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Let us consider the use of this wonderful appendage. It diminishes the gravity of the fish in relation to its bulk, and thereby enables it to continue buoyant or to ascend in water, with little or no muscular effort. Nay, Ichthyologists inform us, that such aquatic natures as are furnished with the largest air bags, swim with peculiar velocity, frequently ascending and descending; and that, when entirely wanting, some peculiar modification in the organs of loco-motion answers the purpose equally as well, or else that the constitution of the animal inclines it to grovel at the bottom of the water. Thus, in the Shark, the want of an air-bag is compensated by a long and powerful tail, as well as by pectoral fins of considerable size. In the Mackerel by a frame-work, and muscular substance remarkably light, and by a tail of unusual strength; in Flat-fishes by a peculiar facility for beating and flapping the water with their broad surfaces in a manner analogous to Flying-fish, which motion enables them readily to change their places of abode ; in Lampreys, by a disposition to remain quietly buried in mud, in shallow streams, or inlets of the sea.
It is, therefore, evident that this curious appendage, is not only essential to the loco-motion of certain fishes, but that repeated changes are effected in the quantity of air which it contains, in order to enable them to rise or descend at pleasure. How these changes are produced, has not yet been ascertained, though it is conjectured with considerable probability, that the gaseous contents of the bag are increased, or diminished, by secretion, or absorption, according to the necessities, or inclination of the animal. Nor is the rapidity of these changes a sufficient objection to this ingenious supposition. Tears are instantaneously seen to rush into the human eye, on any sudden affection of the mind; and nothing can be more rapid than the slight effusion which adds so powerful an expression to the emotions of the soul.
Some naturalists conjecture that the air contained in the swimming bladder, varies in accordance with the habitats of different species, as the natatory vessel of the Sword-fish appears to be filled, under the tropics, with pure oxygen gas. The conjecture is supported by facts of recent date. Humboldt employed eight months, in conjunction with M. Provenzal, in experiments relative to the respiration of fishes: during the course of which, he frequently observed that they absorbed not only oxygen, but also azote; and that the quantity of the latter differed in fishes of the same species, although the oxygen inhaled, was very far from being equalled by the carbonic acid exhaled, from the whole surface of the body. These facts tend to prove that the proportion of the oxygen and azote vary in the air vessel, according as the vital action of the gills and of the skin is modified by the
greater or less pressure sustained at different depths. Hence the contents of the swimming vessel, in the example already given, lead to the supposition that the Sword-fish inhabits the lower strata of the ocean.
Such, my friend, is the general construction of aquatic natures ; such are a few of their most distinguishing characteristics. They constitute a large proportion of what Cicero has happily denominated the insatiable variety of nature; or rather, they evince the beneficence of that Being who controls and regulates her operations. We
with astonishment on the vast bulk and admirable proportion of those vessels, which, notwithstanding they are so great and driven of fierce winds, are yet turned about with a very small helm, whithersoever the governor listeth. And yet what are these oak leviathans—the pride of polished nations, the terror and astonishment of savage tribes, when compared with the sublimer works of Deity! To an eye that could follow the rapid course of one of those living creatures that
pass through the paths of the great waters, and embrace in a comprehensive glance, the whole of its admirable mechanism, what exquisite arrangement, what just precision, what perfect symmetry would be discoverable in every part! An exterior perfectly adapted to its mode of life: the heart, the engine that works the whole machine, appearing like a hollow muscle, invested with spiral fibres, running in different direc
tions, and throwing out at every stroke a quantity of vital fluid with immense velocity. Still going, day and night, at the rate of one hundred thousand strokes every twenty-four hours; having at each stroke, a great resistance to overcome, yet still continuing the action, during, perhaps, a period of two hundred years, without weariness, and without disorder.
The visual organ is also beautifully adapted to receive the rays of light through an aqueous medium. In some few instances it is prominent, capable of rapid motion, and revolving in a transparent sphere, for the evident purpose of protection ; in others it is covered with a shining gold-coloured membrane ; in others with a transparent horny convex case ; in others with a lid that may be drawn over at the pleasure of the wearer, containing a circular perforation, the aperture of which is closed by means of one muscle, and opened by five, that rise from the bottom of the orbit. Teeth, arranged in exact accordance with the habits of aquatic animals,the points turned backwards, like those of a woolly or cotton card, and continually renewed, in contradistinction to those of terrestrial ones ; evidently for this reason, that the period of duration assigned to the inhabitants of the water, is far more extended than such as live on land. The organs of hearing admirably constructed for receiving sonorous
vibrations in a dense medium, destitute of an external concha, because unnecessary, but furnished internally with hard and calcareous substances, placed in a fluid, and provided with so large a number of nerves, as to render the sense of hearing remarkably quick. The organ of smelling, a beautiful arrangement of ligaments, laminæ, nerves and glands, so attached to a wonderful apparatus, situated on the lower part of the snout, as to render even the undulations of the water sufficient to warn the animal of any approaching danger. Senses of taste and feeling, imperfect, if not altogether wanting, because unnecessary to such creatures as select their food, by those of seeing, and of smelling. The body covered with scales of minute and exquisite workmanship, varying in different species, according to local situation, or the character of their marine enemies. The construction of each, adjusted with the nicest precision, to the ocean sites they are designed to occupy.
Colours varying from the tints of aurora, or those of the rainbow, to the brown, mottled, and sober coating of such as hide among sea-weeds or gravel.
One question may possibly have arisen in your mind during the perusal of these observations, why such circuitous arrangements ? why the ministry of so many means ? could not the Deity have at once effected what he has brought about by such exquisite contrivances ?-Undoubtedly he could; but this was