If in the birds of the equatorial zone a part of the plumage sparkles with a gem-like brilliancy, all the colours of the rainbow combine to decorate the raiment of the tropical fishes, and no human art can reproduce the beauty of their metallic lustre, which at every movement in the crystalline waters exhibits to the enchanted eye new combinations and reflections of the most splendid tints.
The gaudiest fishes live among the coral reefs. In the tepid waters, where the zoophytes, those sensitive flowers of the ocean, build their submarine palaces, we find the brilliant Chetodons, the gorgeous Balistinæ, and the azure Glyphysodons gliding from coral branch to coral branch like the playful Colibris, that over the Brazilian fields dart from one lustrous petal to another.
Oxygen is as necessary to fishes and other marine creatures as it is to the terrestrial animals, but as they are obliged to draw it from a denser element, which absorbs but a small volume of air, their gills are necessarily differently constructed from the lungs of the creatures breathing in the atmosphere. In most species, comprising all the bony fishes, and the sturgeons, among those which have a cartilaginous skeleton, we find on either side of the throat five apertures, separated from each other by four crooked, parallel and unequal bones, and leading to a cavity, which is closed on the outside by an operculum or cover. In this cavity, and attached to the bones, are situated the delicate membranes, bearded like feathers, which serve to aërate the blood. The water constantly flows through the gills in one direction, entering by the branchial apertures of the throat, and emerging through the operculum. This is, in more than one respect, a most wise provision of Nature; for if the fishes were obliged to receive and reject the water by the same aperture, as we do the air, each expiration would evidently drive them backwards, and consequently retard their movements. It is also evident that the delicate fringes or folds of the gills would soon get into disorder if the water were carried through them in two opposite directions.
In most of the cartilaginous fishes, such as the sharks, rays, and lampreys, the gills are differently formed, the water not passing into a cavity closed by a cover, but flowing directly outwards through five (in the shark) or seven (in the lamprey) vents or spiracles. In these species also the gills are fixed, their margins being attached. Though the whole breathing apparatus of a fish is comprised in a small compass, its surface, if fully extended, would occupy a very considerable space; that of the common skate, for instance, being equal to the surface of the human body. This single fact may convince us of the numberless ramifications and convolutions of the gills, in which the water is elaborated and attenuated in the course of giving out its air; and how wonderfully Nature has contrived to effect her purpose with the greatest economy of space.
Theoretic representation of the Circulation in Fishes.
Respiration is a species of combustion, and this must necessarily be very slow in an element which contains so small a portion of oxygen. No wonder that the circulation of the blood in fishes is equally tardy. Their heart, in comparison with ours, is but half a one, as it merely serves to force the venous blood into the gills—whence the aërated blood does not flow back to the heart as with us, to be rapidly and strongly propelled through the body, but proceeds immediately to the arteries. Evidently only a cold blood could be formed under such circumstances. It may seem strange that, when fishes are taken out of the water, they die from want of air; such, however, is the case. Their delicate breathing membranes collapse in the atmosphere, the blood can no longer flow as before into the innumerable small vessels with which they are interwoven, and, by rapidly drying in the air, they soon entirely lose the faculty of breathing. Thus those fishes whose gill-cover has a large aperture, die soonest in the air, while those where the opening is narrow, and more particularly those species where the gills communicate with a cellular labyrinth containing water, which serves to keep them moist, are able to live a much longer time in the atmosphere.
The Anabas of the Dry Tanks.