In the article we will consider the internal and external structure of the fish. In addition, we briefly describe the features of the evolutionary development and reproduction of these organisms. Pisces is perhaps the most successful group of vertebrates. It is not only more numerous than other groups, but also the richest in species: there are probably no less than 23,000. The external structure of the fish and the habits of these animals are extremely diverse. Some of them live in the sea, others in fresh water; some stay at great depths, others at the surface; some peacefully feed on algae, plankton or invertebrates, others are aggressive predators that attack not only their own kind, but also some land animals.
Fish breathing
All fish breathe, pumping water through thinly branched gills, the numerous folds of which form a huge surface that can absorb oxygen from the water and emit carbon dioxide. Water is pumped through the movements of the oral cavity and pharynx, and in bony species, the gill cover, and its perineal current is opposite to the movement of blood in the gills. As a result, when meeting with fresh water, the blood is enriched with oxygen and freed from carbon dioxide.
Fish body shape
Fish unusually vary in shape and size. This, of course, reflects the diversity of ways of their adaptation to the most diverse habitats that can only be found in sea and fresh waters. So, the body of fish living among algae can be covered with characteristic outgrowths or have a complex color. Both devices serve the purpose of camouflage, as well as the ability of some fish to change color in order to merge with the surrounding background. The inhabitants of turbid waters are characterized by the presence of a tactile antennae around the mouth, helping them in finding food. The body shape of the carnivorous fish is a smooth streamlined contour with a huge mouth with sharp teeth.
In contrast, flat flounders are slow and collect their prey at the bottom of the sea. We will tell you more about these unusual fish. All flounders are united in a squad of flounders. The body of these fish is literally curled to the side. The skull bones, for example, have shifted so that the eyes are on one side of the head. In adult fish, the sighted side becomes dorsal; it is highly pigmented, while the blind side remains white. Some species, moving from a rocky bottom to a sandy one and vice versa, change color accordingly.
The evolutionary development of fish forms
The external structure of the fish was developed during the course of many millions of years of evolution along the path of developing maximum speed and maneuverability in water. Many predatory bony species are excellent swimmers. They can move at a speed of three to six lengths of their body per second and turn around within a distance equal to only one body length. The evolutionary leap for bone fish was the development of their swimming bladder. Cartilaginous fish, whose skeleton consists only of cartilage, do not have a swim bladder and therefore drown if they stop swimming. The pectoral fins provide them with lift. Nevertheless, many cartilaginous fish switched to a near-bottom lifestyle. Due to the swim bladder, bone bones acquired constant buoyancy. The external structure of bone fish allows them to use pectoral fins as brakes or oars for backward movement. This increases their maneuverability and allows you to master a greater number of environmental niches. The locomotor capabilities of these animals are very diverse, and this is reflected in their muscle proteins. Fast swimmers, such as tuna or fish experiencing increased stress, such as migrating salmon, have mostly red muscles, and slowly swimming fish, like flounder, have white.
Fish fins
It is necessary to dwell on the fins in more detail, describing the external structure. Fish, as you know, use them for movement. However, not only for him. Fins of bone fish perform different functions. So, the dorsal and anal do not allow the fish to turn over. The pectoral fins often serve as brakes. Abdominal interferes with the raising of the front end of the body during slow immersion of fish. Paired fins allow these animals to float, dive and turn around. The external structure and movement of fish are closely related. An effective steering wheel is the tail fin. Shark fins serve as stabilizers when moving: the dorsal, anal and caudal interfere with course deviation. For sharks that do not have a swimming bladder, the functions of the caudal and pectoral fins, which provide sharks with the necessary buoyancy due to their lifting force, are much more significant.
Loss of protective cover
Finishing to describe the features of the external structure of fish, we note that the progressive feature of modern species should be considered their loss of heavy protective cover. For ferocious predators, cartilaginous fish, there is no need for such protection, and instead they developed strong, rough skin. In bone fish, the ancient carapace was replaced by a light cover from scales overlapping each other, protecting the animal, but not restricting its movements.
Internal structure of fish
Fish, the ancestors of vertebrates, naturally have a vertebral column - a support for providing muscle movement. The fins support hard beams consisting of bone, cartilage, or mutated scales. Bonefish is located above the intestines in bone fish. The abdominal aorta carries blood to the gill arteries. The gills are protected by a gill cover. The brain is well developed. Sharks have especially pronounced olfactory bulbs. Paired kidneys lie under the spine, the liver behind the heart. The intestine ends with an anus located in front of the urogenital pore.
Sensory organs of fish
To coordinate movements when hunting, avoiding danger or uniting (for collective protection), fish need well-developed sensory organs that reliably inform them about the environment. So, sharks to detect prey developed a very delicate sense of smell. Many fish have keen eyesight, and they clearly distinguish between yellow and green underwater.
Many species have good hearing, used for communicative purposes to perceive the signals of a mating partner or pack. The hearing organ is part of the labyrinth of the inner ear of these animals - an extremely important organ that evaluates the position of the body in space and angular acceleration, which is necessary to maintain balance during swimming. The fish of the sea and river have another unique organ - the lateral line, which works like an ear of vertebrates, but does not perceive sound waves, but various turbulences in the water caused by movement.
Side line
This organ serves as a kind of distant tactile receptor. The lateral line stretches from head to tail on both sides of the body of the fish. It is a channel filled with liquid, which is connected to the external environment through penetrating scales. Behind each hole is a sensitive organ - neuromast. It consists of a gelatinous mass - a cupule - and a group of sensitive hair cells whose nerve fibers are sent to the brain.
Fish brain
Information from the sensory organs enters one or another part of the brain, among which there are those that are responsible for the automatic start of respiratory and cardiac rhythms. The organs of hearing and balance are connected by the nerves to the hindbrain, and the large olfactory bulbs, the organs of smell, or chemoreception, are connected with the hemispheres of the forebrain. Chemoreception is believed to play an important role in navigation, nutrition, and mating. The most perfect part of the brain of fish that controls their behavior is the visual, associated with the eyes. The cerebellum coordinates sensory information and movement.
So, we have described the external and internal structure of fish. In conclusion, let us say a few words about their reproduction.
How fish breed
These animals breed in different ways. Some have internal fertilization; in others, the external. In addition, among the fish there are hermaphrodites. But whatever the method of reproduction, the fish are very prolific. So, cod (pictured above) at a time sweeps up to 8 million eggs, although the majority - only tens of thousands. Juveniles, usually of microscopic size, at first enter into plankton. The bulk of it dies before it reaches maturity, however, a significant portion of individuals survive. Scientists estimate, for example, that there are about 10 12 herrings in the Atlantic. Sea fish are an important source of protein for humans.