HOW FISHES BREATHE. 23 



our streams is an instance of breathing by the 

 intestine. In this case air is swallowed by the 

 fish, which, frequently rising to the surface, 

 thrusts its mouth above the level of the stream 

 and gulps down a mouthful of the precious fluid. 

 This passes at once to the intestinal tube, when 

 the oxygen is quickly extracted by minute blood- 

 vessels. Certain cat-fishes and carps also trans- 

 form the intestine into an accessory breathing- 

 organ. Some fishes, it is contended, are more 

 easily drowned than such an essentially land 

 animal as the adult frog. For Milne Edwards, 

 the great French naturalist, assures us that a 

 frog, though immersed iu a wire cage in the 

 bed of a stream, will if it be supplied with food 

 thrive prodigiously, respiration being carried on 

 through the delicate skin; whilst the fish en- 

 closed in the same cage would speedily die. 



The wonderful little walking-fish (Perioph- 

 ihalmus) passes the greater part of its life on 

 land, skipping about the mud-flats of mangrove 

 swamps. To render this amphibious existence 

 the more perfect, the gill-chamber has become 

 somewhat enlarged, and whilst the fish is out 

 of the water the chamber is kept filled with 

 air. Gill-breathing is said to be further sup- 

 plemented by respiration through the skin of 

 the tail. 



Now let us take a peep at the lung-like struc- 

 tures. Before we can rightly understand these, 

 however, we must consider the organ to which 

 they themselves are due the air-bladder or more 

 correctly perhaps the <7as-bladder. The air-bladder 

 must be familiar enough to many of my readers. 



