52 



SPECIAL SENSES. 



revealed, with the utmost fidehty, in the expression of his 

 eye, and it has been rightly called " the window of the 

 soul." 



82. Many of the invertebrate animals have the eye 

 constructed upon the same plan as that of the vertebrate 

 animals, but with this essential difference, that the optic 

 nerve which forms the retina is not derived from a ner- 

 vous centre, analogous to the brain, but arises from one 

 of the ganglions. Thus, the eye of the cuttle-fish contains 

 all the essential parts of the eye of the superior animals, 

 and, what is no less important, they are only two in number, 

 placed upon the sides of the head. 



83. The snail and kindred animals have, in like manner, 

 only two eyes, mounted on the tip 

 of a long stalk, (the tentacle^) or 

 situated at its base, or on a short 

 pedestal by its side. Their struc- 

 ture is less perfect than in the cuttle- 

 fish, but still there is a crystalline lens, 



and more or less distinct traces of the vitreous body. Some 

 bivalve mollusks, the scollops for example, have likewise 

 a crystalline lens, but instead of two eyes, they are furnished 

 with numerous eye-spots, which are arranged like a border 

 around the lower margin of the animal. 



84. In spiders, the eyes are likewise simple, and usually 



eight in niuiiber. These 

 little organs, usually called 

 ocelli^ instead of being 

 placed on the sides of 

 the body or of the head, 

 occupy the a^iterior pari 

 of the back. All the essen- 

 tial parts of a simple eye 



the corner, the crystalline lens, the vitreous body, are found in 



Fio-. 15. 



Fig. 16. 



