Gills are the outgrowths of the body of animals that are designed to breathe in water. Most often they are branched filaments, equipped with a network of blood vessels and devoid of muscle.
What animals have gills
With the help of gills, most aquatic inhabitants get oxygen from water: fish, many aquatic invertebrates (for example, polychaete worm, pearl barley mollusk, branchipus crustacean, mayfly larva) and some larval amphibians (for example, tadpoles).
In cyclostomes (predators or fish parasites), respiration is carried out through the gill sacs.
The annelids have primitive gills. In most higher crustaceans, these respiratory organs are located on the side walls of the body and the upper parts of the thoracic legs. Aquatic insect larvae have tracheal gills, which are thin-walled outgrowths on different parts of the body, in which there is a tracheal network.
Of the echinoderms, the gills have starfish and sea urchins. All primary-water chordates (fish) have rows of paired openings (gill slits) located in the pharynx. In intestinal breathers (motile benthic animals), tunicates (small marine animals with a saccular body covered with a membrane) and skullless (a special group of invertebrates), gas exchange occurs during the passage of water through the gill slits.
How animals breathe with gills
The gills are made of leaves (filaments), inside them there is a network of blood vessels. The blood in them is separated from the external environment by very thin skin, thus creating the necessary conditions for the exchange between gases dissolved in water and blood. The gill slits in fish are separated by arcs from which the gill septa extend. In some bony and cartilaginous species, the gill petals are located on the outer side of the arches in two rows. Actively swimming fish have gills with a much larger surface than sedentary aquatic animals.
In many invertebrates, young tadpoles, these respiratory organs are located on the outside of the body. In fish and higher crustaceans, they are hidden under protective devices. Often the gills are located in special body cavities, they can be covered with special folds of skin or leathery lids (gill caps) to protect them from damage.
The gills also function as the circulatory system.
The movement of the operculum during breathing is carried out simultaneously with the movement (opening and closing) of the mouth. When breathing, the fish opens its mouth, draws in water and closes its mouth. Water acts on the respiratory organs, passes through them and goes out. Oxygen is absorbed by the capillaries of the blood vessels located in the gills, and the used carbon dioxide is released through them into the water.
