THE OYSTER. 37 



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

 pores into the water tubes, and through them into the 

 cloaca, where 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 

 they have lived and multiplied. 



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

 oyster must strain many gallons of water to get its 

 daily bread; but the gills, with their hundreds of thou- 

 sands of microscopic water pores, are most efficient 

 strainers. 



The surface of the gills is covered by an adhesive 

 excretion, for entangling the microscopic organisms 

 contained in the water, and as this circulates over and 

 through the gills, they stick fast like flies on fly-paper. 

 The cilia which drive the water through the gills, push 

 the slime, with the food which has become entangled 

 in it, towards the mouth, which is well up towards the 

 hinge ; for it is hardly necessary to say that what the 

 oystermen call the mouth is only the opening between 

 the halves of the mantle. 



