MECHANICS OF RESPIRATION 



for the majority of species found in temperate countries, 

 but not those of the more tropical lands. Besides 

 oxygen to breathe, the latter need a constant temperature, 

 a temperature that is high even in winter, usually be- 

 tween 75 and 90 degrees Fahrenheit. Indeed, most of 

 them can get on with a fairly low degree of oxygen, 

 provided that the heat indispensable to them is forth- 

 coming. In order to avoid the expense of canalizing 

 water that has first been warmed, the difficulty is solved 

 satisfactorily by planting an aquatic vegetation in the 

 tanks and letting it grow there, keeping the whole at a 

 sufficient temperature. 



Several advantages are thus secured. Branches and 

 foliage, floating in the water, make an ornamental 

 frame, in which the fishes, as they move about in all 

 directions, unite with their environment to give a har- 

 monious representation of natural phenomena. Again, 

 these vegetable growths purify the water on the spot 

 and all the time, so that it need not be renewed, except 

 at intervals sometimes of weeks, sometimes of months. 

 Daylight, acting on the chlorophyll of their green parts, 

 causes it to decompose the carbon dioxide which, pro- 

 duced by the fishes' respiration, gradually dissolves in 

 the water as a waste product. The oxygen, thus set 

 free, is reconditioned and again ready to be breathed. 

 The same oxygen can be used over and over again 

 until the time comes when the greater part of it has been 

 actually incorporated in the substance of the creatures 

 whose life it has supported. 



Such a system of managing an aquarium, thanks to 

 the care and constant supervision which it entaib, shows 

 how great an importance in life is attached to this act of 

 respiration and the absorption of oxygen which is the 

 essential factor in it. On land, creatures are immediate- 

 ly in contact with it; they have only to draw upon it in 

 its ordinary state, in the form of gas. They gather it 

 at the source, always find it in more than sufficient 

 quantity. The state of affairs in the water is entirely 

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