PART III. 



CHAPTER XLV. 



SUB-KINGDOM VERTEBRATA. 



General Characters. 



The Vertebrata, or highest types of the entire animal kingdom, are 

 distinguished as a whole from all the preceding sub-kingdoms (col- 

 lectively designated as the Invertebrata) by the general presence of 

 an internal skeleton, and more especially of a cylindrical longitudinal 

 axis, termed the notochord, which is usually replaced in the adult 

 by a series of cartilaginous or bony segments collectively constitut- 

 ing the vertebral column. This axis, or column, separates the smaller 

 dorsal or neural tube of the body from the larger ventral or visceral 

 (haemal) tube ; and the body itself, together with its appendages, is 

 always symmetrical to this axis, and is never externally divided into 

 segments. Limbs may be completely wanting, but when present 

 they never exceed two pairs, and are always turned away from the 

 dorsal or neural aspect of the body. 



That the Vertebrata have been derived from the Invertebrata at 

 an extremely early epoch of the earth's history is practically certain ; 

 and, although we are unable to point to the direct ancestors of the 

 sub-kingdom, yet we have an inkling of this relationship exhibited 

 by the presence of a notochordal structure in the earlier stages of 

 the Ascidians, while there are also certain features in the organisa- 

 tion of the Annelids suggestive of their being allied to the primitive 

 stock whence the Vertebrates took their origin. Since, however, it 

 is probable that these primitive types were soft animals, it is unlikely 

 that any light will be thrown on the origin of Vertebrates by means 

 of Palaeontology ; and if the problem is ever to be solved it will be 

 by the aid of Embryology. 



