FKESH-WATER SNAILS. 13 



The Western rivers teem with species of snails having 

 opereula. 



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



Fig. 16. — Aperttjeb of Shell closed by Opehcultim, 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.'' 



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

 ently ia 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 



' If the class is sufficiently adranced, the teacher may here explain about 

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



