196 FISHES CHAP. 



form individual vertebrae may be quite different to that which 

 takes place in the Dog-Fish, and may even be accompanied by 

 their more or less complete fusion. 



In the more primitive types of vertebral column, such as 

 are characteristic of many fossil and not a few existing Fishes, 

 arcualia alone are present, and remain associated with a per- 

 sistent notochord which has grown with the growth of the 

 animal. In the more specialised Fishes, on the contrary, the 

 need of an axial support for the body, which, while retaining 

 the necessary flexibility, must possess greater strength, has 

 resulted in the development of a series of solid cartilaginous, 

 calcified or bony, discoidal joints or segments, the centra, which 

 surround and more or less completely replace the notochord, and, 

 while supporting, form also a bond of connexion between the 

 dorsal and ventral arches. Notwithstanding their superficial 

 resemblance, an important developmental distinction is to be 

 noted in the mode of formation of centra in different Fishes, 

 which enables one kind to be distinguished as " chorda-centra" 

 and another as " arch-centra." ^ Chorda-centra are centra formed 

 by the conversion of the chordal sheath into a series of ring-like 

 cartilaginous segments, which subsequently, by a process of inward 

 thickening, become biconcave, disc-like structures, and more or less 

 completely replace the notochord, except in the spaces between 

 them. Arch-centra, on the other hand, owe their formation to 

 the growth of the bases of the primary arcualia round the noto- 

 chord, external to the chordal sheath, and their subsequent fusion 

 to form annular segments, which, later, become biconcave centra. 

 Of Fishes which possess vertebral centra the Elasmobranchs alone 

 have chorda-centra ; the Holostei and the Teleostei, and very 

 probably the Crossopterygii also, having arch-centra. The Dipnoi 

 and the Holocephali, and the Chondrostean Teleostomi are acent- 

 rous — that is, they are devoid of vertebral centra and possess a 

 persistent notochord. ISTeither in their embryonic development nor 

 in their evolution in time are the different vertebral components 

 synchronous in their appearance. Developmentally, the arcualia 

 are the first to be formed, and of these those on the dorsal aspect 

 of the notochord appear earlier than their representatives on the 

 ventral side, while the centra are the last of all ; and in a general 

 way the palaeontological sequence agrees with the embryological. 



' Gadow, op. cit. p. 190. 



