CHAPTER VI.

OF THE CLASS OF FISHES.

515. THE fourth and last class of the Vertebrated sub-kingdom, comprises the animals known as FISHES. These are destined to live altogether in the water; and this circumstance has impressed a peculiar character on their entire organization: but the most important differences which they present, when we compare them with the other Vertebrata, consist in the conformation of their apparatus for respiration and circulation. They have no lungs at any period of their lives; and they breathe by gills only. Their heart contains but two cavities; and receives only venous blood. This liquid, after having been brought into contact with oxygen, passes into a dorsal vessel, where no new force accelerates its course through the different parts of the body. Their circulation cannot, therefore, be as active as among the higher animals; and their blood is cold like that of Reptiles. Their skin is covered only with scales, which, in some instances, are scarcely discoverable, so that the skin appears quite bare; they have no mammary glands like the Mammalia, and they are reproduced by means of eggs; lastly, their members have the form of fins.

516. The external form of Fishes varies; but their body is generally but little divided. Their head, which is as broad as the trunk, is not separated from it by any narrowing like the neck of the higher Vertebrata; and their tail, by its size at its origin, is not distinguished from the rest of the body. Some of these animals are quite destitute of fins; but in general we find a considerable number of these organs, placed,-some on the central line of the back or the abdomen, and consequently single, -others on the sides, and arranged in pairs (Fig. 245). These last represent the four limbs of the other Vertebrated animals.

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EXTERNAL ORGANS OF FISHES.

The anterior members, which correspond with the arms of Man and the wings of Birds, are fixed on each side of the trunk, immediately behind the

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a ì v 0

head, and are called pectoral fins. The abdominal members are less separated from each other, and are generally found on the lower side of the body; they may be situated more or less forwards or backwards, from the under part of the throat to the commencement of the tail; these are called ventral fins. fins occupy, as we have already said, the central line of the body; and are divided into dorsal fins, anal fins, and caudal fins, according as they are placed along the back, under the tail, or at its extremity. They have all very nearly the same structure, and consist almost always of a fold of skin, supported by bony or cartilaginous rays; very nearly in the same manner as the wings of bats and dragons are sustained by the fingers or by the ribs, of the animals.

FIG. 245.-BEARDED MULLET, showing position of fins; p, pectoral fin; v, ventral fin; d1, first dorsal; de, second dorsal; c, caudal; a, anal; o, opening of gill-covers.

The single

517. On the external surface of the body, large openings are seen on each side; which are placed immediately behind the head, and which serve as an outlet for the water that has passed through the gills. In general there is only one on each side; and its anterior edge is moveable, and resembles a flap or valve. There exists on each side, along the whole length of the body, a series of pores, which is termed the lateral line; these are the openings of glands imbedded in the skin, by which the thick mucus is secreted, that covers the skin of Fishes, and gives to it its peculiar slipperiness.

518. The skin is sometimes almost entirely bare, but it is generally covered with scales. Occasionally these scales have the appearance of coarse grains, at other times they exist as large tubercles or plates of a considerable thickness; but in general

SKELETON OF FISHES.

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they present the aspect of very thin laminæ or plates, arranged in the manner of tiles upon a roof, and held together in the folds of the skin. We may compare them to our nails; but they contain a much larger quantity of calcareous salts. The colours with which these animals are adorned, astonish us by their variety and splendour. Sometimes they can only be compared to the most brilliant gold and silver; sometimes these present tints of the richest green, blue, red, or the deepest black. The silvery matter, which frequently gives to them so beautiful a metallic splendour, is secreted by the skin, and is composed of a number of small polished laminæ.

519. The Skeleton of Fishes is usually bony; but amongst several of these animals, such as the Ray and the Shark, it remains permanently in a fibro-cartilaginous or cartilaginous state; and there are even some in which this frame-work possesses still less solidity, and remains perfectly membranous: certain Lampreys are in this condition; and in this manner they form a transition between the Vertebrated and Invertebrated animals.The bones never have any medullary canal; and the cartilage which constitutes their foundation is not the same as that of the Mammalia and Birds; for, when boiled in water, it does not give out any gelatine.

520. The skeleton is composed of the head, to which is

joined a highly-developed apparatus which is subservient to

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BONES OF THE HEAD OF FISHES.

respiration; of the trunk; and of the members.-The structure of the head is very complicated. At first is seen a central or median portion, composed of a great number of bones united together by sutures, and forming a kind of immoveable keel, with which are connected the bones of the jaw, the cheeks, &c. This median portion, of which the general form is very nearly that of a pyramid with three sides, having its summit directed forwards, has at its back part the cavity of the cranium; in which is placed the apparatus for hearing, as well as the brain. Its middle side is hollowed out to form the orbital cavities, or; and in front are seen the apertures belonging to the olfactory apparatus, n; and a kind of large knob, formed by the vomer, and serving to support the upper jaw (Fig. 247.) We may distinguish the bones corresponding with the occipital, the temporal, the sphenoid, the parietal, the frontal, the ethmoid, and the vomer; but most of these are composed of several pieces, which never acquire the union that takes place at an early period among the Mammalia and Birds.-At the anterior extremity of

FIG. 247.-Bones of the head of Pike; c, cranium; or, orbit; n, nasal cavities; im, intermaxillary bone; m, superior maxillary bone; t, lateral partition, separating gills from mouth; p, io, op, bones of operculum, or gill-cover.

this cranial portion of the head is placed the upper-jaw, which is sometimes fixed there in an immoveable manner, but in general preserves great freedom of motion; there may be distinguished in it on each side an intermaxillary bone, placed near the medial line, and a maxillary bone, which extends sideways, and which is moveable upon the first.

521. A chain of small bony pieces extends on each side, from

BONES OF THE HEAD OF FISHES.

5

the anterior angle of the orbital cavity to its posterior angle, and thus completes the circle of the orbit. Further inwards is seen also on each side a kind of vertical partition, (Fig. 247 t,) which is suspended to the skull, and which separates the orbits and the cheeks from the mouth. It is formed by bones corresponding with the palatine, pteregoid, and tympanic bones of

FIG. 248.-Bones of the head of the Perch, after the removal of the jaws, lateral partition, and operculum, on one side, to show the interior of the mouth, and the hyoid apparatus; c, cranium; or, orbit; v, vomer (armed with teeth); im, superior maxil. lary; dp, teeth implanted on the palatine arch; mi, lower jaw; 7, lingual bone; b, lateral branches of the hyoid apparatus; s, process for the attachment of these to the lateral partition; r, r, branchio-stegal rays; a, branchial arches; ph, superior pharyngeal bones; ar, articular surface by which the lateral partition is attached; o to h, bony framework supporting the pectoral fin, p; o and o', scapula divided into two pieces; h, humerus; ab, bone of the fore-arm; ca, bone of the carpus; co, coracoid bone.

the higher Vertebrata. At its under part it gives attachment to the lower jaw; and behind it is prolonged in such a manner, as to form a kind of moveable flap, which protects the respiratory apparatus, and is termed the operculum or gill-cover. The lower-jaw is formed of three pieces on each side. Within the lateral partitions just described, and lying at the bottom of the mouth, is found a framework of very complicated structure,