Because, unfortunately, we are not sure that there are any "fishes." The terms "beast, bird, and fish," notwithstanding common usage and the sanction of Scripture, are devoid of scientific accuracy. For "beast" includes turtles and alligators, and excludes the aquatic mammals, whales, porpoises, manatee, and dugong. "Bird" includes bats and pterodactyls, and excludes the ostriches and penguins, which cannot fly. So "fish" is not only held by some persons to embrace the aquatic mammals, but also, when employed in a stricter sense, it includes forms differing among themselves in many important points.
At any rate, the "fish-like vertebrates" present the following well-marked groups:
1. Amphioxus lanceolatus; the lancelet. A single genus with perhaps a single species, but so peculiar as to have received the following appellations: Branchiostoma, Cirrostomi, Pharyngobranchii, Leptocardia, Acrania, Entomocrania, Dermopteri.
2. Myzonts, or Marsipobranchii; the hag-fishes and lamprey-eels.
3. Plagiostomes, or Elasmobranchii; sharks and skates.
4. Holocephala; the Chimœra and Callorhynchus.
5. Ganoids; the sturgeons (Acipenser and Scaphyrhynchus); the spoonbill (Polyodon); the mud-fish (Amia); the gar-pike (Lepidosteus); and the Polypterus and Calamoichthys of Africa, with many fossil forms.
6. Dipnoans; the mud-fishes of Africa, South America, and Australia (Protopterus, Lepidosiren, and Ceratodus).
All of the above were formerly, and are now popularly, regarded as fishes.
But the fishes, proper, or ordinary fishes, are now called:
7. Teleosts; the perch, salmon, cod, mackerel, and all others not included within the other six groups.
Some have included Amphioxus with the Myzonts; others the Plagiostomes with the Ganoids. The most natural combination seems to be that of the Ganoids with the Teleosts; and to this larger group the term Pisces has been applied. But for the present it is safer to recognize the distinctions, and to make our generalizations more exact.
What, then, is a gar-pike? Is it a Ganoid or a Teleost? Curiously enough, the prefix "gar" (signifying a dart or pointed weapon) is employed to designate two fishes, of which one (Belone) is a marine Teleost, and the other (Lepidosteus) is a fluviatile Ganoid. Both have long jaws with sharp teeth, but in other respects they are very unlike. It will be better to call Belone the "gar-fish" and Lepidosteus the gar-pike.
The general appearance of the gar-pike is sufficiently indicated by
- These common names are very perplexing. Thus the true pike is Esox. The name dog-fish is popularly applied to Menobranchus, a batrachian; to Amia, a ganoid; and to Acanthias, a shark.