ABERRANT ANIMALS-THE MOLLUSKS AND ECHINODERMS 263 



face of its body. The movement of the snail 

 closely resembles the gliding movement of 

 planaria. In some species the foot is actu- 

 ally ciliated to aid the gliding motion; in 

 others movement occurs by rhythmic mus- 

 cular contraction of the foot. The land snail 

 (Fig. 12-13) has a definite head which 

 bears two pairs of tentacles, one short pair, 

 supposedly the center of the sense of smell, 

 and a longer pair with a simple eye at the 

 tip of each. In water forms the eyes are 

 situated at the base of the tentacles. 



Judging from its coiled shell, one would 

 expect the snail to have an asymmetrical 

 body. This is only partly true, however, for 

 the head and the elongated flattened foot 

 are bilaterally symmetrical, whereas the 

 remainder of the body, which composes the 

 visceral mass, is asymmetrical, parts of 

 the digestive and circulatory systems being 

 coiled. The shell of the gastropods is uni- 

 valved, that is, one piece, but the single 

 valve may vary in shape from tiny flat- 

 tened spirals to long spindle shapes or even 

 turban or slipper-like forms. 



Originally snails were aquatic forms, but 

 as some species migrated to land one of the 

 two gills was lost and the mantle was 

 gradually modified until one fold of it 

 appeared as a primitive lung. These ani- 

 mals, possessing one lung and one remain- 

 ing gill, belong to the order Pulmonata. 

 The more primitive marine forms that have 

 retained the two gills are members of the 

 order Prosbranchiata, while the fresh-water 

 forms with the right gill remaining are in 

 the order Opisthobranchiata. Some of the 

 land snails have returned to fresh water, 

 but the lung has remained. Such forms 

 must come to the surface of the water occa- 

 sionally for a supply of oxygen, particu- 

 larly during hot weather. 



Snails occur in nearly all parts of the 

 world, with the possible exception of ex- 

 tremely cold climates, although one species, 

 Vitrina glacialis, is found living high in the 

 snow-covered Alps. Some of the fresh-water 

 snails, such as species of Lymnaea and 



Fig. 12-13. Land snails such as this one {Helix aspersa) 

 have a part of their mantle modified into an air 

 breathing organ so they are at home on land. In the 

 top picture of the dorsal view, the eyes appear at 

 the tips of the two large tentacles. The ventral view 

 of the snail crawling up a glass plate is shown in 

 the bottom picture. Note the second pair of smaller 

 tentacles and the mouth at the anterior end. 



Helisonm, are able to survive for several 

 weeks in cakes of ice, providing they are 

 frozen gradually. Movements of these spe- 

 cies may be observed through the ice. For 

 the most part, water snails are active 

 throughout the four seasons of the year. 

 Land snails, on the other hand, are active 

 only during the warmer parts of the year 

 and are most active at night or immediately 

 following a light rain. As cold weather ap- 

 proaches they seek a protected place for 

 hibernation. During this period, a mem- 

 brane is formed, covering the aperture of 

 the shell to protect the animal. 



In general, snails are harmful to man. 

 The herbivorous land snail, for example, 

 often damages vegetation considerably. 

 Some forms serve as intermediate hosts for 

 parasitic flatworms, while others are para- 



