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 Crossopterygu 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. Neither 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. 
1 Gadow, op. cit. p. 190. 
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