18 FIRST BOOK OF ZOOLOGY. 



as it crawls around on the sides of the jar, he will see at in- 

 tervals the mouth open, and a glistening tongue appear, as 

 the snail laps up the scum which forms upon the surface of 

 the glass. 



NOTE FOR TEACHERS AND PUPILS. Let the teacher here explain to the pupils 

 what is mean by an object's being magnified. 



If the teacher has a common magnifying-glass, let each pupil in turn observe 

 its magnifying effect, by looking at a common house-fly, or the printed page of a 

 book. If a microscope can be shown to the class, it will be better still. 



Let it be explained, also, what is meant by an object's being enlarged two, 

 or three, or more times. To be enlarged two or three times, is to make the 

 object two or three times as long as it was before, and of proportional size. 



In pictures of large animals, in the picture of an elephant, for example, the 

 object has often to be reduced in size in the figure. 



In representations of very small animals, however, the figure has to be en- 

 larged in order to show parts plainly that could not otherwise be seen. Thus 

 in Fig. 24, b and c are greatly enlarged, to show the little snail within the 



egg- 



17. In searching for snails, the pupil will come across 

 snail-like animals, which have no coiled shells on their backs. 

 Let the pupil examine the under side of damp boards or 

 plank walks in gardens, and he will be sure to find them. 



They are very common in old gardens in cities. These 



FIG. 21. A SLUG CONTRACTED. 



creatures will be found clinging to the board or upon the 

 ground, and will present this appearance (Fig. 21). Soon, 

 however, they will stretch out their tentacles, and commence 



