88 OBGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEBAL. 



action of light is of the highest interest, but it cannot be taken as 

 proving a direct participation of the visual purple in the visual 

 process, inasmuch as the visual purple is not present in those parts 

 of the eye in which alone a distinct image is formed, viz., the macula 

 lutea and, generally, the outer segments of the cones. 



The pigment of the eye seems to be of importance for absorbing 

 the superfluous rays of light which would be injurious to the per- 

 ception of an image. It is distributed partly immediately outside 

 the retina, forming the choroid coat of the eye, which extends also 

 inwards between the individual retinal elements ; and partly in front 

 of the lens, giving rise to a transversely placed curtain, the iris 



which is pierced by 

 an opening, thepivpil, 

 capable of contrac- 

 ting and dilating. In 

 the higher grades of 

 development the 

 whole eye is, as a 

 rule, enclosed in a 

 hard, connective tis- 

 sue coat, the sclerotic, 

 and thus marked oft' 

 as an eye bulb. 



The arrangements 

 by which the shining 

 points of an object 

 act in regular ar- 



Fio 85. Diagrammatic representation of the compound eye 

 of aLibellula. C, cornea; K, crystalline cone ; P, pigment ; 

 S, nerve rods of retina ; Fb, layer of fibres : Gz, layer of 

 ganglion cells ; Rf, retinal fibres ; Fk, crossing of fibres. 



regular 

 rangement on corre- 



sponding points of the 

 optic nerve and so render possible the perception of an image vary, 

 and are closely dependent upon the whole structure of the eye. 

 Leaving out of consideration the simplest eyes, such as we find in 

 Worms and the lower Crustacea, two types of eye are to be distin- 

 guished. * 



1. The first form occurs in the so-called facetted eyes* (figs. 85 & 

 86) of Arthropods (Crustacea and Insects). The retina of such eyes 

 has a hemispherical form, the convex surface being directed out- 

 wards, and consists of large compound nerve rods, the retinulaj 



* See Joh. Miiller, "Zur vergleichenden Physiologic dcs Gesiehtssinnes," 

 Leipzig, 1826. H. Grenacher, " L'ntersuchuugen iiber das Sehorgan dcr Arthro 

 poden," Gottingcn, 1879. 



