PHYLUM VII. CHORD AT A. 



The group of Chordates has been formed from animals 

 taken from other divisions and united with the Vertebrates. 

 The Tunicata were formerly classed as Mollusca, the Enter- 

 opneusta as both Echinoclerms and Worms. Yet when all 

 these are studied more accurately, they are seen to have 

 three important characteristics which are common to all 

 and which mark them off sharply from all other phyla. 

 These features are: (1) the possession of gill-slits; (2) a 

 nervous system lying wholly on one side of the alimentary 

 canal; and (3) a notochord which has given the name to 

 the phylum. 



Gill-slits are pockets in the sides of the gullet or pharynx 

 which lead from the throat to the exterior. Blood-vessels 

 run in the partitions between the slits, sending fine branches 

 to the gills, which are usually developed on the sides of 

 the slits. 



The notochord is a gelatinous skeletal structure, cylin- 

 drical in shape, which lies between the alimentary canal 

 and the central nervous system. In the lower groups it 

 forms the sole skeleton; in the vertebrates it is more or 

 less completely replaced by the vertebral column, to be 

 described below. 



There are four divisions of the Chordata, only three of 

 which need description here: Tunicata, Leptocardii, and 



Vertebrata. 



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