CHAPTER I 



, THE CHORD ATE PHYLUM 



In classifying living organisms it was found necessary to divide 

 them into Plant and Anirnal kingdoms. Each of these, in turn, 

 was divided into large groups, or phyla, each phylum separated 

 from some other by only a few fundamental structural charac- 

 teristics. On this basis all animals with a vertebral column were 

 placed in the phylum Vertebrata. However, as research in natural 

 history and embryology continued, several primitive groups of 

 animals were discovered which lacked vertebrae but which were 

 evidently closely allied to the vertebrates and not to any other 

 known phylum. As a result the phylum now includes the verte- 

 brates and these primitive forms, and is called the Phylum 

 Chordata. 



Considered either as to range in size or complexity of struc- 

 ture, the Chordates form the most diverse phylum of animals 

 which we know. Its members range in size from animals barely 

 visible to the naked eye to the whale and the extinct dinosaur; 

 in structure from worm-like, swimming creatures to the elephant 

 and the bird; and in physiological organization and function 

 from stationary animals with a degenerate nervous system (their 

 only apparent activity being the digestion of passively acquired 

 food, and the reproduction of their kind) up to the anthropoid 

 apes and man. 



The chordates have only three characteristics which are not 

 found in any other group, and as two of these disappear in the 

 adults of some vertebrates, we must state that embryologically 

 all chordates have (1) a notochord which gives the group its 

 name; (2) a dorsal hollow nerve cord; and (3) gill slits in the 

 pharynx. Thus the phylum is linked by fundamental develop- 

 mental characters, not by structures which are acquired during 

 growth and later development. Man, for example, is as surely a 



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