THE PHYLA HEMICHORDATA AND CHORDATA 



placental mammals, although they are not ancestral to these mammals. 

 Study of mammalian and reptilian embryos shows that the embryonic mem- 

 branes of the mammal so closely resemble those of the reptile and bird that 

 they must have been derived from the reptilian source. There is no yolk in 

 the egg of a placental mammal, but a yolk sac forms during development 

 (p. 162). 



Fig. 18.38. Representative Cetacea. A, hump-backed whale, Megaptera nodosa; 

 this is a "whalebone whale," feeding with the aid of plates of baleen suspended 

 from the upper jaw. fi, sperm whale, Pkyseter, one of the toothed whales, pur- 

 suing the squid which form the greater share of its food. The structural adapta- 

 tions to marine life in these mammals may be compared with those of fishes, 

 ancient marine reptiles, and modern marine Carnivora such as seals and sea lions. 

 {A, photograph of a model; B, photograph of a painting by F. L. Jacques; both 

 courtesy American Museum of Natural History.) 



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