A LAND SNAIL 113 



the apex, the opening is called the mouth, and its axis, the 

 columella. How many turns does the spiral make ? The apex 

 corresponds to the umbo of the lamellibranch ; it is the oldest 

 part of the shell, the point from which its growth has proceeded. 

 Note the parallel lines of growth. The ventral edge or mouth 

 of the shell is thus its youngest part. The animal can with- 

 draw its entire body within the shell, but when it is walking 

 or feeding it protrudes its head and foot. The visceral mass, how- 

 ever, containing all of its viscera, is always covered by the shell 

 and has thus its exact shape, i.e., it is an elongated cone which 

 has suffered a dextral twisting so as to form a closely coiled 

 spiral. As a matter of fact, however, it is the visceral mass 

 which has been primarily twisted; the shell is twisted because 

 it covers the visceral mass. If the spiral were to be imagined 

 uncoiled and extending straight up above the foot, the apex 

 would be the uppermost and the foot the lowermost portion of 

 the body ; the apex is thus, morphologically, the dorsal and the 

 foot is the ventral aspect of the animal. 



As in the pelecypod, the visceral mass is enclosed in a 

 mantle, which is a fold of the dorsal integument, but unlike the 

 pelecypod it is a single fold and not a double one. This 

 fold falls about and covers the visceral mass on all sides, as 

 does a thimble the finger it is on, and secretes the shell on its 

 outer surface. The ventral edge of the mantle is provided 

 with muscles, so that it can be protruded beyond the mouth of 

 the shell or retracted within it. This edge is called the collar. 

 Find it in your specimen. On the right side of the animal note 

 the deep notch and the round hole in the collar. This is the 

 respiratory pore, which opens into the respiratory chamber. This 

 chamber is the mantle cavity. Probe it gently and determine 

 its extent. The animal being terrestrial has no gills, but 

 respires by means of a lung, which is a highly vascularized 

 portion of the wall of the mantle cavity. In a live animal note 

 its power to open and close the respiratory opening. 



