18 FIRST BOOK -OF ZOOLOGY. 



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

 vals 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 meant by an object 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 the class, it will be better still. 



Let it be explained, also, what is meant by an object 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 bigness. 



Oftentimes the object has to be reduced in size in the figure, as in pictures of 

 large animals, in the picture of an elephant, for example. 



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, were they 

 drawn natural size, that is, a size corresponding to the actual size of the animal. 

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



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 



