THE VERTEBRATA 165 



The Mammalia are a terrestrial group. Excep- 

 tions are the Cetacea (Whales), Sirenia (Dugongs), 

 and Seals or Sea-Carnivora, but all of these are 

 air-breathers ; even the Whale can only stay 

 under water for a limited period of time. Hence 

 we see that none of them are really animals be- 

 longing to the water ; they are land animals 

 adapted for life in the water. 



This brings us very near to the last chapter in 

 the Story of Animal Life. We have seen that 

 our story began with the One-celled Animals, and 

 went on with the tale of the Two-layered Animals, 

 in which each layer was built up by cells in part- 

 nership. From Two-layered Animals we passed 

 to Three-layered Animals, and from them to 

 Three-layered Animals with a " body-cavity." 

 When we reached the latter, we found amongst 

 them traces of the ancestry of the vertebrates. 

 From the lowest of the Vertebrata, the Lancelet, 

 we passed on to the Lamprey, and from that to 

 the true fishes. In the latter we found the parent 

 type of all the other Vertebrata, possessing gills 

 in the adult, while the latter only possess them, 

 or traces of them, in early stages of growth. The 

 Amphibia formed a group to themselves, in which 

 we traced the loss of gills in the adult. In the 

 Reptiles, four-legged egg-laying animals, we found 

 not only a close relationship with birds, but also, 

 through the four-legged egg-laying Ornithorhyncus, 

 a relationship with the Mammalia. The last group 

 comprises all the furry animals, and culminates 

 in the order Primates, in which the great Cuvier 

 included Man. 



Another volume of this series, "The Story of 

 the Earth," has already dealt with the distribution 

 of animal life in time ; while " The Story of Ani- 



