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fin,organ of locomotion characteristic of fish and consisting of thin tissue supported by cartilaginous or bony rays. In some fish, e.g., the eel, a single fin extends from the back, around the tail, and along the ventral surface. In the majority of fishes, however, there are one, two, or three dorsal fins, a distinct tail fin, and an anal fin. These are called median, or unpaired, fins. In addition to these unpaired fins, most fish also have paired fins. These are the pectoral fins, placed just back of the gills, and the pelvic, or ventral, fins, variable in position and sometimes lacking entirely. The tail is an important organ of locomotion and the paired fins are used for steering, checking speed, balancing, and for slow movements. An adipose fin (fatty tissue without support) is found behind the dorsal fin in some fish, e.g., the salmon and the catfish. See climbing perchclimbing perch,
or walking fish,
any of several fish of the family Anabantidae, adapted to living in oxygen-depleted water or on dry land. They are not related to the true perch.
..... Click the link for more information. ; flying fishflying fish,
common name for members of the Exocoetidae, a family of carnivorous or herbivorous fish of warmer seas. Flying fishes usually swim in schools. They average 7 to 12 in. (17.
..... Click the link for more information. .
an organ of locomotion of aquatic animals. Invertebrates having fins include pelagic gastropods and cephalopods, and arrowworms (Chaetognatha). The fin of a gastropod is a modified foot; in cephalopods lateral folds of skin effect locomotion. Arrowworms have lateral and caudal fins, which are formed by folds of skin.
Among extant vertebrates, fins are found in cyclostomes, fishes, some amphibians, and mammals. Cyclostomes have unpaired fins: an anterior and a posterior dorsal fin (in lampreys) and a caudal fin.
Fishes may have paired or unpaired fins. The paired fins may be anterior (pectoral) or posterior (pelvic). In some fishes, such as the Gadidae and Blenniformes, the pelvic fins are anterior to the pectoral fins. The skeleton of paired fins consists of cartilaginous or bony rays, which articulate to the skeleton of the girdles of the limbs. The principal function of paired fins is to direct the fish’s movement vertically. In a number of fishes the paired fins perform the functions of swimming, gliding in the air (in flying fishes), crawling on the bottom, or moving on dry land (in fishes that periodically emerge from the water). For example, fishes of the tropical genus Periophtalmus use their pectoral fins to climb trees. The skeleton of an unpaired fin—a dorsal fin (often divided in two or, sometimes, three parts), an anal fin (sometimes divided into two parts), or a caudal fin—consists of cartilaginous or bony rays that lie between the lateral muscles of the body. The skeletal rays of the caudal fin are connected to the posterior end of the spine; in some fishes the rays are replaced by spinal processes of the vertebrae.
The peripheral parts of the fin are supported by slender rays of horny or bony tissue. In acanthopterygian fishes, the anterior rays become thickened and form hard spines that are sometimes connected to poison glands. To the bases of these rays are attached the muscles that expand the lobe of the fin. The dorsal and anal fins serve to regulate the direction of the fish’s motion. In addition, they sometimes may serve as organs of progressive locomotion or may perform such functions as attracting prey. The caudal fin, which varies greatly in shape in various fishes, is the principal organ of locomotion.
During vertebrate evolution the fins of fishes probably arose from an unbroken fold of skin that passed along the back of the animal, bent around the posterior end of its body, continued on the ventral side to the anal opening, and then divided into two lateral folds that extended to the gill slits. Such is the location of the fin folds in the lancelet, an extant primitive chordate. It may be presumed that in the process of animal evolution, skeletal elements formed in some parts of the folds, but in the interstices the folds disappeared, leading to the development of unpaired fins in cyclostomes and fishes and of paired fins in fishes. This presumption seems to be supported by the existence of lateral folds or rows of spines in the oldest vertebrates (some Agnatha and Acanthodii) and by the fact that in extant fishes paired fins are of greater length in the early stages of development than in the adult state.
Among amphibians, unpaired fins in the form of a cutaneous fold lacking a skeleton are found as permanent or temporary formations in most aquatic larvae, adult caudates, and larvae of acaudate amphibians. Among mammals, fins are present in cetaceans and sirenians, who transferred to aquatic life. The unpaired fins of cetaceans (the vertical dorsal fin and the horizontal caudal fin) and sirenians (the horizontal caudal fin) have no skeleton. They are secondary formations and are not homologous to the unpaired fins of fishes. The paired fins of cetaceans and sirenians, consisting only of anterior fins (the posterior ones are reduced), have an internal skeleton and are homologous to the anterior limbs of all other vertebrates.
REFERENCESRukovodstvo po zoologii, vol. 2. Moscow-Leningrad, 1940.
Shmal’gauzen, I. I. Osnovy sravnitel’noi anatomii pozvonochnykh zhivotnykh, 4th ed. Moscow, 1947.
Suvorov, E. K. Osnovy ikhtiologii, 2nd ed. Moscow, 1947.
Dogel’, V. A. Zoologiia bespozvonochnykh, 5th ed. Moscow, 1959.
Aleev, Iu. G. Funktsional’nye osnovy vneshnego stroeniia ryby. Moscow, 1963.
V. N. NIKITIN
ii. A projecting flat plate or structure, such as a cooling fin.