FRESH. WATER SNAILS. 13 



The Western rivers teem with species of snails having 

 operciila. 



11. If the pupil has any of these operculated snails alive, 







FIG. 16. APERTURE OF SHELL CLOSED BY OPERCULUM, o. 



he will observe that they do not come to the surface to 

 breathe air. 



Instead of a lung, they have a cavity containing an or- 

 gan, or part, called the gill, by means of which the snail is 

 capable of getting from the water what the air-breathing 

 snail gets from the air, namely, oxygen. 1 



It will be seen that the head of the snail is shaped differ- 

 ently in the snails having an operculum, the mouth being at 

 the end of a sort of proboscis or rostrum. (See Fig. 15.) 

 The shells, too, are, as a general thing, more solid. 



1 2. Thus far the pupils have examined those snails which 

 live in fresh water. Some of these were air-breathers, and 

 came to the surface of the water at intervals to breathe 

 air. He has studied other fresh-water snails which did not 

 breathe air directly, but performed this function by means of 



1 If the class is sufficiently advanced, the teacher may here explain about 

 oxygen and what the blood requires, and gets by respiration. 



