Eyes of Mollusca and Arthropods. 725 



other words the argentea, iustead of absorbing, like pigment, the rays 

 of light, reflects them, so that the intensity of the light irritation is doub- 

 led. The aigentea has, then. a function not necessaiy for the percep- j 

 tion of light and it may be absent or present according to the habits ' 

 of the animai. Nocturnal ones, which must make the most of the little j 

 light at their disposai, are conscquently well provided with the reflect- 

 ing surfaees. Those lishes which are best })rovidcd with reilectors are 

 either nocturnal, or live in deep water where there is little light. 



Many animals, as Cardium^ Area and Pecten^ are provided with 

 numerous eyes, rather for the reception of great quantities of light than 

 for fine destiuctions of its gradatiou. Such animals are likewise pro- 

 vided with reflecting surfaees for collecting and economizing the light. 



In the very simple, isolated omniatidia, the central cell is colorless, 

 and beneath the retinidium is a layer of refractive granules which , as 

 we may easily see upon a superficial examination, act as minute reflec- 

 tors to the rays of light which have passed the overlying retinidia. We 

 have here the explanation of the almost universal absence of pigment 

 in the retinophorae , and the predomiuance which their retinophorae 

 have acquired over those of the pigment cells. 



The brilliant sparkle of the faceted eyes of Area is due to the layer 

 of refractive globules at the base of the retinophorae. In Pecten the 

 rods are reversed, and it is therefore impossible for the retinophorae to 

 act as reflectors; therefore a single but large, cellular reflector, the ar- 

 gentea, is formed back of the rods. In this case, the argentea has be- 

 come so perfectly developed in its reflective powers and curvature that 

 rays of light, forming inverted Images upon the rods, are perfectly re- 

 flected by the argentea , and again brought to a focus so as to form a 

 second image upon the first (PI. 32, fig, 149). It is extremely 

 probable that the argentea of the Vertebrate eye fulfils the same 

 function as that of Pecten. 



Just as the formation of inverted Images upon the retina is an in- 

 cidental result of the best concentrationof light upon a gi ven surface , 

 so is the formation of a second image by the argentea an incidental 

 result of the most perfect reflection of the light. It is also worthy of 

 notice that both the reflected and incident rays are approximately parallel 

 with the rods through which they pass, and therefore parallel with the 

 axial fibres, but at right angles to the fibrillae of the retinidia. 



In the Insecta andCrustacea: the frequent presence of the swelling 

 at the base of the retinophorae, or the pedicel, composed of alter- 

 nating layers of differently refractive plates, the arrangement of whose 



