Sea Porcupine, a common name of the osseous fishes of the order plectognathi (with comb-like gills), family diodontidoe or gymno-donts, and genera diodon, tetraodon, etc., so called from the spines with which the body is studded. This order, which contains the sun fish, trunk fish (see Trunk Fish), and file fish, has the internal skeleton partly ossified, and the skin covered with ganoid scales or spines; the maxillaries and intermaxillaries are wholly or in part united, and the upper jaw in most is immovably fixed to the cranium; there are no pancreatic caeca, no well developed ventrals, no duct to the air bladder, and only vestiges of ribs. In the family of gymnodonts the teeth are incorporated with the bone of the jaws, and resemble a parrot's beak with or without mesial division, their plates consisting of hard dentine adapted for bruising and cutting the crustaceans, mollusks, and sea weeds upon which they feed. The skin is thick, leathery, and armed with spines which stand out in every direction when the body is inflated by filling with air the stomach, or more properly a large sac beneath this organ communicating with the oesophagus; the air is forced into this sac by swallowing; when thus distended the fish loses all command over its fins, and rolls over belly upward, floating at the mercy of the wind and waves; as it is a considerable time before the air can be sufficiently expelled to allow the fish to resume the full control of its movements, many are caught in this helpless condition; they emit a blowing sound when taken, from the expulsion of the air; the tail is short and feeble; the spinal cord, according to Owen, is very short.

Some of the family have no external openings to the nostrils, the nerve of smell being expanded on cutaneous tentacles. The flesh of some is poisonous. They are very tenacious of life, on account of the small size of the gill openings, and have a disagreeable odor which is retained even in alcohol for years; they are mostly inhabitants of tropical seas, and are rarely more than 2 ft. in length, with the diameter of the inflated body more than half of this. - In the genus diodon (Linn.) there is no mesial division of the jaws, and the teeth are apparently only two; the spines are long, thin, sharp, with two root-like processes, and capable of erection. There are nine species, of which three are described by Mitchill as occurring on the coasts of the United States, under the name of balloon fishes; these are the D. maculo-striatus, about 6 in. long, greenish spotted and striped with dark; the D. pilosus, smaller, with most of the body furnished with soft, flexible bristles of a golden color; and the D. verrucosus, with a warty and spiny skin.

The atinga (D. hystrix, Bloch), of the East Indian, S. African, and South American coasts, is the best known to seamen; it is caught in nets or on hooks, and is very difficult to handle from the sudden erection of the spines and the active motions of the bodv. - In tetra-odon (Linn.) there is a mesial suture in the jaws, so that there appear to be two teeth above and two below; the spines are very short, and the head, back, and tail are generally smooth. The T. electricus (Paterson), with electric properties, has the skin entirely smooth. (See Electric Fishes.) There are several species on the American coast, of which the most common is T. turgidus (Mitch.), 6 to 14 in. long, olive-green above and whitish below; the abdomen lax, covered with prickles and capable of considerable distention; it is not uncommon about Martha's Vineyard, and on the Massachusetts and New York coasts, where it goes by the names of puffer and swell fish. Other names for this and the preceding genus are globe fish, urchin fish, and spine-belly.

Balloon Fish (Diodon pilosus).

Puffer or Swell Fish (Tetraodon turgidus).