It is evident that the two families most decidedly tropical in their distribution, the morays (_Murænidæ_) and the snake-eels (_Ophichthyidæ_), have diverged farthest from the primitive stock. They are most "degenerate," as shown by the reduction of their skeleton. At the same time they are also most decidedly "eel-like," and in some respects, as in coloration, dentition, muscular development, most highly specialized. It is evident that the presence of numerous vertebral joints is essential to the suppleness of body which is the eel's chief source of power.
So far as known the numbers of vertebræ in eels range from 115 to 160, some of the deep-sea eels (_Nemichthys_, _Nettastoma_, _Gordiichthys_) having much higher numbers, in accord with their slender or whip-like forms.
Among the morays, _Muræna helena_ has 140; _Gymnothorax meleagris_, 120; _G. undulatus_, 130; _G. moringa_, 145; _G. concolor_, 136; _Echidna catenata_, 116; _E. nebulosa_, 142; _E. zebra_, 135. In other families the true eel, _Anguilla anguilla_, has 115; the conger-eel, _Leptocephalus conger_, 156; and _Murænesox cinereus_, 154.
=Variations in Fin-rays.=--In some families the number of rays in the dorsal and anal fins is dependent on the number of vertebræ. It is therefore subject to the same fluctuations. This relation is not strictly proportionate, for often a variable number of rays with their interspinal processes will be interposed between a pair of vertebræ. The myotomes or muscular bands on the sides are usually coincident with the number of vertebræ. As, however, these and other characters are dependent on differences in vertebral segmentation, they bear the same relations to temperature or latitude that the vertebræ themselves sustain.
Thus in the _Scorpænidæ_, _Sebastes_, and _Sebastolobus_ arctic genera have the dorsal rays xv, 13, the vertebræ 12+19. The tropical genus _Scorpæna_ has the dorsal rays xii, 10, the vertebræ 10+14, while the genus _Sebastodes_ of temperate waters has the intermediate numbers of dorsal rays xii, 12, and vertebræ 12+15.
=Relation of Numbers to Conditions of Life.=--Fresh-water fishes have in general more vertebræ than marine fishes of shallow waters. Pelagic fishes and deep-sea fishes have more than those which live along the shores, and more than localized or non-migratory forms. To each of these generalizations there are occasional partial exceptions, but not such as to invalidate the rule.