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THE WORLD OF ANIMAL LIFE 



Gills of Fish 



it may get a supply of oxygen before it is distributed through the 

 body. Instead, then, of inhaling air as other animals do, the fish 

 passes water through its gills, which absorb a great part of the 

 oxygen which it contains. And this action is repeated incessantly. 



Most fish produce their young 

 from spawn, as the jelly-like eggs of 

 fish are called, and one female will 

 lay thousands of eggs in a season. 



While many fish deposit their 

 eggs and leave the hatching to chance, 

 some are very careful of their young. 

 They build nests in which they lay 

 their eggs, and guard them faithfully 

 until the young can take care of 

 themselves. The common stickleback of our ditches and ponds 

 is a good example of the nest-building fish. 



The forms of fishes are indescribably varied. While some are, 

 from our view, intensely ugly, many are extremely beautiful in form 

 as well as in colour. Some are as gorgeously coloured as the most 

 brilliant of birds. 



Many fishes are, like birds, migratory. They spend the wimter 

 in the deeper parts of the ocean, and in the spawning season they 

 come to the shallower water near land. Some, for example the 

 salmon, migrate from the freshwater rivers to the salt sea for 

 a part of the year and return to their streams for the other part. 



It is believed that fishes, in proportion to their size, have much 

 longer lives than most other animals. Indeed it is thought that, 

 if not destroyed by their enemies, they attain to a very great 

 age. Pike and other fish have been known to attain ages varying 

 from fifty to eighty years. 



It will be enough for us, so far as classification is concerned, if 

 we try to understand that the great class of fishes is divided into 

 four sub-classes, as follows — 



I. The Lung- Fishes — Dipnoi. 



II. The Chimferoids — Holocephali. 



III. The Bony Fishes and Ganoids — Teleostomi. 



IV. The Sharks or Rays — Elasmobranchii. 



