CHAPTER XIX.

CLASS PISCES.

THE class Pisces (Latin, piscis, a fish) includes the familiar animals which are properly known as fishes. As the type of this vast and universally distributed group, we may take the common Perch (Perca fluviatilis) of the rivers and lakes of Britain and Northern Europe.

The general shape of the body in the Perch (fig. 33) is somewhat spindle-shaped, tapering towards both extremities, and thus adapting the animal for rapid and easy movement in a watery medium. From the tip of

Fig. 33.-The common Perch (Perca fluviatilis). o Gill-cover, with the gillslit behind it; p One of the pectoral fins, the left; v The left ventral fin; d The first dorsal fin; d' The second dorsal fin; c The caudal fin or tail; a The anal fin; 7 Lateral line.

the nose for about a quarter of its whole length the thickness rapidly increases, and then it more slowly diminishes towards the tail for the remaining three-quarters of its length. The body is also considerably compressed or flattened from side to side, and its height is much greater than its width.

The body is covered with a closely-fitting armour of scales developed in the skin. These scales (fig. 36, B) are thin, flexible, horny plates, which overlap one another like the tiles of a house, and which have their hinder edges fringed with a comb-like row of spines. Running from the back of the head to the root of the tail is a peculiar row of scales (fig. 33, 7), which constitute what is called the "lateral line." Every scale in this line is perforated by a small hole, and communicates internally with a peculiar system of canals. It used to be believed that the slime which covers the body was secreted by these canals, and poured out by the lateral line of scales; but it is more probable that the slime or mucus which covers the body is really the outer layer of the skin.

The fish propels itself through the water by means of the "fins," all of which have the common character of being expansions of the skin in the form

of membrane, supported by numerous "rays" or thin bony spines, much as an umbrella is supported by its ribs. Some of the "rays" (fig. 34, A) are undivided bony spines, and are termed "spinous rays;" others are branched towards their ends, and are divided by joints into numerous short pieces, when they acquire the name of "soft rays" (fig. 34, B). Some of the fins have only spinous rays; some have only soft rays; and some possess both kinds of rays combined with one another.

A

B

Fig. 34-A, "Spinous ray" of a fin, in the form of a simple spine. B, "Soft ray" of a fin.

Whilst all the fins agree in consisting of membrane supported by bony "rays," they are nevertheless divided into two very distinct groups, quite different in their real character. Some of the fins, namely, are placed in pairs on the sides of the body, and are really the limbs of the animal. Others are unpaired that is to say, they are simply placed in the central line of the body, and have no fellows. The "paired" fins are four in number, making

two pairs. The front pair (fig. 33, p) represents the forelegs, and is termed the "pectoral fins" (Latin, pectus, the breast). They are placed on the side of the body, just behind the head. The hinder pair (fig. 33, v) represents the hind-legs, and is termed the "ventral fins" (Latin, venter, the belly), being placed on the lower surface of the fish, behind and below the pectorals. Both the pectoral and ventral fins of the fish are employed chiefly in steering the animal, and more especially in ascending and descending in the water. All the "rays," lastly, of the pectoral fins are "soft" and jointed; and all those of the ventral fins have the same character, with the exception of the first ray, which is "spinous."

The "unpaired" fins are divided into three sets: one placed above or on the back, one placed at the end of the body, and one placed below. The unpaired fins along the back are two in number (fig. 33, d, d'), and are known as the first and second "dorsal fins" (Latin, dorsum, the back). The first dorsal is supported by fifteen "spinous" rays, the second dorsal by rays of which all but the first are soft." The function of the dorsal fins is chiefly to enable the fish to maintain its equilibrium and vertical position in the water. The unpaired fin at the end of the body is commonly called the "tail," but is technically known as the "caudal fin" (Latin, cauda, the tail). It is entirely supported by "soft" rays, and is symmetrical in form, consisting of two equal lobes (fig. 33, c). The position of the tail is vertical, so that it strikes the water from side to side. Along with the whole flexible hinder end of the body, it constitutes the main organ of progression of the fish, its powerful strokes driving the animal forwards, much as a boat can be propelled by a single oar placed over the stern. Lastly, a single unpaired fin (fig. 33, a) is placed upon the lower surface of the body in front of the tail, and is known as the "anal fin" (Latin, anus, the vent), because it is placed near the vent. All its rays are "soft," except the first two, which are rigid spines.

The skeleton of the Perch (fig. 35) is very complicated, and can only be briefly alluded to. Its most important

portion is the "backbone," which is seen to form a long axis or central stem, round which the body is symmetrically

Fig. 35. Skeleton of the common Perch (Perca fluviatilis). p Pectoral fin; v One of the ventral fins; a Anal fin, supported upon interspinous bones (); c Caudal fin; d First dorsal fin; d' Second dorsal fin, both supported upon interspinous bones; ii Interspinous bones; r Ribs.

built up. The backbone is technically called the "vertebral column," and is composed of a succession of short pieces or segments, each of which is termed a "vertebra" (Latin, verto, I turn). Each separate piece or vertebra of the backbone is deeply cupped or hollowed out at each end, and the entire spine is thus rendered extremely flexible. From the possession of this backbone or spinal column, the Perch is said to be a "vertebrate animal." The essential purpose served by the backbone is to protect the very important portion of the nervous system which is known as the "spinal marrow," or "spinal cord." The unpaired fins are all connected with the backbone by means of a series of little bony spines, which are placed in the central line of the body, and are known as the "interspinous bones" (fig. 35, i).

At the front end of the backbone is placed the skull, which protects in its interior the brain or central portion of the nervous system.

The mouth in the Perch is placed at the front of the

head, and is armed with small teeth, which have a uniform size, and are bent backwards. The throat is pierced with a series of slits, which, as we shall see, allow the water to reach the gills or breathing-organs. Behind the throat the digestive canal consists of a gullet, stomach, and intestine ; and the latter terminates in a distinct vent, placed on the lower surface of the body. There is a large and well-developed liver, and behind the stomach are placed three closed tubes, which are believed to represent another of the digestive glands (the sweetbread or pancreas).

At the sides of the throat, on the under surface of the head, are placed the breathing-organs and heart. The former constitute the "gills," and are adapted solely for breathing air dissolved in water, and not for breathing air directly. Hence the fish dies if removed from its natural element. The gills (fig. 36, A, b b) have the form of a series of fringes, supported upon four bony arches (a),

Fig. 36.-A, Gills and heart of the Perch exposed by the removal of the gillcover on the left side. a First of the four bony arches which carry the gills (bb); b' The lower edges of the gills on the right side; h Heart (after Van der Hoeven). B, Scale of the Perch, magnified.

and having a bright red colour from the number of bloodvessels distributed to them. A similar set of gills, supported upon four similar arches, is placed upon the oppo