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 OF 

 VERTEBRATES. 



A . Skull-less (^Acrania) 1. Tube-hearted 1. LeptocarJia 



/ a. Single-nostriltcd (Monorhina) 2. Round-mouths 2. Cyclostoma 



( , I. 13. Fishes 3 Pisces 



<?lniU<vi 1 ( Amnioii-Iess J 4. Mud-fishes 4. Dipneusta 



Craniuta \ U ' Don1 >' (> - no ' itrill - xJ J Anaumia 1 5. Amphibians 6. Amphibia 



If. (6. Reptiles 6. Reptilia 



With Amnion ] 7. Birds 7. Aves 



Auui/dta (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 



