ORGANS OF THE SENSES. 



a pupil and iris, and a distinct crystalline lens ; and in the 

 eyes of the cyclostomum viviparum, as well as in that anato- 

 mised by Swammerdam, a distinct crystalline lens, a coloured 

 choroid coat, and also an iris were detected and described 

 by Stiebel. The structure of the eye in the helix pomatia and 

 in the murex tritonis has also been described and represented 

 by Muller (Fig. 103). The eye of this helix (103. A. a), is 

 attached to one side of a large moveable bulb (103. A. b), 

 and presents a compressed crystalline lens (103. B. a), a 

 thin aqueous fluid, and a larger vitreous body (103. B. b), 

 covered by the dark choroid coat. This ocular bulb (103. A. 

 b), with its attached eye (103. A. ), can be extended from, 

 or retracted into the sheath of its tubular peduncle, and is 

 supplied with a large nerve (103. A. d), which gives off the 



FIG. 103. 



optic (103. A. c), as is done by the tentacular branch in most 

 other gasteropods. A similar bulb is seen attached to the 

 eyes in some other mollusca (Fig . 93. A. c). Blainville ob- 

 served a large crystalline lens in the eye of the voluta cym- 

 bium, which projected anteriorly like that of a sepia, a small 



