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 The Mammals 



Class Mammalia 



The mammals include the largest and most intelligent of the animals 

 on the earth. They are a very successful group, having replaced the 

 reptiles as the dominant large animals of the earth. Many body organs, 

 such as the heart and brain, reach their height of development in the 

 mammals, culminating a long series of gradual changes that started back 

 in the invertebrates. 



Characteristics of the Mammals 



Hair. Only the mammals have hair on their bodies ; most of them 

 have the body almost covered with hair. Even man, whom we think of 

 as a fairly hairless creature, has hair everywhere on the body except the 

 palms of the hands and the soles of the feet. 



Mammary Glands. The mammals take their name from the mam- 

 mary glands which, in the females, enlarge and produce milk when they 

 bear young. We commonly express this by saying that the mammals 

 nurse their young. Embryonic mammals have a mammary ridge on 

 either side of the ventral surface of the body. In most mammals 

 nipples develop along the entire length of this ridge ; dogs and cats are 

 typical examples. In some mammals, such as horses and cows, nipples 

 develop only at the posterior end of this ridge; in others, such as ele- 

 phants and human beings, only a single pair of nipples may develop 

 at the anterior end. Occasionally, however, supernumerary nipples 

 appear in addition to those which are usually found in the species. 

 Cows may often be seen with an extra pair of nipples anterior to the 

 two normal pairs. These are usually rudimentary and give no milk. 

 Human beings sometimes have from one to three pairs of nipples 

 posterior to the normal pair, formed in a row similar to those found 

 in the dog. 



Diaphragm. This is an internal muscular partition separating the 

 chest from the abdomen which can be raised and lowered to exhale and 



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