MOLLUSKS 137 
SNAILS AND SLUGS. 
Gasteropoda. 
Fully 15,500 species of living mollusks belong to the great 
order of Gasteropoda, which means stomach foot. A wide diversity 
of forms are met with, but when young they all have a single shell; 
although this may degenerate into a mere internal scale, as in the 
case of the slugs, or disappear entirely in adult life as in the case 
of the naked mollusks (Nudibranchiata ). 
A flat, creeping sole, or foot is usually present, and the head 
is distinct and usually provided with feelers and eyes, while within 
the mouth we find a remarkable ribbon-like tongue which is cov- 
ered with rows of horny rasping teeth, and is called the radula. 
The vital organs are contained in a large sac-like body-mass 
that is joined by a relatively slender neck to the upper side of the 
foot. In the simplest forms this body-mass is dome shaped, or 
conical, but in the course of evolution its weight has caused it to 
topple over to the left side of the body, and then in order to present 
the least surface to possible injury it has become coiled usually in 
a right-handed direction. The shell which covers the body-mass 
naturally has ashape similar to that of the body-mass itself, and the 
vast majority of gastropoda shells are right-handed spirals. This 
toppling over of the body-mass and shell upon the left side has 
caused the gill, kidney opening and other external organs of that side 
either to disappear or to migrate toward the right side, so that in 
Gasteropoda we usually find a feathered gill only on the right-hand 
side of the body, underneath the mantle-fold, although in some 
species the gill which was originally on the left side has come 
around and still persists on the right side of the body, and thus 
the animal has two gills on the right side and none on the left. 
In the typical snail, then, the intestine bends back upon itself, 
and opens on the right side of the body near the head, while a lit- 
tle in front of the anus les the opening of the kidney and still 
further forward the gill. 
In a few forms we may find a kidney-duct and a gill back of 
the anus, these having shifted over from the left side of the body; 
but this condition is not often seen, for these organs have usually 
disappeared, instead of travelling around the body from the left to 
the right side, . 
