Superclass Pisces: characteristic, features of internal and external structure

Pisces is the largest group of aquatic chordates, according to the species diversity, which is also the oldest. Pisces inhabit almost all fresh and salt water. All their systems of organs are adapted to live in an aquatic environment. According to the accepted scientific classification, Pisces is referred to the domain of Eukaryotes, the kingdom of Animals and the Chordovye type. Let's take a closer look at the superclass.

Covers of the body

The outer covering of the body of the fish is skin and scales. There are rare exceptions when the scales are absent or altered. The skin is divided into dermis and epidermis. The epidermis of the superclass of Pisces is not keratinized.

In the formation of scales, the main role is played by the dermis. Scales are different depending on the class of fish to which it belongs.

• Placoid scales are found in the class of cartilaginous fish. It consists of dentin, covered with enamel. This kind of scales in the course of evolution turned into the teeth of sharks and rays. If the link of the scales is lost, it will not recover.
• Ganoid scales are characteristic of the sturgeon. It is a bone plate covered with ganoine. Such a shell perfectly protects the body.
• Cosmoid scales are observed in cysteper and lungfish. It consists of cosmin and dentin.

The coloring of fish of the Pisces superclass can be very diverse. Representatives of the fauna can be both painted in one color, and be variegated, can have a soft or, conversely, warning of the danger of coloring.

Musculoskeletal system

The musculoskeletal system allows the fish to move and change position in the environment. The skeleton of a fish is different from the skeleton of a terrestrial animal. Her skull has more than forty elements capable of independently moving. This allows the animal to stretch and push the jaws at times very widely.

The spine consists of separate vertebrae, not intergrown with each other. It is divided into trunk and tail sections. When swimming, the driving force is created by the fin in the fish. They are divided into paired (thoracic, ventral) and unpaired (dorsal, anal, caudal). In the bone representatives of the superclass the fin consists of bony rays, which are joined by a membrane. Muscles help to unfold, fold and fold it, as you like fish.

Swimming of the inhabitants of the aquatic environment is possible thanks to the muscles. They shrink, and the fish moves forward. The musculature is divided into "slow" and "fast" muscles. The first are needed for calm swimming, drifting. The second - for fast and powerful jerks.

Nervous system of fish

The brain of fish is divided into departments. Each of them performs a certain function:

1. The forebrain consists of the intermediate and final. In this department there are olfactory bulbs. They receive signals from external organs of smell. Fish, actively using a scent during hunting, have enlarged bulbs.
2. The middle brain has optical lobes in its cortex.
3. The posterior brain is divided into the cerebellum and the medulla oblongata.

The spinal cord in representatives of the superclass of the Pisces passes along the entire length of the spine.

Circulatory system

Most representatives of the superclass have one circle of circulation and a two-chambered heart. The blood supply system is closed, it transfers blood from the heart through the gills and tissues of the body. The heart of the fish does not divide the arterial blood enriched with oxygen from the poor venous.

The heart chambers of fish go one after another and are filled with venous blood. This venous sinus, atrium, ventricle, arterial cone. Blood can move only in one direction - from the sinus to the cone. This is helped by special valves.

The organs of gas exchange in fish

Gills in fish are the main organ of gas exchange. They are located on the sides of the oral cavity. Bone fishes are covered with a gill cover, others can freely open outwards. When the gills are vented, the water passes into the mouth, then into the gill arches. After that, it again emerges through the holes of the gills in the fish.

The structure of the gills is as follows: they have semipermeable membranes, permeated with blood vessels, and are located on bone arches. Gill-shaped petals, penetrated by the smallest network of capillaries, help the fish to feel even more free under the water column.

In addition to gill respiration, fish can use a different method of gas exchange:

• Larvae of fish can perform gas exchange through the surface of the skin.
• Some species have lungs, in which humidified air is retained.
• Some fish species can breathe air themselves.

How is the digestive system of fish arranged?

Fish grab and hold food with teeth that are located in the mouth (as in most vertebrates). Through the throat through the esophagus, food enters the stomach. There it is processed by gastric juice and enzymes contained in it. Then the food moves into the intestine. The remains of it are thrown out through the cloaca (anus).

What do the inhabitants of the aquatic environment eat? The choice is very wide:

• Vegetable fishes eat seaweed and aquatic plants. Some of them can also eat plankton (for example, silver carp).
• Carnivorous fish can eat plankton, various worms, mollusks, crustaceans and, of course, other, smaller fish.
• Some fish can change their taste preferences during life, for example, at a young age they use only plankton, and in mature fish they use small fish. There are predatory fish that feed on only ectoparasites. They choose to hunt places of congestion "cleaners" and eat them from the bodies of parasitic fish.

Fish excretory system

Characteristics of the superclass Pisces can not be complete without a description of the excretory system of organs. Life in the water leads fish to a number of problems with osmoregulation. And these problems are typical for freshwater and marine fish equally. Cartilaginous fish are isoosmotic. The concentration of salt in their body is lower than in the environment. The osmotic pressure is leveled due to the high content of urea fish and trimethylamine oxide in the blood. Low concentration of salt cartilage class supports because of the rectal gland and removal of salts by the kidneys.

Bony fishes are not isosmotic. In the course of evolution, they were able to develop a mechanism that delayed or eliminated ions. Biology of the Chordovye type helps fish to bring salt out into the sea. This is because the fish lose water. Chloride ions and sodium ions excrete the gills, and magnesium and sulfate are the kidneys.

Freshwater fish have the opposite mechanism. The concentration of salt in the body of such creatures is higher than in the environment. Their osmotic pressure is equalized by the release of a large amount of urea and the capture of the desired ions from the water space by the gills.

Superclass Pisces: how is reproduction multiplying?

Fish have several types of reproduction. Let's consider each of them in more detail.

1. Bipolar reproduction is the most common form. In this case, the two sexes of the fish are clearly separated. Often this can be seen even by external signs (for example, color). Most often secondary mating characteristics are males. They can manifest themselves in the difference in the size of the body of the male and female, the difference in parts of the body (for example, the longer fin). Males in bisexual reproduction can be monogamous, polygamous, or lead chaotic messy connections (promiscuity).
2. Hermaphroditism - in such fish the sex during a life is capable to vary. Proto -andria in the beginning of life are males, then after reorganization the bodies become females. Protoginia is a form of hermaphroditism, when all males are transformed females.
3. Gynogenesis is a method of reproduction for species of fish, represented only by females. It is rare in nature.

Fish can reproduce with the help of live birth, egg production and egg production.

Class Bony fish

Superclass Pisces is divided into two classes: Cartilaginous and Bony fishes.

Bony fishes are the most numerous group of vertebrates. They number more than 19 thousand species. Their skeleton is bone. In some cases, the skeleton may be cartilaginous, but then it is additionally strengthened. Bony fish have a swim bladder. In this class there are more than 40 detachments. We will tell more about the most numerous.

• Order Sturgeon includes ancient bone fishes, such as sturgeon, beluga, sterlet. They are distinguished by the existing snout and mouth on the ventral side of the body. The mouth has the form of a transverse slit. The basis of the skeleton is cartilage. Only sturgeons live in the Northern Hemisphere.
• A detachment of Celery is a marine schooling fish that feed on plankton. Herring, Baltic herring, sardines, anchovies - commercial fish. They lay eggs on the ground or algae.
• Squad Salmon-shaped - freshwater fish that lay eggs on the bottom. They are found in the Northern Hemisphere. They are valuable commercial fish with tasty meat and caviar. The main representatives are salmon, chum salmon, pink salmon, trout, and trout.
• A detachment of Carpiformes is a freshwater fish without jaw teeth. Food they grind with pharyngeal teeth. The detachment includes commercial fish (roach, bream, tench, ide) and fish that are artificially bred in water bodies (carp, white cupid, silvery crucian carp).
• Squad-lungs are the oldest detachment. They can breathe gills and lungs (hollow outgrowths on the wall of the esophagus). They have adapted to life in hot countries and drying reservoirs. Bright representatives of the detachment are an Australian bastard and an American scaly cat.

Cartilaginous fish

The main difference between cartilaginous and bony fish lies in the structure of the skeleton, the absence or presence of gill covers and a swim bladder. The class of cartilaginous fish is represented by the inhabitants of the seas, which have a cartilaginous skeleton throughout life. Since there is no swimming bubble, representatives of this class are actively swimming, so as not to go to the bottom. As in sturgeon, the mouth looks like a transverse slit, there is a snout.

Cartilaginous fish include only two orders. These are Sharks and Skates. Sharks have a torpedo-shaped body, they are active swimmers and terrible predators. Their powerful jaws are strewn with sharp teeth. In this case, the largest sharks eat plankton.

Ramps have a flattened body with gills at the belly. The fins of the fish are greatly enlarged. Feed on skates with bottom animals and fish.

Use of fish resources and their protection

Fish is of great importance in human life, being one of the main food products. Every year around the world, about 60 million tons of fish are caught. Most of all they catch herring, cod and mackerel.

Recently, the catch of fish has decreased significantly. This is due to the deteriorating environmental situation in the world. Stocks are depleted due to excessive fishing, the destruction of certain species of fish, pollution of their spawning grounds, poisoning with salts of heavy metals. Gradually mankind passes from uncontrollable catching to cultivation of fish as a commercial object.

The best successes in the cultivation of fish are farms that are rooted in history. They exercise full control over the cultivation of products from the larva to the marketable produce. Fish are bred in artificial ponds for various purposes: feeding, overgrown, wintering and so on. There are also special ponds for spawning. They are always small in size and well warmed up.