VISION. 



481 



cula. Of the latter we have examples in the 

 common slug and snail, where these tentacula, or 

 horns, are four in number, and are capable of 

 being protruded and again retracted, by folding 

 inwards like the finger of a glove, at the pleasure 

 of the animal. According to Muller,* the eye of 

 the Helix poinatia, represented at e, (Fig. 418), 

 is situated a little to one side of the rounded 

 extremity, or papilla (p), of the tentaculum, and 



418 



is attached to an oval bulb of a black colour. 

 It receives only a slender branch (o) from a 

 large nerve (n n) which is distributed to the 

 papilla of the tentaculum, and appears to be ap- 

 propriated exclusively to the sense of touch. 

 The bulb, with the eye attached to it, is repre- 

 sented, in this figure, as half retracted within the 

 tubular sheath of the tentaculum (s s) ; but it 

 can exercise its proper function only when fully 

 exposed, by the complete unfolding and protru- 

 sion of the tentaculum. This eye contains, 

 within its choroid coat, a semi-fluid and per- 

 fectly transparent substance, filling the whole of 



* Annales des Sciences Naturelles; xxii. 12. 

 VOL. II. I I 



