ANATOMY
The living species of fish are usually divided into three classes: the Agnatha, the jawless fishes, comprising the hagfishes and lampreys; the Chondrichthyes, the cartilaginous-skeleton fishes, such as sharks and rays; and the Osteichthyes, the bony-skeleton fishes, comprising all other living fishes. The skeletons of these three groups vary in fundamental ways. In the hagfishes and lampreys the backbone is basically a notochord, a rodlike structure composed of unique notochordal tissue. In sharks and rays the notochord is surrounded and constricted by spaced rings of cartilage, the vertebrae, to form a backbone. The remainder of the skeleton is also cartilaginous, not bony, but in many forms the cartilage is partly calcified, and thereby hardened, by the addition of calcareous salts. In primitive bony fishes, such as the sturgeon, the vertebrae spaced along the notochord are still largely cartilaginous, but in most advanced bony fishes the vertebrae are bony and are united to form the backbone, and the notochord is no longer present.
Some fishes, such as lampreys, lack ribs; others have either a single or a double pair of ribs attached to each trunk vertebra. Among the higher bony fishes there also may be small, riblike intermuscular bones, which often render such fish difficult to eat.
The body appendages of fish are of two kinds, cirrhi and fins. Cirrhi are flaps of flesh that may appear on any part of the body; they often serve as camouflage. Fins are either median or paired. Median fins are situated along the centerline of the body, at the top, the bottom, and the end. The top, or dorsal, fin may consist of one to several fins, one behind the other, and may include a fleshy fin, called the adipose fin, near the tail. The bottom, or anal, fin is located on the belly behind the vent, or anus. The end fin is called the tail, or caudal, fin.
The dorsal and anal fins may be supported by cartilaginous rods, as in the lampreys, by cartilaginous rods and horny rays, as in sharks, by horny rays, as in the spiny-finned fishes, or by bony rays (derived from scales) in the soft-rayed fishes. The tail fin may be protocercal, the body continuing straight back as a middle support between the upper and lower lobes of the tail; heterocercal, with the end of the body turning up and continuing to the tip of the upper lobe; or homocercal, in which the last few vertebrae are fused and joined with other bony elements (hypurals) to support the tail-fin rays. A modification of the heterocercal tail so as to resemble the protocercal type is called diphycercal.
The paired fins correspond to the arms and legs of land vertebrates. The pectoral fins are situated at the front of the body behind the gill openings and generally function to provide maneuverability, but may be highly modified to fulfill other functions. The simplest internal support for the pectoral fins occurs in the sharks, where a U-shaped cartilaginous skeletal structure, called the pectoral girdle, joins and helps support the two pectoral fins. In the higher bony fishes the pectoral girdle is composed of bone and is more complex in structure. The pelvic fins, also called the ventral fins, are located along the bottom of the body but vary considerably in their placement. They may be located in the middle of the belly, as in salmon; below the pectorals, as in the largemouth bass; or in front of the pectorals, as in cods. Pelvic fins also serve as maneuvering structures and also may be modified to serve other uses. The supporting pelvic girdle is lacking in many bony fishes; in most fishes in which the pelvic girdle is present it is represented by a single skeletal element on each side of the body.
The scales of fish are colorless; a fish's coloring arises from structures beneath or closely associated with the scales. Not all species of fishes have scales, or the scales may be so small as to make the fish appear scaleless. Scales also may be present only on small areas of the body. The arrangement of scales may be imbricate (overlapping like the shingles on a roof) or mosaic (fitting closely together or just minutely separated).
Four basic scale types can be distinguished on the basis of structure. Placoid scales, also called dermal denticles, are found on sharks and rays and are toothlike in structure. Indeed, modified and enlarged placoid scales have become the teeth of sharks. The placoid scale consists of an upper layer of enamellike substance called vitrodentine, a lower layer of dentine, a pulp cavity, and a disklike basal plate embedded in the skin. Placoid scales do not increase in size as do the scales of bony fishes, and new scales must be added as a shark grows.
Cosmoid scales are found on the primitive coelacanth. They also occur on lungfishes, but in a highly modified, single-layered form. The cosmoid scale of the coelacanth is a four-layered bony scale. The upper layer is enamellike vitrodentine; the second layer is a hard, dentinelike substance called cosmine; the third layer is spongy bone, and the lowest layer is dense bone.
Ganoid scales, as found on gars, are typically squarish (rhombic) in shape and consist of a single bony layer, a layer of cosmine, and a covering of a very hard enamellike substance called ganoin.
Leptoid scales are believed to have been derived from ganoid scales by the loss of the ganoin layer; they consist of a single layer of bone. Leptoid scales are found on the higher bony fishes and occur in two forms: cycloid (circular) and ctenoid (toothed), the latter bearing tiny comblike projections. The single-layered cosmoid scale of lungfishes also may be classified as leptoid, although of a different derivation.
