=Pelagic Fishes.=--Among the free-swimming or migratory pelagic fishes, the number of vertebræ is usually greater than among their relatives of local habits. This fact is most evident among the scombriform fishes, the allies of the mackerel and tunny. All of these belong properly to the warm seas, and the reduction of the vertebræ in certain forms has no evident relation to the temperature, though it seems to be related in some degree to the habits of the species. Perhaps the retention of many segments is connected with that strength and swiftness in the water for which the mackerels are preeminent.
The variations in the number of vertebræ in this group led Dr. Günther to divide it into two families, the _Carangidæ_ and _Scombridæ_.
The _Carangidæ_ or _Pampanos_ are tropical shore fishes, local or migratory to a slight degree. All these have from 24 to 26 vertebræ. In their pelagic relatives, the dolphins (_Coryphæna_), there are from 30 to 33; in the opah (_Lampris_), 45; in Brama, 42; while the great mackerel family (_Scombridæ_), all of whose members are more or less pelagic, have from 31 to 50.
The mackerel (_Scomber scombrus_) has 31 vertebræ; the chub mackerel (_Scomber japonicus_), 31; the tunny (_Thunnus thynnus_), 39; the long-finned albacore (_Germo alalonga_), 40; the bonito (_Sarda sarda_), 50; the Spanish mackerel (_Scomberomorus maculatus_), 45.
Other mackerel-like fishes are the cutlass-fishes (_Trichiuridæ_), which approach the eels in form and in the reduction of the fins. In these the vertebræ are correspondingly numerous, the numbers ranging from 100 to 160. _Aphanopus_ has 101 vertebræ; _Lepidopus_, 112; _Trichurus_, 159.
In apparent contradiction to this rule, however, the pelagic family of swordfishes (_Xiphias_), remotely allied to the mackerels, and with even greater powers of swimming, has the vertebræ in normal number, the common swordfish having but 24.
=The Eels.=--The eels constitute a peculiar group of soft-rayed ancestry, in which everything else has been subordinated to muscularity and flexibility of body. The fins, girdles, gill-arches, scales, and membrane bones are all imperfectly developed or wanting. The eel is perhaps as far from the primitive stock as the most highly "ichthyized" fishes, but its progress has been of another character. The eel would be regarded in the ordinary sense as a degenerate type, for its bony structure is greatly simplified as compared with its ancestral forms, but in its eel-like qualities it is, however, greatly specialized. All the eels have vertebræ in great numbers. As the great majority of the species are tropical, and as the vertebræ in very few of the deep-sea forms have been counted, no conclusions can be drawn as to the relation of their vertebræ to the temperature.