Fish (Pisces) - A Brief Description Of The Superclass

Fish (Pisces) - A Brief Description Of The Superclass
Fish (Pisces) - A Brief Description Of The Superclass

Video: Fish (Pisces) - A Brief Description Of The Superclass

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Video: Vertebrate Diversity: The Fish 2023, January
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Fish are vertebrates constantly living in water, breathing with gills and moving with the help of fins. Paired limbs, if any, are never a five-toed limb type. Unpaired fins are supported by a special skeleton. There is only the inner ear.

Most fish are mobile, good swimmers. The main type of translational movement is lateral wavy movements of the whole body or only a powerful tail. Paired fins - pectoral and abdominal - function as stabilizers, supporting planes, rudders, and less often as organs of movement; unpaired fins provide stability to the body. The high activity of fish, the maneuverability of their movements are associated not only with the improvement of the motor system, but also with the development of the brain and sense organs.

The mouth in fish is limited by movable jaws. The respiratory organs are the gills. The olfactory openings are paired. All fish, except for lungs, have one circle of blood circulation. The heart has two chambers: the atrium and the ventricle. Body temperature is variable. The swim bladder serves as a hydrostatic organ. Most fish have special lateral line organs in their skin. Fish, as a rule, are dioecious, but there are also hermaphrodites. They usually reproduce by spawning, but there are also viviparous ones.

Sea fish, photo photograph pixabay
Sea fish, photo photograph pixabay

In the modern fauna, there are about 20 thousand species of fish, most of which live in the seas. Fish are of great importance as producers of valuable food products and technical raw materials.

Modern fish are usually divided into two classes: Cartilaginous and Bony. Each of them, in turn, is divided into subclasses. Almost all cartilaginous fish are marine. The most famous representatives of cartilaginous fish are numerous species of sharks and rays.

In teleost fishes, the caudal fin is equal-lobed, and only the skeleton of its base is asymmetric. The axial skeleton is well ossified, although in some species the notochord can be preserved between the vertebral bodies. Scales in the form of bony plates, not rhombic in shape.

There are three main groupings of orders of teleost fish. A less specialized grouping, which appeared already from the Lower Cretaceous, has retained some ancient structural features and will include fish, mainly herring-like, salmon-like, pike-like, carp-like or eel-like appearance.

In the Cretaceous period, a second group appeared - parapercoid fishes - transitional to the later developed progressive spiny fishes.

The third group - spiny-finned fish - are the most progressive percoid and batrachoid fish of perch-like, mackerel, flounder and other types of structure.

The shape of the caudal fin in fish, diagram drawing
The shape of the caudal fin in fish, diagram drawing

The sizes of fish vary within very wide limits: from 1 cm (one of the goby species) to 16-20 m (whale shark). Some fish weigh up to 1.5 tons or more.

The body shape of fish is extremely diverse due to the variety of habitats and lifestyle. Most fish have a streamlined body that facilitates their movement in the water. Such fast-swimming fish as sharks, salmon, walleyes, etc., have an elongated torpedo-like body. Fish that do not make large movements (bream, crucian carp, carp) often have a high, laterally compressed body. In bottom fish, the abdominal side of the body is often flattened, which provides greater contact with the bottom, where food is located.

Organs of movement. Fish move in water by bending the body and using paired and unpaired fins. The fins are based on the internal supporting skeleton. The outer lobes are supported by bony or cartilaginous fin rays. The latter are hard undivided or soft segmented, soft rays are branched.

Paired fins are two pairs: pectoral and ventral. The former are located behind the gills, and the latter are located in different parts of the abdominal surface. In codfish, they are located on the throat in front of the pectorals. The absence of paired fins is due to their loss during evolution.

The paired fins support the fish's body in a horizontal position and serve as rudders and rudders.

The pelvic fins of gobies are spliced ​​and form a suction cup, with the help of which the fish are kept on the bottom in places with a fast current or strong surf. The very long pectoral fins of flying fish form the bearing planes on which they glide when separated from the water.

Types of fish scales, drawing
Types of fish scales, drawing

Unpaired fins include the caudal fins, one or more dorsal fins, and one, rarely more anal fins. In the forward movement of fish, the tail fin plays the main role: it also serves as a rudder when the animal turns and dives. The dorsal and anal fins are primarily stabilizers of the direction of movement of the fish, but they are also involved in turning the body. For example, a bream can tilt the front end of the body downward by moving its long anal fin, which makes it easier to find food. In pikes, the dorsal and anal fins are pushed back to the tail, which increases the power of the tail strike and, consequently, a rapid rush to the prey from an ambush. In the angler fish living at the bottom of the seas, the elongated front ray of the dorsal fin is located above the upper lip: by its oscillation, the predator attracts prey. In fish, the dorsal fin is changed into a suction cup.Sometimes fins are represented by spines.

Veilsfish are formed by skin from the epidermis and dermis. The outer epidermis is not keratinized. It contains numerous unicellular glands that secrete mucus, which reduces the friction of the fish's body against the water. The dermis has a fibrous structure. In most fish, the body is covered with scales of one structure or another. Sharks and rays have placoid scales. It consists of a dentin base embedded in the skin and an outwardly protruding enamelled spike. On the jaws of these fish, placoid scales have been modified into teeth. In sturgeons, along the ridge, sides and border of the sides and belly, there are five rows of bony plates - beetles, and the tail is covered with rhombic ganoid scales typical of ancient fish. Bony fishes are usually covered with bony scales that look like thin, overlapping plates of various shapes. They sit obliquely in animal skin,forming in most cases the correct rows.

The uneven growth of scales in different seasons of the year leads to the formation of wide summer and narrow winter rings on it; by their number, one can judge the age of the fish. Since there is a relationship between the growth rate of the scales and the body of the fish, the width of the annual rings on the scales can be used to judge the growth rates of fish over the past years. Sometimes fish scales are modified into needles, spines, bony shields and other skin formations. The color of fish is determined by pigment cells in the skin. The silvery color and metallic luster of the scales is due to the presence of acicular crystals of a special substance - guanine, which reflects the incident light.

Fish structure diagram, drawing
Fish structure diagram, drawing

Skeleton. The structure of the skeleton is associated with the height of the organization and the characteristics of the biology of the fish. In sturgeon, lungfish, and some other fishes, a well-developed notochord is preserved throughout life, and the vertebrae are represented only by cartilaginous arches. In adult teleosts, the remnants of the notochord are observed only between the vertebrae. Sharks and rays have a skeleton consisting of cartilaginous elements without bony parts. In sturgeon fishes, the skull, spine and fin belts are cartilaginous at the base, but their cartilaginous skull is covered with an outer shell of flat overhead bones. Bones are also found in the girdle of the pectoral fins. In teleost fish, the skeleton is formed mainly by bone elements. The skeleton of fish is composed of the skull, the spine, associated ribs, bones and cartilage of the fins and their bases.

The skull consists of the cerebral box and the visceral skeleton, which is formed by the bony or cartilaginous jaws, the hyoid arch, and the branchial arches. In most fish, the gills are covered with bony gill covers. The vertebrae of cartilaginous and bony fish have a biconcave body. Their upper arches form the spinal canal, and the lower arches of the caudal vertebrae form the canal along which large blood vessels stretch. Ribs are connected with the vertebrae of the body, freely ending in the other end in the muscles of the walls of the body. Paired fins have belts formed by cartilage or bones that lie in the body of the fish.

The musculature of the body and tail of fish has a metameric structure. Along the body, on the left and on the right, there are wide stripes of longitudinal muscles, separated by connecting layers - myosepts into a number of muscle segments - myomeres. The longitudinal myosepta divides them into dorsal and abdominal regions. Separate muscles are located in the head, fins and their girdles.

Part of the spine of a fish in a longitudinal section, diagram drawing
Part of the spine of a fish in a longitudinal section, diagram drawing

The nervous system of fish more perfect than that of cyclostomes, but still carries many primitive features. The brain of fish is small: for example, in large pikes it is only 1/1300, and in sharks - 1/3700 ​​of body weight. It consists of the anterior, diencephalon, middle and medulla oblongata, as well as the cerebellum. The forebrain does not form hemispheres (except for lung-breathing fish) and has only one ventricle, covered from above by a thin cover without nerve cells. Its anterior end continues into the olfactory lobes with the olfactory nerves extending from them. The forebrain of fish plays the role of the olfactory center and is involved in the coordination of movements. The diencephalon of fish is small. On its roof there is an outgrowth - the pineal gland, which plays the role of an endocrine gland. The lower surface of the diencephalon of fish forms a funnel,to the top of which is adjacent another endocrine gland - the pituitary gland. The optic nerves extend from the bottom of this brain region. The midbrain is relatively well developed. On its upper surface, two optic hillocks protrude, with which the fibers of the optic nerves are connected. The cerebellum, which in fish, like in other vertebrates, is the center of the regulation of movements, is developed in different ways depending on the degree of mobility of the animal. The medulla oblongata of fish is covered from above with an epithelial film. Its side walls are thickened. Ten pairs of brain nerves extend from the fish brain. The spinal cord is thin and stretches to the end of the spine.as in other vertebrates, the center of movement regulation is developed in different ways depending on the degree of mobility of the animal. The medulla oblongata of fish is covered from above with an epithelial film. Its side walls are thickened. Ten pairs of brain nerves extend from the fish brain. The spinal cord is thin and stretches to the end of the spine.as in other vertebrates, the center of movement regulation is developed in different ways depending on the degree of mobility of the animal. The medulla oblongata of fish is covered from above with an epithelial film. Its side walls are thickened. Ten pairs of brain nerves extend from the fish brain. The spinal cord is thin and stretches to the end of the spine.

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