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.c., 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. 
