to the mackerel, which, being surface swimmers, have a good respiration, was higher than usually supposed. Thus the bonito was found to possess a temperature of 90 degrees, Fahr., when the surrounding medium was 80° 5', and may therefore be regarded as an exception to the supposed general rule. Physiologists have shewn that the quantity of respiration is inversely as the degree of muscular irritability. Mr. Yarrell regards it as a law, that those fishes which swim near the surface of the water, have a high standard of respiration, a low degree of muscular irritability, great necessity for oxygen, die almost immediately when removed from the water, and have flesh prone to rapid decomposition—mackerel, salmon, trout, and herring, being examples of this rule; while, on the contrary, such as live near the bottom of the water, have a low standard of respiration, a high degree of muscular irritability, less necessity for oxygen, a more enduring power of life in open air, and flesh which keeps fresh for several days. Of this second rule, carp, tench, eels, and the various kinds of skate and flat-fish, may be mentioned as examples.

Whatever may be the physical temperature of fishes, there is nothing in their history more remarkable than their power of enduring the extremes of heat and cold. The breeding powers of that brilliant species of Chinese carp, commonly called the gold-fish, are greatly accelerated by water kept at a constant temperature of 80 Fahr.; yet Mr. Hoste, a naturalist of Vienna, has seen that species recover freely after being frozen up in ice. Fishes

H

exist naturally in various baths and thermal springs, of which the temperature ranges from 113 to 120 degrees; and Humboldt and Bonpland were witness in South America to fishes being thrown up alive, and apparently in good health, from the bottom of a volcano, along with water and heated vapour, which raised the thermometer to 210°, that is, to within two degrees of the boiling point. Contrast this with Dr. Richardson's account of the species of carp common in the fur countries of North America.

SECTION XV.

The Swimming Bladder of Fishes.

ONE of the most peculiar and characteristic organs of the finny tribe is the swimming bladder, commonly so called. This is a fine pellucid often silvery-coated viscus, of size and shape extremely variable in the different kinds, and in many species altogether wanting. In several genera, it has no opening or canal of communication, and the air which it contains must therefore be the result of secretion. It is composed of an extremely fine internal tunic, and of another of a thicker texture, and peculiar fibrous structure, remarkable for producing the finest kind of isinglass. It is enclosed within the general coating with which the peritoneum invests the other viscera. It is sometimes

simple, as in perch, sometimes furnished with more or less numerous appendages, as in some of the haddock tribe, or branched, as in certain Sciena. Occasionally we find it divided, as it were, into two parts by a restriction, as in the genus Cyprinus, several of the Salmonidæ, and others. It is chiefly among the abdominal fishes that we find it communicating by a tube or tunnel with the intestinal canal, and either directly with the gullet, as in Cyprinus, or with the base of the stomach, as in the herring. That of the sturgeon opens into the former portion by means of a conspicuous orifice. The contents of the swimming bladder are usually found to be azote, mingled with some fractional parts of oxygen or carbonic acid. There is probably a want of uniformity in its composition, which is of itself a proof that the air is secreted rather than drawn in from the atmosphere in the ordinary way. The gas in the carp was found by Fourcroy to be nearly pure nitrogen, while other chemists have found it composed of nitrogen, oxygen, and carbonic acid-the nitrogen in greater, the oxygen in smaller proportion, than in atmospheric air. Some physiologists seem to have regarded the swimming bladder as a true lung, which both admitted and retained the external air; but as we have said, the connecting air-duct is in numerous species entirely wanting, while in many others which remain constantly at prodigious depths, the quantity of oxygen in the swimming bladder is greater than in those the abode of which is near the surface. Indeed, the oxygen is said to increase

in quantity in proportion to the depth at which the species dwells. Carus considers it probable that the vessel in question performs a part analogous to that of the expiratory functions of the lungs in the higher classes, by not only separating excrementitious azote, and superabundant oxygen, from the blood, but even discharging those elements in such species as have this particular viscus provided with an air-duct.

The more obvious uses, however, of this organ, seem to be to maintain the fish in equilibrium, and to lighten or increase the relative weight, so as to cause a sinking or ascension in proportion as the bladder is compressed or expanded. This is probably effected by the contraction or dilatation of the ribs, at least we often see fishes rise or descend in the water without a visible effort of any kind. At all events, it is certain that when the air-bladder bursts, the fish remains at the bottom, usually turning up its belly, or exhibiting other irregularities in its attempts at locomotion. Another curious effect is observable in regard to fishes which have been suddenly brought from a great depth by means of a long fishing line, and which having no time either to compress or partially empty the organ in question, the air which it contains being no longer pressed by the heavy weight of water, either expands so as to burst the bladder, or by its dilatation forces the stomach and œsophagus into the fish's mouth. When it is pierced artificially, the fish almost immediately turns upon its back, and sinks to the bottom. Though of the highest im

portance in the economy of such species as possess it (and these are by far the greater number), yet this swimming bladder is not indispensable to the class of fishes, of which in truth about a fourth part are naturally destitute of it. In the Pleuronectidæ or flat-fish, it is entirely wanting, and these species generally remain at the bottom. The Lamprey is also in a similar predicament, and dwells in the mud. It is however difficult to determine for what reason this organ should have been denied to so many fishes, not only of the more indolent kinds, like the majority of those just referred to, which dwell composedly at the bottom of the water, but to many others which yield to none of their class in the ease and velocity of their movements. Its presence or absence does not even accord with the other conditions of organisation; for while it is wanting in one kind of mackerel, it occurs in another, of which the habits are analogous, if not the same.

SECTION XVI.

The general position and relationships of Fishes, considered as a great class in the Animal Kingdom.

Ir results not less from the preceding general exposition of structure, than from all observation of special organisation, that fishes form a class of creatures distinct from every other, and destined