CHAPTER XLVIII 

 PHYLUM CHORDATA 



The last of the phyla, and from the standpoint of efficiency the 

 highest, is Chordata (kor da' ta; G., chorde, cord, referring to the noto- 

 chord). In addition to the vertebrates, which are, generally speaking, 

 the largest, most conspicuous, and best known animals, the phylum 

 includes several forms of which only zoologists are aware and which, 

 therefore, have no common name. Some of these lower chordates 

 are very unlike the vertebrates and reveal their chordate character 

 only after close study. 



334. Characteristics. — The chordates possess three characteristics 

 which set them apart from all other animals, these being the possession 

 of (1) a notochord, (2) pharyngeal slits, and (3) a tubular nervous 

 system dorsally situated in the body. Of these structures the notochord 

 and the pharyngeal slits remain throughout life in only the lower chor- 

 dates, being in the higher ones replaced by other structures. The 

 nervous system, however, persists with the same general character 

 throughout, in the highest chordates reaching a very high degree of 

 development and functional activity. 



The notochord first appears, both ontogenetically and phylogenetically, 

 as a rod of cells. It is derived from a longitudinal outfolding of the 

 dorsal wall of the archenteron which becomes pinched off and lies at 

 the dorsal side of the alimentary canal (Fig. 201). It is, therefore, 

 entodermal. In somewhat higher types it becomes converted into a 

 more rigid, though still flexible, rod which runs the length of the body, 

 serving to stiffen it. In the vertebrates the notochord develops in 

 the embryo, but in the adult there is formed about it a series of bones 

 which together make up a vertebral column. This is the axis of an 

 internal skeleton, to which is added a large number of other bones 

 providing support and giving attachment to muscles. As the vertebral 

 column becomes more highly developed the notochord practically 

 disappears. Flexibility in the vertebral column is secured either by 

 articulation of the separate bones, or vertebrae, or by the compression 

 of discs of fibrocartilage between them. 



The 'pharyngeal slits are a series of passages on each side of the body 

 toward the anterior end leading from the cavity of the pharynx to the 

 outer surface of the body. They are found in all lower chordates and 

 in the lower vertebrates. In their walls are developed networks of 



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