28 SPECIAL SENSES. 



reflected, with the utmost fidelity, 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 eyes 

 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 eyes of the cuttle-fish contain 

 all the parts essential to the eye of the superior ani- 

 mals, 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. 

 Fig. 15. Their structure is less perfect than 



the eyes of the cuttle-fish, but still there is a crystalline, 

 and more or less distinct traces of the vitreous body. 

 Some bivalve mollusks, the scollops for example, have 

 likewise a crystalline, but instead of two, they are fur- 

 nished with numerous eyes, which are arranged like a bor- 

 der around the lower margin of the animal. 



84. In spiders, the eyes 

 are likewise simple, and 

 usually eight in number. 

 These little organs, usu- 

 ally called ocelli, instead 

 of being placed on the 

 sides of the body or of the 

 head, occupy the anterior Fig. 16. 



part of the back. All the essential parts of a simple eye, 

 the cornea, the crystalline, the vitreous body, are found in 



