CHAPTER XVIII. 



PHYLUM VIII. CHORDATA. 



329. 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. 



330. General Characters of the Chordata (Protoverte- 

 brata and Vertebrata). The Protovertebrata are allied with 

 the typical vertebrates and separated from the invertebrates by 

 the possession, either in the larval 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 Fig. 155). 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 cham- 

 ber. 



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



331. In the Group of Pro to vertebrates 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 outgrowth of the gut extends into the proboscis, where it 

 forms a solid rod which in its origin suggests the notochord; 

 a portion of the nervous system is dorsal ; and gill-slits occur. 



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