CHAPTER XVII 



PEOTOCHOEDATA 



Classification adopted — 



Hemichorda (Enteropneusta) (Balanoglossida 



ICephalodiscida 



Cephalochorda 



/-Larvacea 



Urochorda (Tunicata) 



TAscidiae simplices 

 Ascidiacea \ Ascidiae compositae 



lAscidiae Luciae 

 iThaliacea 



Although the second and third volumes of this work are destined 

 to treat of the embryology of the great group of Vertebrata or 

 Cliordata to which we ourselves belong, yet it is necessary to deal at 

 the close of this first volume with the embryology of the lowest 

 members of this group, for two reasons. First, because we must show 

 how the Vertebrata are related to the Invertebrate groups whose 

 embryology has been discussed in this volume ; and, secondly, because 

 one of the three groups which make up the division of Vertebrata 

 known as Protochordata are still frequently regarded as Tnvertebrata. 



The Vertebrate affinities of the Enteropneusta are denied by 

 many zoologists, and although all now admit that the Urochorda 

 are degenerate Vertebrata, yet these animals are so degenerate that 

 their adult structure shows more similarity to that of a Podaxonian 

 or Polyzoan than to that of any ordinary Vertebrate. The name 

 Protochordata is, of course, nothing more than a convenient collective 

 term for the poor relations of the vertebrate phylum which fall far 

 below the rest of their brethren in structure and activity. It is by 

 no means implied that its three subdivisions — Hemichorda, Cephalo- 

 chorda, and Urochorda — are specially closely allied to one another. 

 What indeed their relationship to one another actually is, will come 

 out when their life-histories are studied. 



We prefer the term Vertebrata to Chordata for the following 



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