EXTERNAL FORM AND ATTRIBUTES OF FISHES.
To aid the Angler in his scientific researches, as well as to add to the interest of the ordinary observer, we now proceed to a brief exposition of the principal characteristics of the class of fishes, and shall, at an after period, expatiate upon the more peculiar attributes of each particular kind, when we come to treat of the species in their order.
We need scarcely say to the student of nature, that the form and functions of fishes are as admirably adapted for easy movement through the water, as are those of birds for that aërial motion called flight. Suspended in a liquid element of almost equal specific gravity with themselves, external organs resembling those of birds in size, would have been disproportionate and unnecessary; but the air-bladder (the functions of which, by no means entirely understood, have never been satisfactorily explained in all their bearings) is known to possess the power of contraction and dilatation, the exercise of which is followed by a corresponding descent or ascent of the animal’s body. Thus a small central and inconspicuous organ effects, in the easiest and most simple manner, the same object which even the soaring eagle or giant condor can only accomplish by great exertion of the wings, and after laborious and frequently repeated gyrations. We shall ere long, however, have occasion to remark in more detail, that the air-bladder, although essential to the economy of such species as possess it, is by no means indispensable as a general attribute of the class, as in many tribes it is entirely wanting. It is not even a generic characteristic, as it does not exist in the red mullets of the British seas, though possessed by the corresponding species of Asia and America—while of our two kinds of mackerel, the so called Spanish species (Scomber colias) is distinguished by a swimming bladder, and the common mackerel (Sc. scomber) does not possess that organ.
Fishes being without a neck, and the portion called the tail being usually equal at its origin to the part of the body from which it springs, the prevailing shape is somewhat uniform and continuous, diminishing gradually towards either extremity. Of this, the most elegant and characteristic form of fishes, the salmon and mackerel exhibit familiar examples. Yet a vast variety of shape, as well as of size and colour, is naturally presented by a class which now contains some seven or eight thousand known species; and no further illustration of the subject will be deemed necessary by him who has seen and remembers the difference between an eel and a skate.
The mouth of fishes either opens from beneath, as in the rays, or at the extremity of the muzzle, as in the great majority of species, or from the upper surface, as in a small foreign group called Uranoscopus, or moon-gazer—an odd name for species, some of which have been alleged to bury themselves to the depth of twenty feet in sand—a bed not easily obtained, and in no way fitted for astronomical observation. It also varies much in its relative dimensions, from the minute perforation of the genus Centriscus, to the vast expanded gape of the ugly angler-fish. We mean nothing personal in the last allusion.
The teeth of fishes are frequently very numerous, and are sometimes spread over all the bony parts of the interior cavity of the mouth and pharynx, that is, on the maxillary, inter-maxillary and palatal bones, on the vomer, tongue, branchial arches, and pharyngeal bones. In certain genera they exist on all those parts; while in others they are wanting on some, or are even entirely absent on all. The denominations of the teeth are derived from their position, that is from the bones to which they are attached, and are consequently as numerous as the varieties of their situation. In the upper portion of the mouth of a trout, for example, there are five rows of teeth. The single middle-row is placed upon the central bone of the mouth called the vomer; a row on each side of it is fixed on the right and left palatal bones, while the outer-rows or those of the upper-jaw, properly so called, are situate on the maxillary bones. In the under portion of the mouth there are four rows, that is, one on each side of the tongue, and another external to these on each side of the lower-jaw. As to the form of teeth in fishes, the majority are hooked and conical, and more or less acute.
In the majority of osseous fishes, besides the lips, which, even when fleshy, having no peculiar muscles, can exert but little strength in retaining the aliments, there is generally in the inside of each jaw, behind the anterior teeth, a kind of membranous fold or valvule, formed by a replication of the interior skin, and directed backwards, of which the effect is to hinder the alimentary substances, and especially the water gulped during respiration, from escaping again by the mouth. This structure does not, as formerly supposed, constitute a character restricted to the genus Zeus, but exists in an infinity of fishes.
The food seized by the teeth of the maxillæ, and detained by the valve just mentioned, is carried still further backwards by the teeth of the palate and tongue, when these exist, and is at the same time prevented by the dentations of the branchial arches from penetrating between the intervals of the branchiæ, where it might injure those delicate organs of respiration. The movements of the maxillæ and tongue can thus send the food only in the direction of the pharynx, where it undergoes additional action on the part of the teeth of the pharyngeal bones, which triturate or carry it backwards into the œsophagus. The last-named portion is clothed by a layer of strong, close set, muscular fibres, sometimes forming various bundles, the contractions of which push the alimentary matter into the stomach—thus completing the act of deglutition.