External structure of fish. External and internal structure of fish. Features of the external structure of fish

In the article we will consider the internal and external structure of the fish. In addition, we briefly characterize the features of the evolutionary development and reproduction of these organisms. Pisces, perhaps, is the most successful group of vertebrates. It is not only more numerous than other groups, but also the most rich in species: there are probably at least 23,000 of them. The external structure of the fish and the habits of these animals are extremely diverse. Some of them live in the sea, others live in fresh water; Some are kept at great depths, others are at the surface; Some peacefully feed on algae, plankton or invertebrates, others - aggressive predators, attacking not only themselves, but also in some land animals.

Breath of fish

All fish breathe by pumping water through finely branched gills, whose numerous folds form a huge surface that can absorb oxygen from the water and release carbon dioxide. Water is injected by the movements of the mouth and pharynx, and in the bone species - the gill cover, and the paracircular current is opposite to the movement of blood in the gills. As a result, meeting with fresh water, the blood is enriched with oxygen and released from carbon dioxide.

Fish body shape

Fish vary enormously in shape and size. This, of course, reflects the diversity of ways to adapt them to the most diverse habitats that can be found in marine and fresh waters. So, the body of fish living among algae can be covered with characteristic outgrowths or have a complexly dissected color. Both adaptations serve the purposes of disguise, as does the ability of some fishes to change color to merge with the surrounding background. For the inhabitants of turbid waters is characterized by the presence of tactile tendrils around the mouth, helping them in finding food. The shape of the fish body is predatory - a smoothed streamlined contour with a huge mouth with sharp teeth.

In contrast, flat flounder are slow and collect their prey on the seabed. We will tell you more about these unusual fish. All flatfish are united in a detachment of flounder. The body of these fish is literally collapsed. The bones of the skull, for example, have shifted so that the eyes are on one side of the head. In adult fish, the seeing side becomes dorsal; It is strongly pigmented, while the blind side remains white. Some species, moving from the rocky bottom to sandy and vice versa, respectively, change color.

Evolutionary development of fish forms

The external structure of the fish was developed during many millions of years of evolution along the path of development of maximum speed and maneuverability in water. Many carnivorous bone species are excellent swimmers. They can travel at a speed of three to six lengths of their body per second and unfold within a distance equal to just one body length. An evolutionary leap for bone fishes was the development of a swim bladder in them. Cartilaginous fish, whose skeleton consists only of cartilage, do not have a swim bladder and therefore drown if they stop swimming. Lifting force is provided by the pectoral fins. And yet many cartilaginous fishes have shifted to a benthonic lifestyle. Bony ones, thanks to the swim bladder, acquired a constant buoyancy. The external structure of the bony fishes allows them to use the pectoral fins as brakes or oars to move backwards. This increases their maneuverability and allows them to master a greater number of ecological niches. Locomotor capabilities of these animals are very diverse, and this is reflected in their muscle proteins. In swimmers, like tuna or fish, experiencing increased stress, such as migrating salmon, the muscles are predominantly red, and in slow-moving fish, like flounder, white.

Fins of fish

It is necessary to elaborate on the fins, describing the external structure. Fish, as you know, use them for movement. But not only for him. Fins of bony fishes perform different functions. So, the dorsal and the anal do not allow the inverting of the fish. Pectoral fins often serve as brakes. Ventrals prevent elevation of the anterior end of the body when the fish slowly immerse. Paired fins allow these animals to float, dive and unfold. The external structure and movement of fish are closely related. An effective steering wheel is the tail fin. Shark fins serve as stabilizers for movement: dorsal, anal and caudal prevent a deviation from the course. For sharks that do not have a swim bladder, much more important is the function of the caudal and pectoral fins that provide the sharks with the necessary buoyancy due to their lifting force.

Loss of protective covers

Concluding the description of the external structure of fish, we note that the progressive feature of modern species should be considered the loss of heavy protective coatings. For ferocious predators, cartilaginous fishes, there is no need for such protection, and instead they developed a durable rough skin. In bone fishes, the ancient shell was replaced by a light covering of overlapping scales protecting the animal, but not constraining its movements.

Internal structure of fish

Fish, the ancestors of vertebrates, naturally have a vertebral column - a support for the muscles providing motion. Fins support hard rays, consisting of bone, cartilage or modified scales. Above the intestines of the bone fish is a swim bladder. The abdominal aorta carries blood to the gill arteries. Gills are protected with a gill cover. The brain is well developed. In sharks, olfactory bulbs are especially pronounced. Paired kidneys lie under the spine, the liver behind the heart. The intestine ends with the anal opening, located before the genitourinary time.

Organs of the senses of fish

To coordinate movements during hunting, avoiding danger or uniting in a pack (for the purpose of collective protection), fish need well developed sense organs that reliably inform them of the environment. Thus, the sharks developed a very subtle sense of smell to detect the prey. Many fish have sharp eyesight, and they can clearly distinguish underwater yellow and green colors.

Many species have a good hearing, used for communicative purposes to perceive signals from a mate or flock. The hearing organ is part of the labyrinth of the inner ear of these animals - an extremely important organ that estimates the position of the body in space and angular acceleration, which is necessary to maintain equilibrium during swimming. Fish of the sea and the river have one more unique organ - a lateral line that works like the ear of vertebrates, but does not perceive sound waves, but various swirls in the water caused by movement.

Lateral line

This organ serves as a kind of distant tactile receptor. The lateral line extends from head to tail on both sides of the body of the fish. It is a fluid-filled channel that is connected to the external medium by piercing holes. Behind each hole is a sensitive organ - a neuromast. It consists of a gelatinous mass - the cupula - and a group of sensitive hair cells, the nerve fibers of which are sent to the brain.

The Brain of Fish

Information from the senses enters the one or another part of the brain, among which there are those who are aware of the automatic triggering of respiratory and cardiac rhythms. The organs of hearing and balance are connected by nerves to the hindbrain, and large olfactory bulbs, olfactory organs, or chemoreceptions, with the hemispheres of the forebrain. It is believed that chemoreception plays an important role in navigation, nutrition and mating. The most perfect department of the fish brain, controlling their behavior, is the eye, connected with the eyes. The cerebellum coordinates the sensory information and movement.

So, we described the external and internal structure of the fish. In conclusion, say a few words about their reproduction.

How do fish multiply

These animals multiply in many ways. Some have internal insemination; In others it is external. In addition, among the fish there are hermaphrodites. But whatever the method of reproduction, the fish are very prolific. So, the cod (in the photo above) at one time sweeps up to 8 million eggs, although most - only tens of thousands. Molody, usually of microscopic size, is included in plankton for the first time. The bulk of it dies before it reaches maturity, but a significant portion of the individuals survive. Scientists believe, for example, that there are about 10 ¹² herrings in the Atlantic. Fish of the sea serve as an important source of protein for humans.