CHAPTER V. 



TRACES OF UNITY IN THE VERTEBRA AND 

 ANNELLUS. 



As commonly defined, the vertebra and the annellus 

 have little or nothing in common. The former is made 

 up of a series of rings arranged in the same plane around 

 a solid centre or "body," and the muscles belonging to it 

 are on the outside. The latter is a single, centreless 

 ring, with its special muscles on the inside. It is diffi- 

 cult, however, to rest satisfied with this way of looking 

 at the matter. It is certain that the line of demarcation 

 between the vertebra and the annellus is drawn with 

 little firmness in many places, and not at all in others : 

 it is by no means certain that it is to be found even 

 where it is supposed to be most firmly drawn, that is, 

 across the " body " of the vertebra. And there is no 

 great difficulty in making good this statement. 



In the thoracic vertebrae of chelonian reptiles the 

 place of the " bodies " is occupied by narrow bony belts 

 or arches, while the most body-like parts of the vertebrae 

 are over the spinal cord instead of wider it. In certain 

 extinct ganoid fishes of the Devonian or Old Red Sand- 

 stone period, as in the Coccosteus, there is a blank 

 space between the neural and haemal spines where the 

 bodies of the spinal vertebrae ought to be a plain 

 proof that in the living animal this space was occupied 

 by tissues too perishable to allow of calcification, by 



