ANATOMY OF VERTEBRATES. 



41 



under part of the centrum, as, e. g. the two which articulate with 

 the basioccipital in the Arapaima gigas. As the centrum of the 

 atlas retains its normal relations to the other elements, and the 

 ordinary mode of articulation with the body of the second verte- 

 bra, this shows no c odontoid process ' in fishes. 



The number of vertebra? varies greatly in the different osseous 

 fishes : the Plectognathi (Diodon, Tetrodon) have the fewest and 

 largest: the apodal fishes (Eels, Gymnotes) have the 

 most and smallest, in proportion to their size. It is not 

 easy to determine the precise number, on account of the 

 coalescence of some of the vertebra;, or at least of their 

 central elements, in particular parts of the column. In- 

 stances of anchylosis of some of the anterior vertebra?, 

 analogous to that noticed in the cartilaginous Sturgeons, 

 Chima?ra?, Rhinobates, and some Sharks, occur also 

 amongst the osseous fishes, as in many Siluroid and Cy- 

 prinoid species, in Loricaria and Dactylopterus. Fig. 35 

 represents the four singularly elongated anchylosed ante- 

 rior vertebra? in the Tobacco-pipe fish (Fistularia tabac- 

 caria). A coalescence of several vertebra? is more con- 

 stant at the opposite end of the column in osseous fishes, 

 in order to form the base of the caudal fin, when this is 

 symmetrical in form, as in fig. 33, and in most existing 

 species of Teleostomi. But this modification is arrested 

 at different stages in the piscine class. In Cyclostomi 

 the gristly parts of the vertebra? continue distinct, with 

 gradual reduction in size to the taper end of the long tail : 

 in Protopterl the bony representatives of the caudal ver- 

 tebra? behave in the same way: the notochord persists in 

 both orders. In 3furmiidce > where it is changed into cen- 

 trums, these also gradually diminish in size, and remain distinct 

 to the tail-end. The continuous vertical fold of skin bordering 

 the compressed, long, and slender termination of the vertebral 

 column is not specialised as a caudal fin. 1 In Plagiostomi, Holo- 

 cephali, Sturionida, and many Ganoidei, the caudal fin, fig. 

 29, c, is formed chiefly by the haemal spines and appendages, 

 developed to support a lower ' lobe ;' the vertebra? continue 

 distinct to the end of the tail, which bending upward, seems to 

 form an upper lobe longer than the lower : to this unsymme- 

 trical tail-fin the term ' heterocercal ' is applied. By decreased 



1 This primitive embryonal basis of the piscine tail-fin is not to be confounded, 

 because it is symmetrical as to shape, with the extreme stage of developemental modi- 

 fication constituting the true ' homocercal ' type of most existing fishes. 



Anchylosed 

 anterior ver- 

 tebra, Pipe- 

 Fish 



(Fistularia) 



