PHYLUM XII, CHORDATA 



In the phylum Chordata life has an instrument through which 

 it not only can take cognizance of its environment through the 

 special organs of touch, taste, smell, hearing and sight, but has 

 a centrally controlled system of nerves and a complex muscular 

 system to respond quickly and accurately to this environment. 

 The members of this phylum, comprising the most highly evolved 

 animals, possess in common throughout life or only in the young, 

 (i) the notochord (primitive backbone), a cord of cells devel- 

 oped from the endoderm, extending along the middle line of 

 the dorsal side of the body between the digestive canal and the 

 extension of the central nervous system (spinal cord of higher 

 forms) ; (2) branchial clefts, a series of clefts perforating the 

 walls of the pharynx ; and (3) the neurocoele, one or more cavities 

 within the central (dorsal) nervous system. 



The main advance of the typical Chordata (Vertebrata) 

 upon the Invertebrata lies in the development of the second 

 body cavity, which lodges the central nervous system, — the 

 spinal cord and its anterior enlargement, the brain. This 

 cavity is dorsal to the one containing the digestive canal, heart 

 and lungs. 



Derivation of name. — Chordata > Latin chorda, a cord, 

 referring to the universal presence of the notochord at some stage 

 in development. 



This phylum is divided into the following sub-phyla : — 



1. Adelochorda 



2. Urochorda 



3. Vertebrata 



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