300 ANIMAL PHYSIOLOGY. [LESSON 93. 



others, appear to be destitute of eyes ; in the naked Aplysia they 

 are but minute black dots. 



1358. In some of the Gasteropods (Harpa elongata, Fig. 381) 

 the eyes are placed on tubercles, at the bases, or near the bases of 

 the tentacles. Such is the arrangement in the beautiful Cyprea 

 tigris from the South Seas (Fig. 426). 



1359. In this animal the two long tentacula (a, a) present, near 



FIG. 426. 







Cyprea Tigris, or Leopard Cowry. 



their bases, two prominent, round, black, and movable eyes (c, c), 

 with smooth, transparent, glistening corneas. The tentacula being 

 placed above the mouth (b) and in front of the syphon (d), the eyes, 

 which are raised on tubercles at some distance from the base of tHe 

 long, slender tentacula, have a considerable range of vision. Above 

 the expanded foot (g, g) is seen the inner surface of the mantle (e), 

 turned up over a portion of the shell (&), and covered with small 

 ramified tentacular extensions (f), which warn the animal of danger 

 from behind. 



1360. In the Carinaria Mediterranea the optic nerves are seen 

 passing directly to the eyes from the cerebral ring (Fig 380, i). 



1361. In the general plan of their formation, the eyes of the 

 Molluscous animals form a near approach to the ordinary condition 

 of these organs in Fishes, and the higher vertebrated classes. 



1362. In all the vertebrata the eyes are two in number, and 

 symmetrically disposed on the sides of the head, and the differences 

 which they present relate chiefly to the density of the media through 

 which the various animals receive the rays of light, and the extent 

 of development of the external protecting parts of these delicate 

 organs. 



1363. From the imperfect development of the nervous system in 

 Fishes, and the obscurity of the element through which they move, 

 their organs of vision are of great size, and, from the density of the 



