371

CHAPTER XXI.

Functions and Instincts. Fishes.

2

THE animals we have hitherto considered have been destitute of an internal jointed vertebral column and its bony appendages; and though some, as the Cephalopods and some slugs,' have a kind of internal bone, and in one Order of Polypes the axis is sometimes articulated, yet these, especially in the latter instance, merely indicate an analogical relation, but no affinity. In none of these instances is this internal bone perforated for the passage of a spinal marrow, as in a real vertebrated column; we now, however, enter that superior section of the animal kingdom, the individuals belonging to which, with scarcely any exception, are built upon the column in question, incasing a spinal marrow, and terminated at its upper extremity by a bony casket, calculated to contain and protect the most precious and wonderful of all material substances, the cerebral pulp, by which the organs of sense perceive; the will moves the members; the mind governs the outward frame;

I VOL. i.

p.

305.

2 Ibid. p. 177.

and, in the king of animals, an immortal spirit, is enabled to seek and secure a higher destiny.

This change in the structure of animals was rendered necessary by an increase in their bulk, for though there are some of the invertebrated Sub-kingdom, as the fixed Polypes and several of the Cephalopods, that are of as large dimensions, and a few of the vertebrated, as the humming birds,' and the harvest mouse, that are not so large as some insects; yet the generality of those distinguished by a vertebral column form a striking contrast, as to magnitude, with those that are not. Besides this, as these animals, by the will of their Creator, were to be endowed as they ascended in the scale, with gradually increasing intellectual faculties, it was necessary that the principal seat of those faculties should be differently organized. A different organ of respiration also, as well as of circulation, in the great body of vertebrates, required an internal cavity defended from the effects of pressure.

Having premised these general observations, we are next to consider what animals form the basis of the vertebrated Sub-kingdom. Most modern zoologists appear to be of opinion that the Fishes occupy this position, and, taking all circumstances into consideration, this seems the

station assigned to them by their Creator; still there are characters in some of the Reptiles that seem to connect them more immediately with the Insects. The metamorphoses, particularly of the Batrachian Order, are of this description; as is likewise the carapace, or shell of the Chelonians, of which the vertebral column and ribs form the basis. Those extraordinary animals, the hag1 and the lamprey, half worms and half fish, by means of the leech, evidently connect the Fishes with the Annelidans.3 Perhaps those butterflies of the ocean, the flying fishes, with their painted wing-fins with branching rays, may look towards the Lepidoptera amongst Insects, but there is no direct connection at present discovered between the two Classes.

The characters of the Class of Fishes areBody with a vertebral column, covered with scales, and moved by fins. Respiration by permanent gills. Heart with only one auricle and one ventricle; blood red, cold.

Fishes are distinguished from the other vertebrated animals, especially birds and beasts, by their mode of respiration; the latter breathing the atmospheric air, are furnished with lungs,

1 Gastrobranchus. (Myxine. L.)

3 Sir E. Home, Philos. Trans. 1815, 265. 4 Exocœtus volitans, &c.

2 Pteromyzon.

which receive that element, oxygenate the blood, and again expel it in a different state; while the former, which must decompose the water for respiration, breathe by means of gills, found also in many invertebrates; these are usually long, pointed plates, disposed like the plumules of a feather, or teeth of a comb, in fishes attached to bony or cartilaginous bows; each of them, according to Cuvier, covered by a tissue of innumerable blood-vessels; but, according to Dr. Virey,' having a minute vein and artery. In the gill of a cod-fish, which I have just examined under a microscope, a vein and artery traverse each plate longitudinally at the margin, which appear to be pectinated, at right angles on each side, with innumerable minute branches, and resemble, in this respect, the gills of Crustaceans.* Thus the blood is oxygenated by the air mixed with the water, and carried to the heart, whence it is distributed to the whole body. So that the aërated water produces the same effect upon the blood in the branchial vessels, as the air does upon that in our lungs.

We know, by experience, how soon an animal that breathes by lungs, if it remains only a few minutes under water, and is cut off from all communication with the atmosphere, is suffocated and dies; and that all aquatic animals

2

'N. D. D'H. N. iv. 330. Latr. Cours. D'Ent. t. 2. f. 2.

that have not gills, or something analogous, as all the water-beetles, the larves of gnats, &c. are obliged, at certain intervals, to seek the surface for respiration. Whence we may learn what an admirable contrivance of Divine Wisdom is here presented to us, to enable the infinite host of fishes to breathe as easily in the water as we do in the air.

When we sum up all the diagnostics of the Class we are considering, we can trace, at every step, so that, almost, he that runs may read, Infinite Power in the construction, Infinite Wisdom in the contrivance and adaptations, and Infinite Goodness in the end and object of all the various physical laws, and in all the structures and organizations by which they are severally executed, which strike the reflecting mind in this globe of ours. What else could have peopled the waters, and the air, with a set of beings so perfectly and beautifully in contrast with each other, as the fishes and the birds. Sprung originally from the same element, they each move, as it were, in an ocean of their own, and by the aid of similar, though not the same, means. The grosser element they inhabit required a different set of organs to defend, to propel and guide, and to sink and elevate the fish, from what were requisite to effect the same purposes for the bird, which moves in a rarer and purer medium; yet as both were fluid mediums, consisting of the