A SNAIL. 87 



which closes up the aperture of the shell like a lid when the animal 

 has withdrawn into the interior. 



e) The snail has a distinct head bearing a pair of eyes and one or two 



pairs of tentacles. Are the eyes, in the species you are studying, 

 at the base of a pair of tentacles or at the tips ? Which position of 

 the eyes is associated with two pairs of tentacles ? 



f) Look for the mouth on the under side of the head and closely 



applied to the surface of the glass. Is it longitudinal or transverse 

 with respect to the longer axis of the body ? The animal feeds by 

 rasping the surface of a leaf, or that of the algae-covered glass, with 

 a curious tongue-like radula. Watch the process if possible. In 

 some European localities the land snails are abundant enough to be 

 very destructive in gardens. 



g) The mantle enwraps that portion of the body within the shell. It 



can be seen only at the edges of the aperture. 



Exercise 3 . Draw an enlarged side mew of the snail extended as in creeping. 

 If the subject moves about too much, use a specimen killed as sug- 

 gested in the technical note. 



Look for the eggs of the snail on vegetation in the aquarium or on the 

 glass. They appear as dark specks enclosed hi globules of gelatinous 

 substance. In molluscs of the sub-class to which land and fresh- 

 water snails belong the sexes are united, the animal being hermaphro- 

 ditic. 



Whether living in fresh water or on land, snails are air-breathing, the 

 so-called lung being a sort of sack richly supplied with blood-vessels. 

 Watch the fresh-water snails getting the air. 



Interesting points of comparison with snails are furnished by the 

 closely related slugs, common enough in most localities. These 

 possess no shell, or at least but the rudiments of a shell. 



