430 THE SENSORIAL FUNCTIONS. 



have organs which appear to exercise this sense, 

 situated sometimes at the base, sometimes at the 

 middle, and frequently at the extremity of the 

 tentacula. 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 MuUer,* the eye of 

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



418 



is situated a little to one side of the rounded extre- 

 mity, or papilla (p), of the tentaculum, and is at- 

 tached to an oval bulb of a black colour. It re- 

 ceives only a slender branch (o) from a large nerve 

 (n n) which is distributed to the papilla of the ten- 

 taculum, and appears to be appropriated exclusively 

 to the sense of touch. The bulb, with the eye 

 attached to it, is represented, in this figure, as half 

 retracted within the tubular sheath of the tentacu- 

 lum (s s) ; but it can exercise its proper function 

 only when fully exposed, by the complete unfold- 

 ing and protrusion of the tentaculum. This eye 

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

 perfectly transparent substance, filling the whole 

 of the globe ; and Muller also discovered at the 

 anterior part, another transparent body, having the 



* Annales des Sciences Naturelles ; xxii. 12. 



