Subphylum VERTEBRATA CRANIATA 



THE present work deals with the Vertebrata Craniata, that highest 

 branch of the Phylum Vertebrata, or Chordata, which comprises 

 the classes Cyclostomata, Pisces, Amphibia, Eeptilia, Aves, and 

 Mammalia. 



Besides the Subphylum Craniata, there are included in the 

 Phylum Vertebrata the Cephalochorda, represented by Amphioxus and 

 a few allied genera, the degenerate Tunicata, and the Enteropneusta. 

 The latter group is an aberrant one, whose right to a position in 

 the Phylum is less thoroughly established. 



That the six Classes of Craniata mentioned above belong to a 

 single branch derived from some common ancestral form, which 

 had acquired many new characters, and had advanced very con- 

 siderably beyond the grade of structure found in their nearest 

 allies the Cephalochorda, cannot be doubted when we study their 

 anatomy. 



Not only do the Craniata differ from the Cephalochorda in the 

 possession of a large brain more or less completely enclosed in a 

 cartilaginous cranium, to which character they owe their name ; 

 but they have made a great advance in almost every detail of their 

 organisation. The comparison given below shows clearly the great 

 gap which separates the lowest known Craniate from Amphioxus. 

 In making this comparison it must, of course, be remembered that 

 Amphioxus is itself a highly specialised animal, differing in all 

 probability very considerably from the early vertebrate ancestor 

 from which both these surviving branches have diverged. 



The Craniata have in common with the Cephalochorda the 

 fundamental characters of the Vertebrata (Chordata), such as : the 

 bilateral symmetry ; the pronounced metameric segmentation of the 

 mesoblast ; the gut opening by an anterior mouth and a posterior 

 anus, and provided with paired lateral gill-slits ; the dorsal tubular 

 central nervous system ; the supporting notochord of endodermal 

 origin, unsegmented and underlying the central nervous system ; the 

 prolongation of the body beyond the anus in the form of a metameri- 

 cally segmented tail, containing tissues derived from the three 

 germ-layers ; the well-developed coelom, and the separate blood- 

 vascular system. 



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