CHAPTER X. 

 VEETEBRATA. 



Aninials more or less resembling the Frog in. 

 structure (seep. 304). 



Classes. — Fishes. Amphibia. Eeptiles. Birds. Mammals. 



The Vertebrata, or animals with backbones, form a 

 group very sharply defined from the rest of the animal 

 kingdom, and yet there are, as has been stated, several 

 interesting links between them and the lower groups. 

 Link bet\\reen Vertebrates and Inverte- 

 brates. — Two of these links, Balanoglossus and the 

 Ascidian, have already been referred to ; a third, 

 Amphioxiis, is included, in classification, among the 

 Vertebrates themselves, although it; can only be looked 

 upon as a sort of poor relation of the true Vertebrates. 

 It is so diSerent from the true Vertebrates, that it has 

 to be put into a sub-division by itself. K\\ other 

 Vertebrata have true heads, clearly distinguished from 

 the region of the spinal column, and a backbone ; but 

 Amphioxus has nothing worth calling a head, and it 

 has no backbone, though it has the essential structure 

 which in other vertebrates precedes the backbone in 

 development, namely, the notochord. To meet this 

 difficulty, the name of Choedata (animals with a noto- 



