CHAPTER XVIII 



PHYLUM CHORDATA 



337. This phylum includes, beside the typical Vertebrata 

 to be described in later chapters (fishes, amphibians, reptiles, 

 birds and mammals), several groups of much more simple 

 organization. These latter forms may be included under the 

 general head Protovertebrata, not because they all show close 

 relationship among themselves, but because of their primitive 

 character, considered as Chordata. They are of very great 

 interest to the biologist on account of the hints they may 

 offer concerning the ancestors of the vertebrates. For detailed 

 description of the manner of. life and the structure of these 

 primitive chordates the student must be referred to advanced 

 text-books of zoology. 



338. General Characters of the Chordata (Protovertebrata 

 and Vertebrata). — The Protovertebrata are allied with the 

 typical vertebrates and separated from the invertebrates by 

 the possession, either in the immature or adult condition, of the 

 following features : 



1. A mid-dorsal, longitudinal rod of cells (notochord) de- 

 rived from the entoderm, but often surrounded by mesodermal 

 structures (see Pig. 157). This lies ventral to and supports 



2. The central nervous system, a mid-dorsal cellular tube 

 with thickened walls derived from the ectoderm! 



3. Gill-slits or perforations connect the cavity of the pharynx 

 with the outside directly or through an atrial chamber. 



4. The heart is typically ventral to the digestive tract. 



339. In the Group of Protovertebrata may be placed: 



I. Balanoglossus, a soft-bodied, worm-like form whose claim 

 to a place among the Chordata rests upon the fact that an out- 

 growth of the gut extends into the proboscis, where it forms a 

 solid rod which in its origin suggests the notochord; a portion of 



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