MOLLUSKS 137 



SNAILS AND SLUGS. 



Gasteropoda. 



Fully 15,500 species of living mollusks belong to the great 

 order of G'aste?'opoc?a, wliich means stomacli foot. A wide diversity 

 of forms are met witli, 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 fNudibranchiataJ. 



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 rilsbon-like tongue which is cov- 

 ered with rows of horny rasping teeth, and is called the radida. 



The vital organs are contained in a large sacdike body-mass 

 that is joined by a relatively slender neck to the upper side of the 

 foot. In tlie 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 a shape 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, kidne}' opening and other external organs of that side 

 either to disappear or to migrate toward the right side, so that in 

 GaateroiMdu 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 iipon itself, 

 and opens on the right side of the body near the head, while a lit- 

 tle in front of the anus lies the opening of tlie kidney and still 

 further forward the gill. 



In a few forms we ma}' 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, 



