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 Generelle Morphologie (1866), 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 OP 

 VERTEBRATES. 



A. Skull-less (^cranio) 1. Tube-hearted 1. Leptocardia 



I a. Single-nostrilled (Monorhina) 2. Round-mouths 2. Cyclostoma 



/ I. [3. FishPB 3. Pisces 



( Amnion-less J 4. Mud-fishes 4. Dipneusta 



b. Double-nostrillod 1 Anamnia (5. Amphibians 6. Amphibia 



Amphirhina < n , 6. Reptiles 6. Reptilia 



[ With Amnion I 7. Birds 7. Aves 



\ Amniota (t. 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 (Gra- 

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



