Pisces is the largest in species diversitygroup of aquatic chordates, which is also the oldest. Fish 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.
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.
The coloring of individuals of the Pisces superclass can be verydiverse. 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 allows the fishto move and change the 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 wide.
The spine consists of separate vertebrae, notfused with each other. It is divided into trunk and caudal divisions. 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 bone representatives of the supraclass, 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 possiblethanks 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.
The brain of fish is divided into departments. Each of them performs a certain function:
The spinal cord of representatives of the supra-class Pisces passes along the entire length of the spine.
Most representatives of the superclass have onecircle of blood 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 completely divide the oxygen-enriched arterial blood from the poor venous blood.
Heart chambers in fish go one after another andfilled 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.
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 of 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 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:
The fish grab and hold the food with their teeth, whichare located in the oral cavity (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:
Characteristics of the superclass Pisces can not becomplete 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 iso-osmotic. 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 the 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 exactly the oppositemechanism. 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.
Fish have several types of reproduction. Let's consider each of them in more detail.
Fish can reproduce with the help of live birth, egg production and egg production.
Superclass Pisces is divided into two classes: Cartilaginous and Bony fishes.
Bone fishes are the most numerous groupvertebrate animals. 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. Let's tell more about the most numerous.
The main difference between cartilaginous and bony fishes lies in thestructure of the skeleton, 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 their entire life. Since there is no swimming bubble, representatives of this class actively swim, so as not to go to the bottom. As in sturgeon, the mouth looks like a transverse slit, there is a snout.
Cartilaginous fishes 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.
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 uncontrolled fishing to the cultivation of fish as a commercial object.
The best successes in growing fish are farms,The roots are far back in history. They exercise full control over the cultivation of products from the larva to the commercial product. Fish are bred in artificial ponds for various purposes: feeding, overgrown, wintering and so on. There are also special ponds for spawning. They always have a small size and get warm well.