280 CLASSIFICATION OF CHOEDATA. 



rod between the nervous system and the gut. Couch describes 

 this fascinating little creature amongst the British Fish. 



Another form to be mentioned here is the worm-like Balano- 

 glossiis, which Mr Bateson has shown to have a dorsal axial 

 rod (notochord) and a dorsal hoUow nervous system, besides a 

 series of gill-slits, like a fish. 



The whole of those animals that possess a dorsal nervous 

 system and a notochord during some period of their existence 

 are best known, then, as Chordata. 



The Chordata can again be divided into two sections — namely, 

 the Acrania and the Graniota. The former contains those lowly 

 intermediate forms which Mr Bateson classifies as follows : — 



1. Hemichordata (Balanoglossus). 



2. Urochordata (Ascidians). 



3. Cephalochordata (Amphioxus). 



Whilst Fish, Amphibia, Eeptilia, Aves, and Mammalia may be 

 placed in a fourth division — 



4. Vertebrata. 



Thus for the old term Vertebrata we now substitute Chordata, 

 and use the term Vertebrata only for those Chordata with a 

 cranium and a distinct skeleton, either cartilaginous or osseous, 

 so that the Chordata tabulate as follows : — 



!1. Hemichordata. 

 2. Urochordata. 



Chordata 



3. Cephalochordata. 



( Pisces. 

 I Amphibia. 

 Crwniota (brain present) 4. Vertebrata ■( Reptilia. 



I Aves. 

 I Mammalia. 



