276 The Evolution of Optics. 



head, as in the bird spider. The spiders which inhabit short 

 tubes, terminated by a large web exposed to the open air, 

 have eyes separated and more spread upon the front of the 

 cephalo-thorax. Those spiders which rest in the center of 

 a fine web, which they frequently traverse, have the eyes 

 supported on slight prominences which permit a greater di- 

 vergence of their axes; this structure is well marked in 

 those species which lie in ambuscade in flowers. Lastly, the 

 spiders called errantes, or wanderers, have their eyes still 

 more scattered, the lateral ones being placed at the margin 

 of the cephalo-thorax." * 



The eyes of vertebrates differ from those we have just 

 been considering both in their type and method of develop- 

 ment. All vertebrate eyes are not identical in structure, 

 but for the most part we find the same essential features as 

 in the human eye, which we have already considered, while 

 each class presents some slight modifications which are 

 usually intelligible from the habits of life of the animal. 



The eyes of fishes 

 (Fig. 5), for example, 

 are flattened in their 

 anterior segment, thus 

 diminishing liability to 

 injury during the rap- 

 id movements of the 

 fish. The eyes are set 

 in bony sockets, and 

 the sclerotic is still 

 further fortified by os- 

 seous or cartilaginous 

 plates. In the water, 

 vision at great dis- 

 tances is impracticable, 

 and we find that the eyes of fishes and most aquatic ani- 

 mals are so formed as to adapt them for vision at compara- 

 tively short range. The flat cornea is compensated by a 

 globular lens, making the animal naturally near-sighted. 

 The pupil is usually round, but in some flat fishes which 

 grovel at the bottom of the water there is a peculiar fringed 

 process from the margin of the iris which can be drawn up 

 or let down at will, thus regulating the amount of light re- 

 ceived. The lachrymal gland is of course absent, being un- 

 necessary in aquatic animals ; but in some fishes as, for ex- 



* Owen. 



