Ninth Section. 



Vertebrata. 



General Review of the Group. 



§ 315. 



The most essential characters of the Vertebrata are the possession 

 of a skeleton traversing the longitudinal axis of the body, and the 

 division of the body into a number of metameres (primitive ver- 

 tebrae). They differ from the Tunicata, with which, alone of all 

 the divisions of the Invertebrata they have any well-marked rela- 

 tions, by this metamerism. They have more distant relations to the 

 Vermes, but then this group obviously has relations to most of the 

 other phyla. 



The axial skeleton separates a dorsal from a ventral division of the 

 body. The former contains the central nervous system, the latter the 

 digestive canal, which is continued on from a respiratory chamber, 

 and which, with the organs differentiated from it, is embedded in a 

 ccelom. Two regions therefore can be made out, which extend 

 along the body ; the upper one is neural, and the lower enteric; 

 the chief trunks of the system of canals for the nutrient fluid 

 belong to the latter region. 



The various divisions are set in order in the following review : 



A. Acrania. 



Leptocardii. 

 Amphdoxus. 



B. Craniota. 



I. Cyclostomata.* 



Myxinoidea. 



Bdellostoma, Myslne. 

 Petromyzontes. 



Petromyzon. 



* The Cyclostomata should be placed apart from all the rest of the Craniota, for 

 their whole organisation shows that they branched off very early from the Craniota. 



