CHAPTER XIII 



MECHANICS OF RESPIRATION 



One of my principal tasks in the management of an 

 aquarium, and it is a matter which always causes me a 

 great deal of concern, is to keep a close watch over the 

 breathing of all my guests, so that no trouble of any 

 kind may arise. Fishes breathe. Like other living 

 beings, they need oxygen. Because they live in water, 

 they can find that oxygen only in the water itself, and 

 there they do indeed find it, absorbing it in a state of 

 solution. Normally water contains in solution the ele- 

 ments of which air is made up, oxygen among them. 

 Some of these elements it takes and absorbs until it can 

 hold no more, whenever both air and water come into 

 contact. The fishes and their neighbours in the water 

 use this dissolved oxygen to breathe. It is essential, 

 therefore, that the degree of solution should be ade- 

 quately maintained, if the respiratory function is to be 

 properly fulfilled. It is my business to see that the 

 water in my tanks always contains a sufficient proportion 

 of dissolved oxygen, in spite of the continual consump- 

 tion of it for which my wards are responsible. 



There are a number of accepted methods of keeping 

 up this supply, and I choose one or another of them as 

 circumstances suggest. The simplest consists in re- 

 newing the water in the tanks either continually or at 

 regular and sufficient intervals. The old water, made 

 unclean by the disappearance of the oxygen consumed, 

 and by various substances formed by the excreta, is 

 replaced by new water, which is free from foul matter 

 and contains dissolved oxygen in the correct proportion. 

 Thus the requirements of respiration are adequately 

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