316 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



recent forms the centra of the vertebrae are exceed- 

 ingly well ossified ; they have, especially in the region 

 of the neck, an exceedingly characteristic form of 

 surface, for they are saddle-shaped, being convex from 

 side to side, and concave from above downwards on 

 their anterior face ; as exactly the opposite arrange- 

 ment obtains on the posterior surface, it is obvious 

 that the vertebrae are able to move on one another, 

 and the neck capable of that mobility which is so 

 notable and useful a possession of the bird, whose 



ayi 



Fig. 130. Anterior and Posterior Surfaces of the Vertebrae of a Snake, 

 showing the form of the Centra, and the Zygosphenes (zys), and 

 Zygantra (zyt). 



anterior appendages are of no use for seizing food 

 or other objects. An exactly analogous arrange- 

 ment to this may be observed among the Ophiu- 

 roidea ; in the larger number of these brittle stars the 

 several ossicles have a certain power of movement on 

 one another, but this is limited by the development of 

 processes and pits analogous to the zygosphene and 

 zygantra of the Ophidian vertebrae. In such Ophiu- 

 roids, however, as are, like Astroschema, capable of 

 twisting or twining their arms round a straight 

 Gorgonian, the saddle -shaped faces are well developed, 

 but the limiting pits and processes are absent. 



In the Mammalia the faces of the centra are 

 often nearly plane, and from the intervening cartilage 

 there is developed (except in the Prototheria, where 



