CHAPTER I. 



CHORDATA. 



Animals with a rwtochord, a hollow dorsally placed nervous 

 system, and a jjharynx opening to the exterior by lateral pcissages. 



The group Chordata is a division of the animal liingdom 

 superior to a phylum. It includes four phyla and is to be 

 compared in its rank to such groups as the Jletazoa and Codo- 

 mata, both of which are phylum-including di\'isions. The four 

 phyla into which the group Chordata is divided are, stating them 

 in the order in which they are dealt with in this work, the 

 Cephalochorda, which includes but a single genus, Amphioxus ; the 

 Vertebrata, which is by far the largest and most important 

 division of the group ; the Tunicata, which includes a con- 

 siderable number of marine forms of low organization ; and 

 lastly the Enteropneusta, which has but a small number of 

 genera mainly of vermiform appearance and is the most out- 

 lying phylum of the group. Indeed, by some higlily competent 

 authorities the Enteropneusta are placed altogether outside the 

 Chordata, largely on account of their early development, which 

 differs in important particulars from that of other Chordata 

 and approaches that of Echinodermata ; and because it is not 

 certain that they possess that typically chordate organ, the 

 notochord. While not presuming to pronounce an opinion 

 on the latter point beyond saying that if the notochord is present 

 in Enteropneusta, its development, structure and relations to 

 other organs differ considerably from those of the notochord in 

 the other phyla, we desire to emphasise quite distinctly our 

 opinion that the Enteropneusta are Chordates. They present 

 most clearly the other characteristic features of that group, viz., 

 the hollow central nervous sj^stem and the perforated pharjTigeal 

 wall — features of organization found in no other group of the 



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