A single family composes this order, which comprehends great numbers both in genera and species; they are anguilliform or snake-like, elongated in form, the skin thick and soft, and have no ventral fins. In this order are included the Ammodytes, Gymnotes, Murænas, and Anguilla, or eels.
The Ammodytes have the body elongated and serpent-like, having a continuous fin extending along the greater part of the back, with another at the opposite side, and a third or forked fin at the end of the tail. The muzzle is also long; the lower jaw longer than the upper. A. lancea (Fig. 363) buries itself in the sand; hence it is called the sand-eel; it hollows out a burrow for itself in the sand with its muzzle to the depth of fifteen or twenty inches, where it hunts out worms, on which it feeds, while it shelters itself from the jaws of many voracious fishes, which eagerly pursue it for its delicate flesh. In appearance the Ammodytes lancea is silvery blue, brighter on the lower parts than on the upper, the radiating fins on the abdomen being alternately white and bluish in colour.
Fig. 363. The Lance (A. lancea).
The gymnotes are long, nearly cylindrical, and also serpent-like, the tail being long in comparison to the other parts of the body. Beneath the tail is a long swimming fin, the only locomotive organ, and it is this nakedness of the back which confers its designation of γυμνὸς, naked, νῶτος, back.
Fig. 364. The Gymnotus Electricus, or Electrical Eel.
The Gymnotes are fresh-water fishes of South America, where they attain a great size. There are several species, but the most remarkable, from its singular physical properties, is the Electrical Eel, Gymnotus electricus (Fig. 364). These properties enable the gymnotus to arrest suddenly the pursuit of an enemy, or the flight of its prey, to suspend on the instant every movement of its victim, and subdue it by an invisible power. Even the fishermen themselves are suddenly struck and rendered torpid at the moment of seizing it, while nothing external betrays the mysterious power possessed by the animal.
The electrical properties of the gymnotus were reported for the first time by Van Berkal. The astronomer Richer, who had been sent to Cayenne in 1671 by the Academy of Sciences of Paris, on the Geodesic Survey, first made known the singular properties of the American fish. "I was much astonished," says this author, "to see a fish some three or four feet in length, and resembling an eel, deprive of all sensation for a quarter of an hour the arm and neighbouring parts which touched it. I was not only an ocular witness of the effect produced by its touch; but I have myself felt it, on touching one of these fishes still living, though wounded by a hook, by means of which some Indians had drawn it from the water. They could not tell what it was called; but they assured me that it struck other fishes with its tail in order to stupefy them and devour them afterwards, which is very probable when we consider the effect of its touch upon a man."
The observations of Richer made little impression at the time on the savants of Paris, and matters remained in this state for seventy years, when the traveller Condamine spoke in his "Voyage en Amérique" of a fish which produced the effects described by Richer. In 1750 a physician named Ingram furnished some new views respecting this fish, which he thought was surrounded by an electric atmosphere. In 1755 another physician, the Dutch Dr. Gramund, writes: "The effect produced by this fish corresponds exactly with that produced by the Leyden jar, with this difference, that we see no tinsel on its body, however strong the blow it gives; for if the fish is large, those who touch it are struck down, and feel the blow on their whole body."