MOLLUSCA. 123 



itself. It consists of one piece only; in other words, it is uni- 

 valve. If a hollow cone with a very small angle is thought of 

 as wound round and round into a right-handed spiral with 

 closely adherent coils, a good notion will be formed of its 

 structure. By the juxtaposition of the inner edges of the coils 

 or whorls, a hollow pillar, the columella, is produced, which forms 

 the axis of the spiral, and opens to the exterior below. The 

 opening of the shell (mouth, or peristome) has a prominent smooth 

 rim, and the blind end is termed the apex. 



During winter the snail passes into a torpid condition (hyber- 

 nates) when the aperture of the shell is closed by means of a 

 thick calcareous plate, the epiphragm or hylernaculum. When the 

 animal is fully expanded, the part of the body protruding from 

 the shell is bilaterally symmetrical, terminated in front by a 

 distinct head, and produced ventrally into the broad sole-like 

 foot, an elongated muscular expansion, tapering behind to a point, 

 and ending in front just below the mouth. The animal crawls 

 by wave-like contractions of this organ. The dorsal part of the 

 body projects as a spirally coiled visceral hump, always contained 

 within the shell, and attached to the columella by a special 

 muscle (spindle muscle) by the contraction of which the animal 

 can be completely withdrawn into its shell. The anterior boun- 

 dary of the visceral hump is marked by a thickened edge, the 

 collar. 



The head bears two pairs of tentacles or feelers, which are hollow, 

 and can be retracted into the body much as the finger of a glove 

 can be turned into the glove's interior. Each of the posterior 

 longer pair, the optic tentacles, bears a small black dot, the eye, on 

 the outer side of its tip. The anterior shorter pair are the olfactory 

 tentacles. In the front of the head is the mouth, guarded by an 

 inferior and two lateral lips. Below the mouth there is a small 

 pore, the opening of the supra-pedal gland. Not far below the 

 right optic tentacle the rounded genital opening is situated. In 

 the thickened anterior edge of the mantle, on the right side, 

 there is a deep depression, into the left side of which the large 

 valvular respiratory aperture opens, and on the right side of this a 

 smaller anus. The respiratory opening leads into a spacious 

 mantle-cavity, which acts as a lung and is roofed in by the mantle, 

 a vascular flap-like outgrowth of the body-wall. 



2. Skin. The skin is smooth on the under side of the foot, 



