n8 ANIMAL LIFE 



breathe air are able to hold out longer than their 

 submerged fellows, and, should the next rain be too 

 long delayed, they will be first to meet the spate and to 

 secure the new supplies of food that the flood carries. 



In order to store the air which gives them such 

 advantage the throat of fish is puckered up into 

 pouches ; some of these are quite shallow pockets, 

 some are long tubes running down the body. In 

 virtue of this supply the climbing perch makes ex- 

 cursions overland before the dew is off the grass, and 

 the mudfish, which tenants every sheet of water in 

 equatorial Africa, passes through the driest season 

 with impunity, rolled up and sleeping. 



The demand for storage of gases is so insistent 

 that we find it obeyed by almost all fish, even in 

 those which breathe solely under water. In them the 

 storing organ is a tube lying under the backbone and 

 opening into the throat during the first part or the whole 

 of life. Into this chamber gas is exhaled from the 

 blood. In marine fish oxygen is chiefly stored; in 

 freshwater fish nitrogen preponderates. But these 

 gas chambers, which originally no doubt served as 

 reservoirs of respirable gas, have lost their primary 

 function, as, indeed, the neutral nitrogen implies, and 

 become transformed in ordinary fish into bladders, 

 which determine their specific gravity by the alterably 

 greater or less quantity of their contents. Thus they 

 have entered into the problem of movement, and, 

 by tending to lighten the fish's back and upset its 

 equilibrium, have led to far-reaching demands upon 



