CHAPTER 



18 



THE PHYLA 

 HEMICHORDATA 

 AND CHORDATA 



The phyla Hemichordata and Chordata consist of bilateral, triploblastic 

 animals, usually with a well-developed coelom which primitively arises, as in 

 echinoderms, by evagination from the embryonic gut. These animals com- 

 monly show evidences of metamerism, at least in some of their structures. 

 The Chordata are further characterized by three fundamental features: (1) a 

 dorsal, tubular central nervous system; (2) gill slits, or pharyngeal clefts, at 

 least during a part of the life cycle; and (3) an extensive primary endo- 

 skeletal structure, the rod-like notochord, from which the name of the phylum 

 is derived. Members of the phylum Hemichordata possess characteristics 

 which are comparable with all of these, but which are present in a relatively 

 rudimentary condition. The hemichords have pharyngeal clefts, sometimes 

 well developed; the central nervous system has a dorsal component, although 

 this is not the principal part of the system; and an endoskeletal structure 

 resembling the chordate notochord is present but very limited in extent. 

 This "half-chord" is responsible for the name Hemichordata. On the basis 

 of these structural resemblances, the hemichords are often regarded as the 

 most primitive members of the phylum Chordata. We shall consider them 

 rather as the non-chordate phylum whose characteristics most closely approach 

 those of true chordates. The hemichords, like the most primitive chordates, 



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