Crustacea is a class of invertebrates of the Arthropoda type. It includes animals with body length from 1 mm to 80 cm. Floating, crawling and fixed attached forms. The body is covered with a chitinous shell and consists of three divisions of head, thorax and abdomen. In lower crustaceans, the legs are used for respiration, movement, and feeding food to the mouth. In the higher these functions are distributed among the individual limbs. The nervous system is represented by the brain and abdominal nerve chain. Breathe by means of gills or in the process of gas exchange involves the entire surface of the body. About 40000 species. Widespread, inhabit marine and fresh waters. Low crustaceans form the basis of zooplankton. Many are objects of fishing. Some crustaceans are parasites or serve as intermediate hosts of parasites.

The size and body shape of crustaceans vary widely. The smallest crustaceans are parasites and belong to the group of tantulocarids; their body length is 0.15-0.3 mm. The smallest arthropod, the parasitic crustacean Stygotantulus stocki, with a body length of less than 0.1 mm, belongs to the same group. Kamchatka crab (Paralithodes camtschatica) reaches a weight of 10 kg, the giant Tasmanian crab (Pseudocarcinus gigas) - up to 14 kg, and the Japanese spider crab (Macrocheira kaempferi) - up to 20 kg and 3.8 m in leg span. Sessile forms with calcareous shells, as well as parasitic crayfish, have a highly modified appearance.

Initially, the body of crustaceans includes 3 divisions: head, thoracic and abdominal. In some primitive species, the thoracic and abdominal parts are segmented almost homonymously (i.e., they consist of almost identical segments). The number of body segments varies greatly: from 5-8 to 50. It is currently believed that during the evolution of crustaceans, as well as other arthropods, there was a decrease in the number of segments. In higher crustaceans, the number of segments is constant: acron, four head segments, eight thoracic segments and six abdominal segments.

The body segments bear a pair of bipedal limbs. In a typical case, a crustacean limb consists of a basal part, the protopodite, bearing two branches: an external one, the exopodite, and an internal one, the endopodite. The protopodite includes two segments: the coxopodite, usually bearing the gill appendage, and the basipodite, to which the exopodite and endopodite are attached. The exopodite is often reduced, and the limb adopts a unibranched structure. Initially, crustacean limbs had several functions: locomotor, respiratory, and auxiliary for feeding, but the majority of crustaceans have morphofunctional differentiation of limbs.

The head consists of a head blade, the acron, and four segments. The head carries the appendages of the acron, the antennae first (antennules) and the limbs of the next four segments: the antennae second, mandibles, or chewing teeth (upper jaws) and two pairs of maxillae (lower jaws). Sometimes the first pair of mandibles are called maxillulae, and maxillae are sometimes called the second pair of mandibles. The antennules are usually unibranched and homologous to the palps of polychaetes. The exopodite of the second antenna is called scaphocerite. The antennae function as sensory organs, sometimes movement, the remaining head appendages are involved in grasping and pulverizing food. Mandibles play a major role in pulverizing food. In the larva, the nauplius, the mandible is a typical bilobed limb with a chewing process. Adults rarely have such a form of mandible, usually both branches are reduced, and the protopodite with a chewing process forms the upper jaw, to which the muscles are attached. 

Maxillae usually have the appearance of delicate leaf-shaped legs with chewing process on the protopodite and to some extent reduced branches. The head can be both fused (syncephalon) and subdivided into two articulated sections: protocephalon, which is formed by fusion of the acron and the first head segment and bears the first two pairs of antennae, and gnathocephalon, formed by fusion of the last three head segments and bearing mandibles and maxillae. The last variant takes place in the orders: gill-feeders, mysids, euphausiids, decapods, rotiferous. The mouth opening in front is covered by an unpaired fold of cuticle - the upper lip. Often in higher crayfish (such as the river crayfish) gnathocephalon fused with the thoracic section, forming a jaw-breast (gnathothorax), covered with a dorsal carapace - carapax. The body of higher crayfish is divided into the following sections: head - protocephalon (acron and one segment), jaw-breast - gnathothorax (three head and eight thoracic segments) and abdomen (6 segments and telson). In other cases, there is a fusion of the entire head, not subdivided into protocephalon and gnathocephalon, with one or more thoracic segments. Thus the cephalothorax is formed, followed by the abdomen. In some crustaceans (e.g., branchiopods), the head is elongated into a downward-pointing beak, the rostrum.

The thorax, like the abdomen, may have a different number of segments. Some crayfish, e.g., gills, have multifunctional abdominal limbs, while in others there is a separation of functions. For example, in the river crayfish, the first three pairs of thoracic legs are bipedal cephalopods used for holding and straining food, the next three are unipedal walking legs, and at the same time the first pair of them is grasping, with a claw at the end, but all thoracic legs at the base carry gills.

The abdomen consists of several segments and the telson; as a rule, it is devoid of limbs. Only in higher crayfish on the abdomen are located bipedal limbs, which perform different functions: in shrimp - swimming, in rotiferous crayfish - respiratory, in males of river crayfish the first two pairs are modified into copulatory organs, and in females reduced the first pair, the remaining abdominal legs are designed to carry young. In most decapods, the last pair of abdominal legs has a lamellar form (uropods) and together with the telson forms a five-lobed "fin". Crustaceans, deprived of abdominal limbs, usually have at the end of the body fork (furca), formed by articulate appendages of the telson. Simultaneously both fork and abdominal legs are present only in the crayfish Nebalia. In crabs, the abdominal section is reduced. In some parasitic crustaceans, the body limbs are significantly reduced or even completely disappear (Sacculina, female Dendrogaster).

Like other arthropods, crustaceans have a tough chitinous exoskeleton (cuticle). The cuticle consists of several layers, its peripheral layers are impregnated with lime, while the inner layers consist mainly of soft and elastic chitin. In small lower forms, the skeleton is soft and transparent. In addition, the chitinous cuticle contains a variety of pigments that give the animal a patronizing coloration. Pigments are also contained in the hypodermis. Some crustaceans are able to change coloration by changing the distribution of pigment grains in the cells (if the pigment is concentrated in the center of the cell, the coloration disappears, if the pigment is distributed evenly in the cell, the coloration will be manifested in the coverts). 

This process is regulated by neurohumoral factors. The function of the external skeleton is not limited to the protection of the animal, various muscles are also attached to the cuticle. Often for their attachment on the underside of the cuticle there are special outgrowths in the form of ridges and crossbars. Mobility of such parts of the crustacean body is provided by special soft membranes located between the fused parts of the body, segments or segments of limbs and appendages. Compacted areas of segments on the dorsal side are called tergites, and on the ventral side - sternites. Already mentioned above carapax is a special fold of coverings. It can be in the form of a shield, bivalve shell or semi-cylinder. Carapax can cover different parts: head, chest (river crayfish, shield) or the whole body (daphnia, barnacles), in higher crayfish its lateral parts cover the gills.

Extremely characteristic of crustaceans should be considered the way they breathe. Like all true aquatic animals, they breathe with gills, through the thin walls of which dissolved in water oxygen penetrates into the blood. Even terrestrial crustaceans-mocrites have gills and use oxygen wetting them a thin layer of moisture. Usually gills serve as gills outgrowths of the thoracic legs, but in equal-legged and rotonogogo crustaceans in the gills turned into abdominal limbs. In some crustaceans (paddlefish, Mystacocaridae, barnacles, barnacles) do not have gills and respiration is accomplished by the entire surface of their body.

The blood is driven by the beating of the heart, which is placed close to the gills. In crustaceans whose gills are located on the abdomen, the heart is located at the back, in others it is located in the thoracic region. Blood vessels are only in representatives of the subclass of higher crustaceans, in the rest of the blood flows through the cavities between the internal organs.

Crustaceans feed on a variety of foods. Some filter the suspended sediment in the water, including small organisms - bacteria, single-celled algae, etc., while others bite off pieces of dead and living animals or plants with their chewing jaws. The food is chewed by the chewing jaws and enters the mouth and from there into the esophagus. The rear part of the esophagus in most crustaceans is converted into a stomach, which sometimes has chitinous teeth that complete the grinding of food. Excrement is ejected through the anus, located at the posterior end of the body.

The organs of excretion are glands opening outward either at the base of the posterior maxillae (maxillary glands) or at the base of the posterior antennae (antennal glands). These glands correspond to the metanephridia of ringworms. Urine excreted by them in many freshwater crustaceans is very poor in salts. Animals remove water from the body cavity, and the concentration of salts in their cavity fluid is higher than in the environment.

Almost all crustaceans are separate-sexed, only barnacles and some decapods are hermaphrodites. Usually males differ sharply from females by their appearance or size. In some crustaceans, males are larger, in others smaller than females. Often males have some sort of adaptation to help them hold onto the female during mating. Mating consists of the male attaching a spermatophore, i.e. a bunch of spermatozoa covered with a shell, near the female's genital opening. When the female begins to lay eggs, a substance is released that dissolves the spermatophore shell; the sperm are released and fertilize the eggs. In other cases, sperm is introduced with the help of special copulatory organs directly into the genital tract of the female. Eggs of representatives of the subclass of gill crustaceans are able to develop parthenogenetically, ie without fertilization. There are species consisting of only females that give rise to a new generation of females.

It is relatively rare for eggs to be laid directly into the water. Usually the female carries them on her body in a special brood chamber or attaches them to the surface of the body or limbs. From the egg develops a nauplius larva characteristic of all crustaceans. The body of the nauplius is not externally segmented. It has three pairs of limbs - anterior and posterior antennae and mandibles. In some crustaceans nauplius comes out of the egg and begins an independent existence, in others this stage is formed inside the egg shell and the larva leaves the egg, reaching a later stage of development. Finally, there are crustaceans with direct development, in which all larval stages take place in the egg, and from it is born already formed a small crustacean. Crustacean larvae are often different from adults and often lead a completely different way of life. For example, the larvae of many bottom-dwelling crustaceans swim in the water column.