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The body of a fish is similar in many ways to the bodies of other vertebrates. For example, fish, like other vertebrates, have an internal skeleton, an external skin and internal organs such as: the heart, intestines and brain, but fish differ in many other respects from the rest of the vertebrates. For example, fish have fins instead of legs and gills instead of lungs. Lampreys and hagfish differ from all other vertebrates, and from all other fish as well in many respects. The physical characteristics of the lamprey and the hag have been discussed in previous parts. This part includes the natural characteristics common to most other fish.

external anatomy

the shape. Most fish have a streamlined body and a somewhat rounded head at the front, and they do not have a neck, so the head is softly fused with the trunk, and then the trunk narrows towards the tail. In addition to this basic symmetry, fish have a variety of shapes, with tuna and other fast-swimming fish distinguished by their torpedo-like shapes. The herring and sunfish that live in fresh water and some other types of fish are flat on both sides. The bodies of many bottom-dwelling fish, such as most types of rays, are flattened from top to bottom, and so on. The bodies of many species are shaped to resemble the objects around them, for example, the anglerfish and the stonefish resemble rocks, and the tubefish resemble tall grasses. This camouflage is called protective camouflage, and it helps the fish to hide from its enemies or prey.

skin and color

The vast majority of fish have fairly tough skin that contains blood vessels, nerves, and connective tissue, as well as special cells, some of which secrete sticky mucus. Which makes the fish smooth. Others have color cells called melanocytes. The melanocyte contains red, yellow, and blackish-brown pigments. These colors may combine to result in other colors such as orange and green. And some species dyes with different colors from other species. Or the pigment cells may be specially arranged. These differences lead to a lot of color variation between species. In addition to the melanocytes, many fish have silver pigments in their skins and scales that, in sunlight, result in a different combination of bright rainbow colors.

The color of most fish matches the color of the surrounding objects. For example, most fish that live close to the surface in the open sea have a blue color that matches the color of the surface of the water. This type of camouflage is called protective coloring, but some brightly colored fish, including those with poisonous spines, do not blend in with the surrounding environment. The bright colors may protect the fish by confusing its enemies or warning them that it has poisonous spines.

Most fish change their colors to match the color changes in their surroundings. Flatfish and other fish with two or more colors can also change their color patterns. The fish receives special stimuli through the eye to cause the desired color change, through nerve signals. The nerve signals resulting from the presence of the stimuli prompt the melanocytes to rearrange the pigments in them to become darker or paler, resulting in the formation of different color patterns.

scales

Most fish have a protective covering of scales, with teleosts having rounded scales at the tip. There are two main types of these scales comb and circular. And the edges of the comb have fine teeth on their surfaces. And fish with a coarse texture, such as bass and perch, have comb-edge scales. The circular scales are smooth surface. It is found in certain fish such as carp and salmon.

Some species of primitive fish, including patchers and beaks, have thick, heavy, glossy Janoid scales. The bodies of the shark and the rays are covered with dental scales resembling fine, compact teeth. Some types of fish do not have scales, including certain types of freshwater eels and catfish.

fins

Movable structures help the fish swim and maintain balance. The fish moves its fins with its muscles, and all modern bony fish have radiating fins, except for a few species without fins. Some primitive fish also have radiating fins. These fins are composed of a tissue of skin supported by a structure of rods called rays, and some radiating fish have soft rays, while others have soft rays and spiny rays, all of which are hard and sharp in touch. Some primitive fish have lobed fins consisting of a fleshy base with a ray-shaped edge. But the lobed fins are less flexible than the radiating fins. Sharks, rays, and chimeras have fleshy fins covered with leather and supported by numerous fin rays that consist of a hard substance called keratin.

The fins of fish are divided according to their location in the body and also according to their structure. Accordingly, the fin is either medial or double. The medial fins are vertical fins located on the back of the fish, under the body or the tail, and include the dorsal, anal and caudal fins. The dorsal fin grows along the back and helps the fish to stay upright. All fish often have one dorsal fin, and many have two or three. Like the dorsal fin, the anal fin grows on the ventral surface, close to the tail, and helps the fish to stay upright and upright.

The caudal fin is located at the end of the tail. The fish rocks its caudal fin from side to side to propel itself through the water to row.

Double fins consist of two identical fins, one on each side of the body. Most fish have double pectoral and pelvic fins. The pectoral or scapular fins of most fish grow on the sides of the body just behind the head, and the pelvic or pelvic fins of most fish are located just below and behind the pectoral fins. However, in some fish the pelvic fin may be located anteriorly to the larynx or almost posteriorly to the caudal fin. Pelvic fins are also called ventral fins, and most fish use their paired fins primarily for turning, standing, or other maneuvers.

Structure and muscles

The body of a fish consists of a head, torso, tail and fins. The spine is the central framework for the trunk and tail, and it is made up of a large number of separate pieces of bone or cartilage called vertebrae. In bony fish, each vertebra has a thorn at the top, and each caudal vertebra has a thorn at the bottom. The ribs are connected to the vertebrae. The skull is mainly composed of the cranium, the mouthpieces, and the nostrils. The pectoral fins of the vast majority of fish are attached to the back of the skull by a structure called the pectoral girdle. The pelvic fins are supported by a structure called the pelvic girdle attached to the pectoral girdle or supported by the muscular tissue of the abdomen. The dorsal fins are supported by bony or cartilaginous structures whose roots extend to the tissue above the spine, and the tail supports the caudal fin, while the anal fin supports bony or cartilaginous structures located below the spine.

Fish, as in all vertebrates, have three types of muscles:

1- skeletal muscles

2- smooth muscles

3- Cardiac muscles.

Fish use skeletal muscles to move their bones and fins. Almost all of the flesh of a fish consists of skeletal muscles that are arranged one after the other in broad vertical strips called muscle segments. The muscle pieces can be easily seen in a fish that has had its skin removed. Each muscle is controlled by a separate nerve. As a result, the fish can bend the front of its body in one direction, while bending its tail in the opposite direction. The vast majority of fish make such movements with their bodies for swimming.

The smooth and cardiac muscles of the fish work in an automatic and involuntary manner. The smooth muscles are responsible for operating the internal organs such as the stomach and intestines, and the heart consists of the cardiac muscles that operate it.

body systems

The internal organs of a fish, as in other vertebrates, are classified into distinct organs according to the function they perform. The main organs include: respiratory, digestive, circulatory, nervous and reproductive systems. Some of these systems are similar to their counterparts in other vertebrates, but others differ from them in several ways.

Respiratory system

Fish, unlike terrestrial animals, get their oxygen from water, and water contains a certain amount of dissolved oxygen. In order for fish to obtain oxygen, they swallow water through the mouth and push it over the gills. Most fish have four pairs of gills, located within a gill chamber located on each side of the head. Each gill consists of two rows of fleshy filaments attached to a gill arch, and water passes into the gill chambers through the gill openings. A piece of bone called the gill cover protects the gills of bony fish, and there are no gill covers for sharks or rays. Its gill openings are clear openings outside the body.

The respiratory process in bony fish begins with the closing of the gill covers and the mouth opening; At the same time, the wall of the mouth expands outward, bringing water into the mouth. Then the wall of the mouth moves inward, the mouth closes and the gill covers open. In this process, the water rushes from the mouth to the gill chambers, where the water passes over the gill filaments, and in the meantime, the gills absorb oxygen dissolved in the water and excrete carbon dioxide, which is formed through metabolic processes, and is excreted during respiration, and then the water passes through the gill openings , and this process is repeated.

Digestive

(Gut). This device converts food into substances that nourish the cells of the body, and by means of which undigested substances are eliminated. In fish, this system begins with the mouth and ends with the anus, which is the opening in front of the anal fin. Most fish have a mouth with jaws, tongue and teeth. A fish cannot move its tongue. The teeth of most fish are attached to the jaws, and fish use their teeth to catch prey or to bite and tear off pieces of the flesh of their victims. Some fish also have teeth on the roof of the mouth or on the tongue. Most fish also have teeth in the pharynx, a short tube behind the mouth, and fish use these teeth to crush or grind their food.

Food passes in all fish through the pharynx on its way to the esophagus, which is another tubular organ, and the fish’s esophagus can easily expand, thus allowing the fish to swallow its food whole. Food passes from the esophagus to the stomach where it is partially digested. The esophagus or stomach of some fish may enlarge to be giblets. The gizzard breaks the food into small pieces before it passes into the intestine. The digestion process takes place in the intestine and the digested food is absorbed through the intestine and from there into the bloodstream. Waste and undigested food pass from the anus.

Circulatory system

The circulatory system distributes blood to all parts of the body. It consists of the heart and blood vessels. The heart of the fish is composed of two main chambers: the atrium and the ventricle, and blood passes through the veins and from there to the atrium, and then passes to the ventricle. The ventricular muscles push blood to the gills through the gill arteries, where the blood gets oxygen and gets rid of carbon dioxide. After that, the arteries carry blood to all parts of the body, while the blood carries the digested food from the intestines, and oxygen from the gills to all the cells of the body.

Nervous system

The nervous system of fish, like the nervous systems of other vertebrates, is composed of the brain, spinal cord, and nerves. However, the nervous system of fish is not as complex as that of mammals and other higher vertebrates. The spinal cord, which consists of soft nervous tissue, extends from the brain and passes into the spine. The brain is a bulge in the front of the spinal cord enclosed in the skull. Nerves extend from the brain and spinal cord to every part of the body. Some nerves called sensory nerves carry messages from the sensory organs to the spinal cord and brain. Other nerves called motor nerves carry messages from the brain and spinal cord to the muscles. A fish can voluntarily control its skeletal muscles, but it does not have voluntary control of smooth and cardiac muscles, and these muscles work involuntarily.

Reproductive system

The reproductive system in fish, as in all vertebrates, consists of testes in males and ovaries in females. The testes produce male sex cells or sperm (male gametes, sperm). There is semen in a fluid called semen. The ovaries produce female sex cells or eggs. The eggs of fish are called roe or fish eggs. Most fish release their eggs through an opening close to the anus. The males of some types of fish have special structures to transfer semen directly to the female. Male sharks, for example, have such a structure called a fishnet, which is found on each pelvic fin. The two masks are used to introduce semen into the female body.

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