THE OYSTER. 39 



effect is inconceivably minute, but the innumerable 

 multitude causes a vigorous circulation, and each one 

 is set in such a position that it drives the water before 

 it from the gill-chamber into one of the water pores, 

 and so into one of the water tubes inside the gill. 

 As these are filled they overflow into the cloaca and 

 fill that. If the mantle were closed, all the water 

 would soon be pumped out of the gill-chamber into 

 the cloaca, but you remember that an oyster at rest 

 always has the mantle open. As fast as the gill-cham- 

 ber is emptied by the hairs, fresh water streams in from 

 outside, to be, in its turn, driven through the water 

 pores into the water tubes, and through them into the 

 cloaca, whence it is driven out between the open shells 

 and away from the oyster. 



So much for the gills as organs for maintaining a 

 current of water. We come now to the way in which 

 they procure food. 



The food of the oyster consists of microscopic organ- 

 isms, minute animals and plants, which swim in the 

 water. They are pretty abundant in all water, but 

 those who do not work with the microscope have very 

 erroneous ideas on the subject. When a professional 

 exhibitor shows you, under the microscope, what he 

 calls a drop of pure water, it is nothing of the sort. 

 It is either a collection made by filtering several barrels 

 of water, or else it is a drop squeezed from a piece of 

 decayed moss, or from some other substance in which 

 microscopic organisms have lived and multiplied. 



Sea water is like fresh water in this respect, and an 



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