Beluga, crucian carp, herring, trout, carp, silver carp, common carp - all known fish. This list goes on and on. And their commercial value is difficult to overestimate. And indeed, the class of fish is very diverse. Modern taxonomy has more than 20 thousand species of these aquatic animals. Thanks to what signs of the structure they were able to master this habitat and occupy a dominant position in it. What class are fish that differ in their structure? You will find the answer to these and other questions in our article.
Signs of fish
No wonder they say about self-confident people: "Feels like a fish in water." Scientists know that the first fish lived in the Silurian period. Outwardly, they looked like modern sharks with moving jaws, on which sharp teeth were located. Millions of years have passed, in the process of evolution, the fish mutated and acquired a number of new adaptive traits.
Like aquatic animals, they all have a streamlined body shape, are fully or partially covered with scales, various types of fins are located on the body, and the gills are the respiratory organs. These are common signs for all representatives of this systematic unit. But to which class the fish belong, you can answer by considering their significant differences. At the moment there are two of them: Bone and Cartilage.
Features of the external structure
The body of absolutely all fish is covered with scales. It protects the skin of aquatic inhabitants from excessive friction of water. After all, most of them spend most of their lives in motion. An additional protection against friction is a large amount of mucus, which is rich in skin. This helps many species survive under the adverse conditions of temporary drought. Not all fish species have full body scales. For example, in sharks, it is located in one row along the surface of the body, resembling their teeth in appearance. The same can be said of the numerous representatives of the sturgeon squad. Most bone fish are protected by scales, like a strong shell. It also performs additional functions: camouflage from predators, warning coloration in predatory and poisonous species, designation of sex, reflection of light in water.
Fin structure
The next characteristic feature of fish is the presence of fins. These formations serve as limbs for movement in water, and some ancient species can even crawl with their help. Fins are combined in two groups. The first are paired: abdominal and chest. They help maintain the balance of fish in the water column. Unpaired are the caudal, anal and dorsal. They work like a rudder, directing the body of aquatic animals in the right direction. As a result of evolution, fish limbs formed limbs of reptiles.
On the body of the fish, you can easily see the lateral line. This is a unique organ of balance and touch, characteristic only for fish.
Internal structure of fish
The organ systems of these animals also have their own characteristics associated with the aquatic environment. The musculoskeletal system is represented by the skeleton. Depending on the class, it is formed by cartilage or bone tissue. All bones of the skeleton of the head are connected motionless, except for the lower jaw. This allows fish to easily capture prey. The structure of this section of the skeleton also includes gill covers and arches, the last of which are attached to the respiratory organs of fish - gills. The axial skeleton consists of separate vertebrae, connected to each other and to the skull motionless. Ribs are attached to the trunk of the spine. The skeleton of the fins is represented by rays. They are also formed by bone tissue. But paired fins also have belts. Muscles are attached to them, causing them to move.
Digestive system of fish of the through type. It begins with the oropharyngeal cavity. Most fish have sharp teeth on their jaws that serve to grip and tear food. In the process of digestion, enzymes of the glands - the liver and pancreas - also take part. In the processes of isolation and salt metabolism, the main role in the fish organism is played by paired kidneys. Outwardly, they open with the help of the ureters.
Fish are cold-blooded animals. This means that their body temperature depends on changes in the environment. This symptom is determined by the circulatory system. It is represented by a two-chamber heart and a closed vessel structure. During their movement, venous and arterial blood mix.
The nervous system is represented by the brain and spinal cord and nerves. And its peripheral part - by nerve fibers. In the brain, the cerebellum reaches a special development. This part determines the fast and coordinated movements of fish. The sensory organs are able to perceive any irritation possible in the aquatic environment. Since the lens of the eyes of fish does not change its shape and position, animals see well only at a short distance. But at the same time, they are able to distinguish between the shape and color of various objects. The organ of sound perception is represented by the inner ear and is associated with the structure responsible for the balance.
Fish breeding also has its own characteristics. These animals are dioecious, with an external type of fertilization.
What is spawning
The process of breeding fish is also called spawning. It happens in the water. The female throws caviar, and the male sprinkles it with seminal fluid. The result is a fertilized egg. As a result of successive mitotic divisions, an adult develops from it.
Sometimes fish breeding is associated with spawning migrations and significant changes in the behavior and structure of fish during this period. For example, pink salmon form large herds, in the composition of which they move from the seas to the upper reaches of the rivers. During this journey, they have to overcome many obstacles, moving against the tide. In these fish, a hump forms on the back, the jaw turns out and bends. Having lost a lot of strength, after the fertilization process, adults die. Surprisingly, young fry come back on their own to the same habitat.
Fish groups
Huge species diversity necessitated the classification of this group of animals. Currently, scientists have precisely identified the signs by which to classify the class of Pisces. Systematic affiliation is determined by the presence of a swimming bladder, gill slits or covers, and the type of scales. So you can distinguish between bone and cartilage fish. There are other signs by which these animals are grouped. For example, fish that move to spawn in other habitats are called migratory. But, given the scope, distinguish between commercial and decorative representatives of these aquatic animals.
Cartilaginous fish
What class are fish with a cartilaginous skeleton, gill slits opening outwards? It’s easy to guess. These are cartilaginous fish. They do not have a swim bladder, so they either live at the bottom or are constantly in motion. Saw fish, white, giant, whale sharks, stingray ... They are known to you. The list of dangerous predators can be continued with the sea devil, electric stingray and sea fox. These marine inhabitants pose a great danger to the life of animals and humans. Although found among the cartilaginous fish and quite innocent specimens. So, the Katran shark eats fish and crustaceans. In addition to the awesome appearance, it poses no danger to humans.
Bone fish
To the question of which class the most numerous fish belong to, perhaps every student will answer. Their skeleton consists entirely of bone tissue. A swimming bladder located in the body cavity allows its owners to stay in the water column. The gills are covered with gill covers, and do not open outward through separate openings. Bone fish have such signs.
Fish value
Representatives of this superclass of vertebrate animals are primarily important commercial value. A person eats their nutritious meat and protein-rich caviar. And the number of recipes for cooking various types does not know the bill. Fish oil has long been used as a treatment for bacterial and viral respiratory diseases. A person annually catches a huge number of individuals and breeds them independently. Meat and bones also make flour. It is used as fertilizer and feed for many pets.
Recently, sport fishing has become increasingly popular, attracting participants from different countries. And each of us is dreaming of catching a goldfish that fulfills all desires!
Thus, to which class the fish belong, you can determine by the features of their structure, organization and lifestyle.