THE MAMMALS OF PENNSYLVANIA 



The General Characters of INLammals 

 The mammals belong to the group of the animal 

 kingdom known as Vertebrates. These are distin- 

 Laiished by the fact that they possess a Vertebral 

 Column or Backbone. The group includes the Fishes, 

 Amphibians, Reptiles, Birds and Mammals. How- 

 ever, the mammals differ from the others in two im- 

 portant respects; in the first place, the mammals are 

 the only animals that possess a coating of hair; second- 

 ly, the young of mammals are nourished by secretions 

 extracted from the body of the mother through the 

 mammary glands which lead to the exterior through 

 the nipples located on the ventral surface of the body. 



All mammals are warm blooded and, although 

 certain forms like the Whales, Seals and Dolphins 

 are adapted to living in the water, all are air-breath- 

 ing. With only two exceptions, the mammals are all 

 viviparous; that is, they bring forth their young alive. 

 The exceptions are the Australian Duck-billed Mole 

 Ornithorhyncus paradoxus and the Spiny Anteater 

 Echidna. Both of these forms are strikingly unique in 

 that they retain the primitive habit of laying eggs, as 

 do birds and reptiles. When the eggs hatch, the young 

 are taken by the mother and nourished in the usual 

 way. However, these two forms do not have well de- 

 veloped mammae and^ mammary glands. The milk in 

 this case is secreted by modified sweat glands and ex- 

 udes through pores located on the ventral surface of 

 the body. It is then licked off by the young animals. 



There are other animals which bear living young, 

 however. This is true of certain snakes, such as the 



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