CHAPTER XXVI 



SUBPHYLUM VERTEBRATA, CLASS 

 MAMMALIA 



A mammal is a warm-blooded vertebrate with hair and 

 mammary glands. There are a few apparent exceptions to 

 this definition. For example, some whales as adults are 

 naked, in one instance having the hair reduced to two bris- 

 tles on the upper lip; but before birth all have an abundant 

 hairy covering. Mammary glands are found in all mam- 

 mals. These are simply specialized skin glands which 

 secrete milk to nourish the young. The ancestors of 

 mammals were probably for the most part land animals, 

 but in recent geological history many have become adapted 

 to live in the water. Even the whales appear to have de- 

 scended from quadrupedal forms which lived along the 

 shores of ancient oceans. 



All mammals have four-chambered hearts, like birds and 

 crocodiles. They also have a single aortic arch carrying 

 blood from the heart to the dorsal aorta, but in this case the 

 one on the left side has persisted instead of that on the 

 right as in birds. In passing from fishes to mammals the 

 aortic arches, which in the simplest vertebrates carry blood 

 to and from the gills, are progressively lost or diverted to 

 other uses; the heart on the contrary becomes more and 

 more complicated (Fig. 105). Fishes (A) usually have 

 four pairs of functional arches which supply the gills, and 

 possess a tubular heart; adult amphibians (B) have three 

 pairs of aortic arches (only one of which connects with the 

 dorsal aorta; the other two being diverted to the head, lungs 

 and skin) ; the reptiles (C) have arches somewhat like am- 

 phibians, but in the crocodiles acquire a four-chambered 

 heart; in the birds (D) and mammals (E) only one arch of a 



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