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EVOLUTION OF ANIMALS 



Part V 



Fig. 37.6. Koala, an Australian marsupial, at ease. It lives entirely in trees and 

 its feet, the spread and separation of the toes, are adapted for clinging to branches. 

 Koalas feed entirely on the leaves of a few species of Eucalyptus trees. Their only 

 water supply is from the trees and their name koala is an old Australian word 

 meaning "no drink." (Courtesy, Australian News and Information Service, New 

 York.) 



contrast to the now scarcely 150 species of marsupials. The great key to their 

 success is the nourishment of the young before birth by means of the placenta, 

 the organ formed partly on the pattern of the old reptilian allantois (Fig. 35.2). 

 By means of the placenta, food and other needs of the growing embryo are 

 provided for and waste products pass through it into the blood of the mother 

 (Fig. 19.18). The young marsupial encounters the setbacks of a seedling that 

 is transplanted midway in its early growth, but for the young placental mammal 

 there is no transplanting. 



