The bones of fishes are less compact than those of the higher orders of animals, but quite elastic; and in some species small bones are distributed through the flesh, giving additional firmness to portions that lack muscular strength. As the peculiar breathing apparatus of the fish is adapted to its element and mode of life, so is every part and appendage of the whole structure. Adaptation reigns through the whole. The elongated tapering body, its scaly covering exquisitely adjusted, the material and position of the fins, all attest the intelligent and beneficent purpose of the Creator.
The Elasmobranchii[36] (strap-gilled) have a cartilaginous skeleton, rough skin, and uncovered gills. The Shark, the Saw-fish and the Ray are representatives of this order.
The Ganoidei (enameled scales) formed one of the largest orders in ancient geological history, but they have now but few representatives, such as the Sturgeon, Gar-pike and Mud-fish. Their characteristics are a skeleton not completely ossified, ventral fins placed far back, and the tail heterocercal, that is, having the upper lobe larger than the lower.
The Teleosti[37] (perfect bone) form the largest order, including nearly all our common fishes. The characteristics of the order are an osseous or bony skeleton; gills protected by a gill cover or operculum; and an equally divided tail. Lowest among the Teleosts is the common eel. It has an elongated, cylindrical, thick-skinned body, and is destitute of ventral fins. There are many species, and they are widely distributed, living in both salt and fresh water. One species is electrical.[38] The cat fishes of this order have long threads hanging from their jaws, and are noted for their peculiar methods of protecting their young; one species is electric, having cells arranged in layers over the body.
Other peculiar types are the Blind or Cave fishes,[39] living in the waters of caves; the Lamp fishes which take their name from the luminous spots arranged along the sides, and which are supposed to light the recesses where they live, and the Flying fishes.
There are at least two genera, and more than thirty species of “flying fish,” a name given to all those which have the pectoral fins so large that they are sustained in short, seeming flights through the air. They do not really fly, as they have not muscular power in their fins to beat the air as birds do. But when extended the fins help bear them up, and the impulse received at the start, sufficient to give them the elevation they reach, may be supplemented by the use of the caudal fin as in swimming. Some naturalists claim to have noticed a movement of the other fins, but the preponderance of testimony is that these are of service only as parachutes or the wings of a paper kite. The Herring, Shad, Salmon, Pike, Perches, Bass, Mackerel and Cod are valuable food fishes, belonging to the Teleosts. The Sword fishes, in which the upper jaw is developed into a long, sword-like projection, used as a weapon of defense and offense, is an interesting member of the order. The Climbing fishes, noted for being able to live out of water, and the Nest Builders, which make homes from the weeds of the Sargossa Sea,[40] are peculiar types.
Another curious Teleost is the Sea Horse. This peculiar animal has its name (Hippocampus) from the shape of the head, that has some resemblance to that of a horse. There is no other resemblance. The short body, without legs, is covered with angular spinous plates. Its fishy part is a long prehensile tail, but it has neither dorsal nor caudal fins.
The last order of fishes is the Dipnoi, or Lung fishes. They are characterized by the possession of two lungs, as well as gills. The gills are used in respiration when the fishes are under water, but when out of it, and burrowing in the mud as they often do, the lungs are put into service. They are also known as mud fishes, from their habits of encasing themselves in the mud.
THE SEA HORSE.