260 ORGANS OF THE SENSES. 



cles extended from the bases of the two tentacula, as seen 

 in the annexed figure of the common cyprcea tiff res (Fig 102.) 



FIG. 102. 



from the South Seas, where the two long tentacula (102. , 

 a,) present near their bases two prominent, round, black and 

 moveable eyes (102. c, c,) with smooth glistening transparent 

 cornese. The tentacula being extended above the mouth 

 (102. b,) and anterior to the syphon (102. 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 large expanded foot (102. g, #), is seen 

 the inner surface of the mantle (102. e), turned up over a 

 portion of the shell (102. h) 9 and covered with small ramified 

 tentacular extensions (102. /), which apprise the animal of 

 danger from behind. In some of the gasteropods, as the 

 haliotis, Umax, helix, and onchidium, the peduncles of the 

 eyes are as long as the tentacula themselves. According to 

 Chiaje the doris and the thetis have pedunculated eyes, and 

 Ehrenberg has observed organs of vision in the hexabranchus 

 and the phyllidia. The position of these small dark eyes at 

 the exterior of the base of the tentacula is already represented 

 in the buccmum undatum (Fig. 22. d, d), and in the harpa 

 elongata (Fig. 92. s, s,) where they are a little further re- 

 moved from the base ; and in the carinaria mediterranea 

 (Fig. 89. f, /), where the optic nerves are seen passing to 

 them directly from the cerebral ring (89. i). The internal 

 structure of these simple eyes of the gasteropods much re- 

 sembles that of the large ocelli of the scorpions and other 

 arachnida. In the eyes of the helix pomatia, which are 

 raised to the ends of long tubular muscular peduncles exceed- 

 ing the length of the tentacula, Swammerdam observed a 

 thin aqueous humour, a more consistent vitreous humour, 



