164 



ANIMALS 



381. Just inside the pupil lies a double convex lens. It 

 originates from the epidermis by an infolding. It is the densest 

 organic structure of the body. It is librous in structure and of 

 glassy transparency. The lens is enclosed in a capsule, which 

 is attached by means of fibres to the muscular ciliary body, a 

 portion of the choroid layer. 



382. The large central cavity of the eye is filled with a trans- 

 parent jelly, the vitreous humor. The smaller space in front 

 of the iris contains a more fluid, aqueous humor. 



>.. 



B.. 



Fig. 82. — Diagrams to show how the concave and convex arrangement of 

 the sensory elements in invertebrate eyes serves to indicate the direction of 

 the hght rays. A, The concave eye, Hke that of Patella; B, the convex eye, 

 like that of the compound eyes of Arthropods. 



383. Vision. — The simplest type of eye described doubtless 

 enables the possessor to distinguish more readily differences in 

 the intensity of light, such as a passing shadow. The animal 

 would not be able, however, to distinguish one object from 

 another by its form, since the eye is not so constructed as to 

 form an image. Where there are a number of such eyes so 

 placed on the body that they "look" in different directions, the 

 stimulation of one more than another would be an indication 

 of the direction of the source of light. In the sKghtly more 

 complex eye, where the elements are arranged radially on a 

 concave surface, there is formed a crude image, because the 



