14 FIRST BOOK OF ZOOLOGY. 



an organ called the gill. And these snails were operculated, 

 that is, they all possessed a little scale called the operculum, 

 which closed the aperture tightly when the snail contracted 

 within the shell. 



He has also learned that the shells grow in size by suc- 

 cessive additions of limy matter deposited around the free 

 border of the aperture, and that the delicate lines which 

 mark the surface of the shell, and which run parallel to the 

 outer edge of the aperture, are lines of accretion, or lines of 

 growth. 



CHAPTEE III. 



LAND SNAILS. 



13. Theee are many other species of snails which live 

 out of the water altogether, though they are generally found 

 in damp places; and these are called land snails, because 

 they live on the land. 



Let the pupils now endeavor to collect some land snails. 

 By going to some hard-wood grove of maple, beech, or oak, 

 and turning over the layers of dead leaves, old rotten logs, 

 or pieces of bark, they will be sure to find some specimens 

 of land snails. Some of them do not grow larger than a 

 pin's-head, others have shells as large as a walnut. 



They are generally light brown in color, and the smaller 

 species often have highly-polished or shiny shells. 



The spire is generally depressed or flattened. In many, 



