Many of our European fishes are richly decorated with vivid colours, but their scaly raiment is generally far from equalling the gorgeous magnificence of the fishes of the tropical seas. 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.