CHAPTER 1 3 



THE ANIMAL CLIMAX— THE CHORDATES 



The last and most diversified group of 

 animals is the phylum Chordata, to which 

 man himself belongs (Fig. 13-1). These 

 animals have struck off on a new line of 

 development which has resulted in maxi- 

 mum size and adaptability. Not only are the 

 chordates the largest animals in existence 

 today, but they have adapted themselves 

 to more modes of existence than any other 

 group, including the arthropods. They are 

 found in the sea, in fresh water, in the air, 

 and on all parts of the land from the poles 

 to the equator. They range in size from 



spend periods of low temperature in a rela- 

 tively inactive condition. Since fish remain 

 in the water where the temperature does 

 not vary greatly, they have no need for a 

 temperature-regulating mechanism. 



All chordates possess at some time in 

 their life cycle three characteristics which 

 are not found among the invertebrates 

 (Fig. 13-2). The first is a dorsal tubu- 

 lar nerve cord, which varies from a more 

 or less undifferentiated tube extending 

 through the entire length of the body of the 

 lower chordates, to a shorter, highly dif- 



the tiniest fish to the great whales, which ferentiated tube, with a greatly enlarged 



anterior portion, the brain, in the higher 

 forms. In some chordates the nerve cord 

 is proportionately about the same as in the 

 invertebrates, while in others, such as man, 

 it assumes greater prominence, both in size 

 and importance. In invertebrates the nerve 

 cord is solid, but in all chordates it is tubu- 

 lar or hollow. 



reach a length of nearly 100 feet and a 

 weight of 100 tons and more (Fig. 13-55). 

 In order to penetrate the cold climates and 

 remain active, the birds and mammals 

 maintain a constant temperature (homo- 

 thermal ) . Those animals without a constant 

 temperature ( poikilothermal ) , such as the 

 amphibians and reptiles, are forced to 



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