BREATHING OF FISH 117 



venes between the blood in the gills and the water 

 which bathes them in a series of successive outgoing 

 spurts. Through this transparent covering the oxygen 

 of the water diffuses into the blood, which, thus 

 invigorated, is carried to the various organs of the 

 body. 



Where the activity of the organs is great the 

 storage of needful oxygen is increased by the develop- 

 ment of a red pigment capable of forming a reserve 

 fund, upon which the muscles may draw for the in- 

 creasing work that is demanded by the ceaseless 

 adjustment of the body and by the rapid, powerful 

 strokes required for pursuit, escape, or migration. 

 The pink colour of salmon-trout and the red of the 

 salmon and of the tunny are stores of this kind. 



This constant irrigation of the gills has brought 

 fish into intimate touch with the qualities of water. 

 The gradual drying of watercourses and ponds in 

 summer, or the fouling of water by the decay of 

 vegetation, reduces the amount of available oxygen, 

 and to avoid the threatening suffocation many fish 

 have adopted the plan of swallowing not water only, 

 but air as well. The common loach rises to the surface 

 to gulp a mouthful of air. The mudfish of Africa 

 and of America does the same. The advantage of 

 such a plan is patent. When the water is pure and 

 abundant, aerial oxygen is a luxury, but when, in 

 dry weather, the stream becomes a few isolated warm 

 pools, which decaying weeds are rendering each day 

 less fit for breathing, then those fish which can 



