The Flounders.—In the flounders, the halibut and its relatives, arctic genera (Hippoglossus and Atheresthes), have from 49 to 50 vertebræ. The northern genera (Hippoglossoides, Lyopsetta, and Eopsetta) have from 43 to 45; the members of a large semi-tropical genus (Paralichthys) of wide range have from 35 to 41; while the tropical forms have from 35 to 37.
In the group of turbots and whiffs none of the species really belong to the northern fauna, and the range in numbers is from 35 to 43. The highest number, 43, is found in a deep-water species (Monolene), and the next, 40, in species (Lepidorhombus, Orthopsetta) which extend their range well toward the north. Among the plaices, which are all northern, the numbers range from 35 to 65, the higher numbers, 52, 58, 65, being found in species (Glyptocephalus) which inhabit considerable depths in the arctic seas. The lowest numbers (35) belong to shore species (Pleuronichthys) which range well toward the south.
Spiny-rayed Fishes.—Among the spiny-rayed fishes the facts are more striking. Of these, numerous families are chiefly or wholly confined to the tropics, and in the great majority of all the species the number of vertebræ is constantly 24,—10 in the body and 14 in the tail (10+14). This is true of all or nearly all the Berycidæ, Serranidæ, Sparidæ, Sciænidæ, Chætodontidæ, Hæmulidæ, Gerridæ, Gobiidæ, Acanthuridæ, Mugilidæ, Sphyrænidæ, Mullidæ, Pomacentridæ, etc.
In some families in which the process of reduction has gone on to an extreme degree, as in certain Plectognath fishes, there has been a still further reduction, the lowest number, 14, existing in the short inflexible body of the trunkfish (Ostracion), in which the vertebral joints are movable only in the base of the tail. In all these forms the process of reduction of vertebræ has been accompanied by specialization in other respects. The range of distribution of these fishes is chiefly though not quite wholly confined to the tropics.
Thus Balistes, the trigger-fish, has 17 vertebræ; Monacanthus and Alutera, foolfishes, about 20; the trunkfish, Ostracion, 14; the puffers, Tetraodon and Spheroides, 18; Canthigaster, 17; and the headfish, Mola, 17. Among the Pediculates, Malthe and Antennarius have 17 to 19 vertebræ, while in their near relatives, the anglers, Lophiidæ, the number varies with the latitude. Thus, in the northern angler, Lophius piscatorius, which is never found south of Cape Hatteras, there are 30 vertebræ. In a similar species, inhabiting the north of Japan (Lophius litulon), there are 27. In another Japanese species, ranging farther south, Lophiomus setigerus, the vertebræ are but 19. Yet in external appearance these two fishes are almost identical. It is, however, a notable fact that some of the deep-water Pediculates, or angling fishes, have the body very short and the number of vertebræ correspondingly reduced. Dibranchus atlanticus, from a depth of 3600 fathoms, or more than 4 miles, has but 18 vertebræ, and others of its relatives in deep waters show also small numbers. These soft-bodied fishes are simply animated mouths, with a feeble osseous structure, and they are perhaps recent offshoots from some stock which has extended its range from muddy bottom or from floating seaweed to the depths of the sea.
A very few spiny-rayed families are wholly confined to the northern seas. One of the most notable of these is the family of viviparous surf-fishes (Embiotocidæ), of which numerous species abound on the coasts of California and Japan, but which enter neither the waters of the frigid nor of the torrid zone. The surf-fishes have from 32 to 42 vertebræ, numbers which are never found among tropical fishes of similar appearance or relationship.
The facts of variation with latitude were first noticed among the Labridæ. In the northern genera (Labrus, Tautoga, etc.) there are 38 to 41 vertebræ; in the semi-tropical genera (Crenilabrus, Bodianus, etc.), 30 to 33; in the tropical genera (Halichœres, Xyrichthys, Thalassoma, etc.), usually 24.
Equally striking are the facts in the great group of Pareioplitæ, or mailed-cheek fishes, composed of numerous families, diverging from each other in various respects, but agreeing in certain peculiarities of the skeleton.
Among these fishes the family most nearly related to ordinary fishes is that of the Scorpænidæ (scorpion-fishes, etc.).
This is a large family containing many species, fishes of local habits, swarming about the rocks at moderate depths in all zones. The species of the tropical genera have all 24 vertebræ. Those genera chiefly found in cooler waters, as in California, Japan, Chile, and the Cape of Good Hope, have in all their species 27 vertebræ, while in the arctic genera there are 31.