126 ORGANIC EVOLUTION CONSIDERED 



■other existing Vertebrata this structure is not its 

 destined to become the permanent or bony vertebi 

 column. On the contrary, it gives way to, or is i 

 placed by this permanent bony structure at a la' 

 stage of development. Consequently, it is very si 

 gestive that so distinctly embryonic a structure 

 this temporary cartilaginous axis of all the otl 

 known Vertebrata should be found actually persisti 

 to the present day as the permanent axis of Amp, 

 oxus. In particular, we must notice that the wall 

 the neck is always perforated by what in Amphios 

 are the gill-openings, and that the blood-vessels 

 they proceed from the heart are always distributed 

 the form of what are called gill-arches, adapted 

 convey the blood round or through the gills for t 

 purpose of aeration. In all existing fish and otl 

 gill-breathing Vertebrata this arrangement is pern 

 nent. It is likewise met with in a peculiar kind 

 worm, called Balanoglossus — a creature so peculii 

 indeed, that it has been constituted by Gegenbaui 

 class by itself. We can see by the wood-cuts, that 

 presents a series of gill-slits, like the homologc 

 parts of the fishes with which it is compared, *. 

 fishes of a comparatively low type of organizatii 

 which dates from a time before the development 

 external gills. Now, as I have already said, these g 

 slits are supported internally by the gill-arches, 

 the blood-vessels which convey the blood to 

 oxygenated in the branchial apparatus; and 1 

 whole arrangement is developed from the anter 

 part of the intestine, as is likewise the respirati 

 mechanism of all the gill-breathing Vertebra 

 That so close a parallel to this peculiar mechani 

 should be met with in a worm, is a strong additioi 

 piece of evidence pointing to the derivation of 1 

 Vertebrata from the Vermes." * 



* Darwin and after Darwin, p. 144. 



