50 THE BOOK OF NATURE STUDY 



other hand withdraw entirely within it and almost pass out of 

 sight round the bend of the last formed whorl. At the anterior 

 end of a well-expanded specimen is the mouth, bound by soft 

 fleshy dorso-lateral lips and by a short horizontal lower lip. 

 Dorsal to the mouth are two pairs of tentacles (" horns " in 

 popular language) ; the more posterior and dorsal of these are 

 the larger, and bear at their extremities a black spot the two 

 eyes. The lower tentacles are tactile, and perhaps also olfactory 

 in function. All the tentacles can be withdrawn into the body. 

 When this occurs the tip sinks down first into the part behind 

 it, and gradually the whole structure disappears much as the 

 finger of a glove might be drawn down into the glove-hand by a 

 string inside attached to its tip. A slip of muscle running up 

 the hollow of the tentacle and fastened to the summit performs 

 this action in the snail. On the right hand side, a little ventral 

 to the large tentacle, is the inconspicuous opening of the repro- 

 ductive organs ; a grooved line leads obliquely down to this 

 from the margin of the shell. The ventral part of the animal 

 is flat and very muscular and is known as the " foot." This 

 special ventral thickening extends along the entire length of the 

 animal, beginning immediately below the mouth and reaching 

 nearly as far behind as in front of the shell ; posteriorly it tapers 

 off to a blunt point. It is by means of this organ that all snails 

 and slugs creep along. The shell is carried on the dorsal surface, 

 with its " mouth " to the right. The cream-coloured " collar " 

 is conspicuous between the edge of the shell and the dorsal surface 

 of the projecting anterior portion of the body. In the "collar " 

 on the right side is a large hole, which is alternately closed and 

 opened in a rhythmic manner. This hole leads into the space 

 which is enclosed between the single mantle-fold and the true 

 dorsal surface of the body. The upper (mantle) wall of this 

 chamber is richly supplied with blood vessels, and the lower 

 can be alternately depressed and elevated so that air passes in 

 and out through the opening. In this way the animal breathes, 

 the blood receiving fresh supplies of oxygen after it has coursed 

 through the body and just before it re-enters the heart. Thus 

 the cavity enclosed by the mantle-fold acts as a lung. Close to 

 the pulmonary opening, but a little behind it on the right side, 



