Due to the fact that every creature is endowed with respiratory organs, we all get what we cannot live without - oxygen. In all terrestrial animals and humans, these organs are called the lungs, which absorb the maximum amount of oxygen from the air. The respiratory system of fish consists of gills, which draw oxygen into the body from the water, where it is much less than in the air. It is precisely because of this that the body structure of a given biological species is so different from all spinal terrestrial creatures. Well, consider all the structural features of fish, their respiratory system and other vital organs.
Briefly about fish
First, let’s try to figure out what kind of creatures these are, how and how they live, what kind of relationship they have with humans. Because now we are starting our biology lesson, the topic "Sea fish". This is a superclass of vertebrate animals that live exclusively in the aquatic environment. A characteristic feature is that all fish are maxillary, and also have gills. It should be noted that these indicators are characteristic for each type of fish, regardless of size and weight. In human life, this subclass plays an economically important role, since most of its representatives are eaten.
It is also believed that fish were at the dawn of evolution. It is such creatures that could dwell under water, but did not yet have jaws, were once the only inhabitants of the Earth. Since then, the species has evolved, some of them have turned into animals, some have remained under water. That's the whole biology lesson. The topic "Sea fish. A brief excursion into history" is considered. The science of marine fish is called ichthyology. Let us now turn to the study of these creatures from a more professional point of view.
General structure of fish
In general, we can say that the body of each fish is divided into three parts - the head, trunk and tail. The head ends in the region of the gills (at their beginning or end - it depends on the superclass). The body ends at the anus line in all representatives of this class of marine inhabitants. The tail is the simplest part of the body, which consists of a rod and a fin.
The shape of the body strictly depends on the living conditions. Fish that live in the middle water column (salmon, shark) have a torpedo shape, less often a swept shape. The same marine inhabitants that float above the bottom have a flattened shape. These include flounder, sea foxes and other fish that are forced to swim among plants or stones. They acquire more maneuverable outlines that have much in common with snakes. For example, eel is the owner of a very elongated body.
Business card fish - its fins
It is impossible to imagine the structure of fish without fins. Pictures, which are presented even in children's books, certainly show us this part of the body of marine inhabitants. What are they?
So, fins are paired and unpaired. Paired and abdominal, which are symmetrical and synchronously move, can be attributed to paired. Unpaired are presented in the form of a tail, dorsal fins (from one to three), as well as anal and fat, which is located immediately behind the dorsal. The fins themselves are made up of hard and soft rays. It is based on the number of these rays that the fin formula is calculated, which is used to determine the specific type of fish. The location of the fin is determined in Latin letters (A - anal, P - pectoral, V - abdominal). Next, the Roman numerals indicate the number of hard rays, and Arabic - soft.
Fish classification
Today, conditionally all fish can be divided into two categories - cartilage and bone. The first group includes such inhabitants of the sea, the skeleton of which consists of cartilages of various sizes. This does not mean at all that such a creature is soft and incapable of movement. In many representatives of the superclass, cartilage hardens, and in its density become almost like bones. The second category is bone fish. Biology as a science claims that this superclass was the starting point of evolution. Once in its framework was a long-extinct cysterae fish, from which all terrestrial mammals may have evolved. Next, we will examine in more detail the structure of the fish body of each of these species.
Cartilage
In principle, the structure of cartilaginous fish is not something complex and unusual. This is an ordinary skeleton, which consists of very hard and durable cartilage. Each compound is saturated with calcium salts, due to which strength appears in the cartilage. The chord keeps its form throughout life, while it is partially reduced. The skull is connected to the jaws, as a result of which the skeleton of the fish has a holistic structure. Fins are also attached to it - caudal, paired abdominal and pectoral. The jaws are located on the ventral side of the skeleton, and above them are two nostrils. The cartilaginous skeleton and muscular corset of such fish is covered on the outside with dense scales called placoid. It consists of dentin, which in its composition is similar to ordinary teeth in all terrestrial mammals.
How cartilage breathes
The respiratory system of cartilaginous superclasses is represented primarily by gill slits. They number from 5 to 7 pairs on the body. Oxygen is distributed into the internal organs thanks to a spiral valve that stretches along the entire fish organism. A characteristic feature of all cartilage is that they lack a swimming bladder. That is why they are forced to constantly be in motion, so as not to go to the bottom. It is also important to note that the body of cartilaginous fish, which a priori live in salt waters, contains a minimal amount of this very salt. Scientists believe that this is due to the fact that in the blood of this superclass there is a lot of urea, which consists mainly of nitrogen.
Bone
Now we will look at what the skeleton of a fish belonging to the superclass of bone looks like, and also find out what else are the representatives of this category characteristic of.
So, the skeleton is presented in the form of a head, trunk (they exist separately, in contrast to the previous case), as well as paired and unpaired limbs. The cranial box is divided into two departments - cerebral and visceral. The second includes the jaw and hyoid arches, which are the main components of the jaw apparatus. Also in the skeleton of bone fish there are gill arches, which are designed to hold the gill apparatus. As for the muscles of this fish species, they all have a segmental structure, and the most developed of them are the jaw, fin and branchial.
Respiratory apparatus of bone inhabitants of the sea
Probably, it has already become clear to everyone that the respiratory system of the superclass of bony fish mainly consists of gills. They are located on the branchial arches. Gill slits are also an integral part of such fish. They are covered by the cover of the same name, which is designed so that the fish can breathe even in an immobilized state (unlike cartilage). Some members of the bone superclass can breathe through the skin. But those who live directly below the surface of the water, and at the same time never deeply sink, on the contrary, capture the air with their gills from the atmosphere, and not from the aquatic environment.
The structure of the gills
Gills - a unique organ that was previously inherent in all the primitive water creatures that lived on Earth. In it there is a process of gas exchange between the hydraulic medium and the body in which they function. The gills of fish of our time are not much different from those of the gills that were inherent in the earlier inhabitants of our planet.
As a rule, they are presented in the form of two identical plates, which are penetrated by a very dense network of blood vessels. An integral part of the gills is coelomic fluid. It is she who performs the gas exchange process between the aquatic environment and the fish organism. Note that this description of the respiratory system is inherent not only to fish, but to many vertebrate and non-vertebral inhabitants of the seas and oceans. But the fact that it is precisely those respiratory organs that are in the body of the fish that are special in themselves, read on.
Where are the gills
The respiratory system of fish is mostly concentrated in the throat. It is there that the branchial arches are located , on which the gas exchange organs of the same name are fixed. They are presented in the form of petals, which allow air and various vital fluids that are inside each fish to pass through. In certain places, the pharynx is pierced by gill slits. It is through them that oxygen passes that enters the mouth of the fish with the water it swallows.
A very important fact is that, in comparison with the size of the body of many marine inhabitants, their gills are very large for them. In this regard, problems with the osmolarity of blood plasma arise in their organisms. Because of this, fish always drink sea water and release it through the gill slits, thereby accelerating various metabolic processes. It has a lower consistency than blood, therefore it supplies the gills and other internal organs with oxygen faster and more efficiently.
Breathing process
When a fish is just born, almost its entire body breathes. Each organ, including the outer shell, is penetrated by blood vessels, because oxygen, which is in sea water, penetrates the body constantly. Over time, each of these individuals begins to develop gill respiration, since it is the gills and all organs adjacent to them that are equipped with the largest network of blood vessels. And then the fun begins. The breathing process of each fish depends on its anatomical features, because in ichthyology it is customary to divide it into two categories - active breathing and passive. If everything is clear with the active (the fish breathes “normally”, collecting oxygen into the gills and treating it like a human), then we will try to deal with the passive one in more detail.
Passive breathing and what it depends on
This type of breathing is peculiar only to fleet inhabitants of the seas and oceans. As we said above, sharks, as well as some other representatives of the cartilaginous superclass, cannot be without movement for a long time, since they lack a swimming bladder. There is one more reason for this, namely, this is passive breathing. When the fish swims at high speed, it opens its mouth, and water automatically gets there. Approaching the trachea and gills, oxygen is separated from the liquid, which nourishes the organism of the marine fast-moving inhabitant. That is why for a long time without movement the fish deprives itself of the opportunity to breathe without spending any effort and energy on it. In conclusion, we note that sharks and all representatives of mackerel are mainly among such fast-moving inhabitants of salt waters.
The main muscle of the fish
Very simple is the structure of the heart of the fish, which, we note, in the entire history of the existence of this class of animals, practically did not evolve. So, this organ is two-chamber. It is represented by one main pump, which includes two chambers - the atrium and the ventricle. Fish heart pumps only venous blood. In principle, the circulatory system in this species of marine life has a closed system. Blood circulates through all the capillaries of the gills, then merges into the vessels, and from there it again diverges into smaller capillaries, which already supply the remaining internal organs. After that, the "spent" blood is collected in the veins (there are two of them in the fish - the hepatic and cardiac), from where it goes directly to the heart.
Conclusion
So our short lesson in biology has come to an end. The theme of fish, as it turned out, is very interesting, fascinating and simple. The organism of these sea inhabitants is extremely important for study, since it is believed that they were the first inhabitants of our planet, each of them is the key to the solution of evolution. In addition, studying the structure and functioning of the fish organism is much easier than any other. And the sizes of these inhabitants of water stochia are quite acceptable for detailed consideration, and at the same time, all systems and formations are simple and accessible even for school-age children.