to the fact that they are normal to 

 the surface of the head and that this surface is strongly convex. Each 

 retinula (Fig. 106, B) consists of a group or bundle of elongated sensory 

 cells (R), the portion of cytoplasm of each cell next the axis of the 

 bundle forming the rod (r) while the deep end of the cell is prolonged 

 into a nerve fibre (n.f). 



Between the outer ends of the retinulae and the cuticle with its 

 underlying layer of cells is the vitreous body. This typically undergoes 

 a subdivision similar to that of the retina, there being at the outer end 

 of each retinula a vitrella, a group of (commonly 4) vitreous cells (v). The 

 portions of these cells next the axis of the group are fused together and 

 of a glassy transparency, constituting the crystalline cone (c.c). 



Each vitrella is separated from its neighbours by cells (p) containing 

 black or dark brown pigment. These together constitute a tubular 

 channel through which the light rays reach the retinula — each retinula 

 receiving its light only through the one of these tubes which is in line 

 with itself. 



In the more highly developed radiate eyes the cuticle covering its 

 surface has undergone a subdivision corresponding with that of the 

 underlying structures, the portion overlying each vitrella having become 



