Hybernation has been observed in many Cyprinoids and Murænoids of the temperate zones. They do not fall into a condition of complete torpidity, as Reptiles and Mammals, but their vital functions are simply lowered, and they hide in sheltered holes, and cease to go abroad in search of their food. Between the tropics a great number of fishes (especially Siluroids, Labyrinthici, Ophiocephaloids, the Dipnoi), are known to survive long-continued droughts by passing the dry season in a perfectly torpid state, imbedded in the hardened mud. Protopterus, and probably many of the other fishes mentioned, prepare for themselves a cavity large enough to hold them, and coated on the inside with a layer of hardened mucus, which preserves them from complete desiccation. It has been stated that in India fishes may survive in this condition for more than one season, and that ponds known to have been dry for several years, and to the depth of many feet, have swarmed with fishes as soon as the accumulation of water released them from their hardened bed.
The principal use derived by man from the class of Fishes consists in the abundance of wholesome and nourishing food which they yield. In the Polar regions especially, whole tribes are entirely dependent on this class for subsistence; and in almost all nations fishes form a more or less essential part of food, many being, in a preserved condition, most important articles of trade. The use derived by man from them in other respects is of but secondary importance. Cod-liver oil is prepared from the liver of some of the Gadoids of the Northern Hemisphere, and of Sharks; isinglass from the swim-bladder of Sturgeons, Sciænoids, and Polynemoids; shagreen from the skin of Sharks and Rays.
The flesh of some fishes is at times, or constantly, poisonous. When eaten, it causes symptoms of more or less intense irritation of the stomach and intestines, inflammation of the mucous membranes, and not rarely death. The fishes, the flesh of which appears always to have poisonous properties, are Clupea thrissa, Clupea venenosa, and some species of Scarus, Tetrodon, and Diodon. There are many others which have occasionally or frequently caused symptoms of poisoning. Poey enumerates not less than seventy-two different kinds from Cuba; and various species of Sphyræna, Balistes, Ostracion, Caranx, Lachnolæmus, Tetragonurus, Thynnus, have been found to be poisonous in all seas between the tropics. All or nearly all these fishes acquire their poisonous properties from their food which consists of poisonous Medusæ, Corals, or decomposing substances. Frequently the fishes are found to be eatable if the head and intestines be removed immediately after capture. In the West Indies it has been ascertained that all the fishes living and feeding on certain coral banks are poisonous. In other fishes the poisonous properties are developed at certain seasons of the year only, especially the season of propagation: as the Barbel, Pike, and Burbot, whose roe causes violent diarrhœas when eaten during the season of spawning.
Fig. 98.—Portion of tail, with spines, of Aëtobatis narinari, a Sting-ray from the Indian Ocean. a, nat. size.
Poison-organs are more common in the class of Fishes than was formerly believed, but they seem to have exclusively the function of defence, and are not auxiliary in procuring food, as in venomous Snakes. Such organs are found in the Sting-rays, the tail of which is armed with one or more powerful barbed spines. Although they lack a special organ secreting poison, or a canal in or on the spine by which the venomous fluid is conducted, the symptoms caused by a wound from the spine of a Sting-ray are such as cannot be accounted for merely by the mechanical laceration, the pain being intense, and the subsequent inflammation and swelling of the wounded part terminating not rarely in gangrene. The mucus secreted from the surface of the fish and inoculated by the jagged spine evidently possesses venomous properties. This is also the case in many Scorpænoids, and in the Weaver (Trachinis), in which the dorsal and opercular spines have the same function as the caudal spines of the Sting-rays; however, in the Weavers the spines are deeply grooved, the groove being charged with a fluid mucus. In Synanceia the poison-organ (Fig. [99],) is still more developed: each dorsal spine is in its terminal half provided with a deep groove on each side, at the lower end of which lies a pear-shaped bag containing the milky poison; it is prolonged into a membranous duct, lying in the groove of the spine, and open at its point. The native fishermen, well acquainted with the dangerous nature of these fishes, carefully avoid handling them; but it often happens that persons wading with naked feet in the sea, step upon the fish, which generally lies hidden in the sand. One or more of the erected spines penetrate the skin, and the poison is injected into the wound by the pressure of the foot on the poison-bags. Death has not rarely been the result.
