Teeth, which are so characteristic of VERTEBRATA, are nowhere found in such variety of form and function as among FISHES. They are not confined to the jaws, but are found by turns in almost every one of the bones that compose the mouth, though not in all species. They are generally simple spines, curved backwards; but innumerable modifications of this form occur. Thus the jaws of the deadly Shark are flat and lancet-like, the cutting edges being notched like a saw; the front teeth of the Plaice and the Flounder are compressed plates; some, as the Wrasse, have flat grinding teeth; others, as the Sheep's-head, have the grinding surface convex; and others, as the genus Chrysophrys, have convex teeth so numerous and so closely packed over a broad surface, as to resemble the paving-stones of a street. The beautiful Chaetodons of warm climates, on the other hand, have teeth which resemble bristles, and these are set close together like the hairs of a brush; while the Perch of our own rivers has them still more slender, minute, and numerous, so as to resemble the pile of velvet. Another of our well-known fishes, the bold and fierce Pike, is armed with teeth scarcely less formidable in size, form, and sharpness, than the canines of a carnivorous quadruped. In number, also, there is a great variety. The Pike, the Perch, the Cat-fish, and many others, have the mouth crowded with innumerable teeth, while the Carp and the the Roach have only a few strong teeth in the throat, and a single flat one above; and the Sturgeon, the Pipe-fish, and the Sandlaunce, are entirely toothless.

The eye in this Class presents a beautiful example of adaptation to the medium in which they live. From the

density of water being so nearly the same as that of the aqueous and vitreous humours, these have scarcely any power to refract the rays of light; and hence a high magnifying power is given to the crystalline lens. Its form is that to which the very highest possible power is assigned

-a perfect sphere, and the density of its texture is very great. But as the power of a lens and the nearness of its focal point are in the same ratio, it was needful to bring the retina, or curtain on which the image is painted, very close to the lens; and this is done by diminishing the vitreous humour behind it, and thus flattening the sphere; while a provision is made for maintaining this shape in certain plates of bone or cartilage, imbedded in the tough coat of the eye, called the sclerotica.

The eye is never protected by an eyelid in fishes; the pupil is very large and incapable of contraction; and another peculiarity is, that (at least in many species) the one eye is moved independently of the other.

The last organ we have space to notice at present is the air-bladder, which is found in most of the bony fishes. It is usually of a lengthened form, attached beneath the spine; but its shape is subject to some variety. Thus, in the Hedgehog fishes it is two-lobed, like a Dutchman's breeches; sometimes it is a double sac; in the great Card family, and in the Electric Eels, it is divided into two compartments by a transverse partition, which, in the former case, is perforated to allow an intercommunication. In one of the Cat-fishes (Pangasius), it is divided into four compartments, and in others into many irregular cells. Thus, the air-bladder closely approaches in structure the lowest form of the lung in air-breathing VERTEBRATA, as

in the Axolotes and the Newts, in which this organ is a simple bladder, and in the Frogs and Toads, in which it is subdivided into large cells; and it may, therefore, be considered as the first rudimentary appearance of an aerial respiratory apparatus.

sure.

With regard to its function, in Fishes, it appears primarily to be connected with swimming. Being condensed by muscular pressure, or allowed to expand, it renders the body either heavier or lighter; and thus enables the fish to swim at any height in the water according to its pleaIn general, the roving and surface-swimming species are furnished with the organ in question, while such as haunt the bottom are destitute of it; and this arrangement well agrees with the function we have ascribed to it. Yet it cannot be denied that there are some most unaccountable deviations from this rule. Thus, the Gurnards, which are ground-fishes, are well furnished with bladders; the two British species of Surmullet are deprived of the organ; while the Tropical species, which have the same habits, are provided with one. Again, of surfacefishes, our common Mackerel has no bladder, while the Spanish and Coly Mackerels, which have exactly the same manners, are each furnished with one.

In many cases the bladder is hermetically sealed, but, in some instances, it communicates by a tube with the stomach, or the gullet. The air which it contains appears to be a secreted gas; it is found to be, in some cases, oxygen, and in others, strange to say, nearly pure nitrogen. The former gas occurs chiefly in species that live in very deep water.

The blood, as already observed, is cold; that is, it com

monly takes the temperature of the surrounding water; in some of the swift oceanic Fishes of the Mackerel family, however, such as the Tunny and the Bonito, the blood is found to be 10° higher than the temperature of the surface of the sea, even within the Tropics: the flesh of these Fishes is dark and dense. The blood-disks are sometimes

circular, sometimes oval. They are larger than those of MAMMALIA and BIRDS; smaller than those of REPTILES, and especially than those of AMPHIBIA.

The irritability of the muscular fibre is considerable, and is long retained. Fishmongers take advantage of this property, to produce rigid muscular contraction, after life has ceased, by transverse cuts and immersion of the muscles in cold water: by this operation, which is called "crimping," the firmness and density of the muscular tissue are increased.

In our next chapter, we shall enter into some details of the instincts and habits of Fishes, and some other matters connected with the Class, of more popular interest than these structural peculiarities, which, we fear, will prove but a dry morsel to many of our readers.

CHAPTER XXVIII.

PISCES (Fishes).

Continued.

As the innate selfishness of our hearts always prompts the question, cui bono?—it may be as well to commence this chapter with a few particulars of the usefulness of FISHES in ministering to our bodily wants. The value of fish as an article of human food has been appreciated in all nations and all ages. The earliest pictorial records of Egyptian every-day life are largely occupied with the capture and preservation of these animals; various forms of nets, the fish-spear, the hook and line, are all in requisition; and strings of fishes, split and salted, and hung out to dry, remind us of scenes familiar enough to the writer of these pages-the cod-fisheries of Newfoundland. Allusions to the hook and line occur in the most ancient of writings the Book of Job; and, in the Mosaic law, "whatsoever hath fins and scales in the seas and in the rivers" was freely given to Israel for food. The most remote and savage tribes feed largely on a fish diet; and the ingenious devices and implements employed by the islanders of the Pacific Archipelago far exceed in variety,