FRESH- WATER SNAILS. 13 



The Western rivers teem with species of snails having 

 opereula. 



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



o 

 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, the snail has a cavity containing an 

 organ, or part, called the gill, by means of which it is capa- 

 ble of getting from the water what the air-breathing snail 

 gets from the air, namely, oxygen? 



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. 



12. 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. 



