CLASS IV. 

 ICHTHYOLOGrY.-FISHES. 



THIS class, destined to live in the water, differ from 

 all other vertebrate animals, by having gills, instead of 

 lungs, through which they breathe. These, placed on 

 the sides of the neck, consist of a number of laminae sus- 

 pended on arches, through which are spread a tissue of 

 innumerable blood-vessels, as may be observed by every 

 one who will take the pains to examine ; these are com- 

 monly called the ears of fishes. These laminae, connected 

 with the cavity of the mouth, suffer a portion of the water 

 swallowed to escape, or, at the pleasure of the creature, 

 the whole mass. Fishes, however, do not take in water 

 by breathing, but only the dissoluble air it contains ; a 

 fact easily proved by observation. If the supply of air 

 contained in water is cut off, the fish die just as a bird 

 or small animal confined under a glass case would, after 

 the pure air is exhausted. Thus, for instance, if a lake 

 or pond should be frozen over entirely, during a hard 

 winter, it is necessary that holes should be cut in the 

 ice, or the fish would all die. If a fish is kept in a glass 

 vessel containing a small amount of water, it must be 

 frequently renewed, or most likely the creature will die, 

 because the air loses its purity ; but if only a little oil is 

 poured on the top of the water, the fish will certainly 

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