CLASSIFICATION OF VERTEBRATES. 97 



nervous and blood-vessel systems. As we have already 

 seen, this process of articulation, or metameric formation, 

 must essentially be regarded as terminal germination. 

 Each distinct trunk-segment, or metameron, represents an 

 individual. Thus the Vertebrates with their internal 

 segmentation stand in a similar relation to their inarticulate 

 Invertebrate ancestors, the Chorda Animals, as do the out- 

 wardly segmented Ringed Worms (Annelida) and Articu- 

 lated Animals (Arthropoda) to the simple inarticulate 

 Worms from which they originated. 



The tribal history of Vertebrates is rendered much more 

 intelligible by the natural classification of the tribe which 

 I proposed first in my GenereUe Morphologie (18G6), and 

 afterwards improved in many ways in " The Natural History 

 of Creation" (Chap. XX., p. 192, etc.). In accordance with 

 that, existing Vertebrates must be divided into at least 

 eight classes, as follows : — 



SYSTEMATIC SURVEY OF THE EIGHT CLASSES OF 



VERTEBRATES. 



A, Skull-less (j4cranto) 1. Tube -hearted 1. Leptocardia 



a. Single-nostrilled [Monorhina) 2. Round-mouths 2. Cyclostoma 



I. 



R QV«iio,i 1 1 Amnion-Ies3 



& sKuiiea I j^ Double-no^rillod ] Anamnia 



3. Fishis 3 Pieces 



4. Mud-fishes 4. Dipneusta 

 ,5. Amphiliians 5. Amphibia 



Amphirhiua ^ ^^ ,q Reptiles 6. Keptilia 



With Amnion I 7. Birds 7. A%es 



Amnuita ( 8. Mammals 8. Mammalia 



The whole Vertebrate tribe may primarily be divided 

 into the two main sections of the Skull-less and the 

 Skulled Vertebrates. Of the earlier and lower section, that 

 of the Skull-less {Acrania), the Amphioxus is alone extant. 

 To the more recent and higher section, the Skulled {Cra- 

 niota), belong all other existing Vertebrates up to Man. The 



