THE CLASSIFICATION OF ANIMALS 



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The relationships of the backboned animals is often shown by a tree. Animals supposed to 

 have appeared early are on lower branches. The very recent animals and man are on the tips 

 of the higher branches. Related animals are found on the same or nearby branches. 



Class III — Pisces. The fishes. Aquatic, cold-blooded verte- 

 brates breathing by means of gills, having an air bladder, a 

 two-chambered heart, and a skinlike exoskeleton of scales. 



Class IV — Amphibia. Cold-blooded vertebrates breathing 

 by means of gills in some stage of their life history. Skin is 

 not covered with scales, the heart is three-chambered. Most 



