Crustaceans (Ass. F. D. MORDUCHAI-BOLTOVSKAYA)

Lower crustaceans (Entomostraca)

Lower crustaceans have a variable number of body segments and usually an unclear - delimited abdomen, which never bears limbs. In fresh and generally inland waters of the Rostov region. lower crustaceans are represented by four orders: branchiopods (Branchiopoda), cladocera (Cladocera), copepods (Copepoda) and shellfish (Ostracoda). These are in most cases small, sometimes microscopic animals that live exclusively in water.

1. Branchiopoda- these are relatively large crustaceans with a clearly dissected body with a large number of leaf-shaped swimming legs equipped with gill appendages (from 10 to 40). They inhabit very small temporary reservoirs and puddles, which usually dry up in the summer. In the reservoirs of the river floodplain. Don, formed during the spring flood, you can often find the most interesting representative of these crustaceans - the shieldfish - Lepidurus apus. This is an extremely peculiar looking animal, up to 4-5 cm, covered on the dorsal side with a greenish armor covering the entire body, with the exception of the posterior part of the abdomen, equipped with two long tail filaments (Fig. 1). Along with Lepidurus, there is also Rpus, which is very close to it, differing from the first in the absence of a plate between the caudal filaments.

Most of the reservoirs in which these crayfish live are completely dry by mid-summer. However, next spring, scale insects appear in them again, as they lay so-called “resting” or “winter” eggs, which are not only equipped with a dense shell that allows them to withstand drying and freezing of the reservoir without harm, but even, apparently, require complete drying for further development.

In the same temporary reservoirs, other representatives of the described order are also found, devoid of armor - branchiopods. Branchiopods have an elongated body with a thin tail (abdomen) and 10-20 pairs of long legs bearing gills; the head is separate from the body and is equipped with stalked eyes and large curved antennae (“antennae”). Of the branchiopods, Branchinella spinosa was found in the reservoirs of the Don floodplain. In the salt lakes of the Mana-Chey basin, another branchiopod is common - brine shrimp (flrtemia salina v. principalis, Fig. 2). Artemia is a well-known inhabitant of salt waters, remarkable in that it cannot exist in fresh waters, but thrives in salt waters even at a salt concentration at which all other animals die. In this case, Artemia can develop in huge quantities. In some salty reservoirs of the Manych Valley, the entire mass of water, devoid of any animals, is filled with the floating remains of the leaf-shaped legs of Artemia.

In addition to shieldfishes and branchiopods, among the branchiopods there is also a group of forms equipped with a bivalve shell, similar to mollusk shells, but usually very thin and transparent. In floodplain lakes and swamp-like reservoirs you can often find these small ones (rarely more than 1a/a cm) crustaceans that swim quickly with the help of numerous (10-30 pairs) legs.

In the Rostov region. The species Leptestheria, Caenestheria, and Cyzicus were found from this group.

2. Cladocera or Cladocera- the overwhelming majority are very small animals, having an almost unarticulated body with a small number of swimming legs (no more than 6). The body is covered with a transparent, thin shell and in front bears a pair of branched antennae - antennae, which serve for movement, which occurs spasmodically. The head is usually equipped with one big eye, often of a rather complex structure. Cladocera inhabit absolutely all fresh water bodies and are one of the most widespread groups of crustaceans. The extremely wide distribution of Cladocera is largely due to the presence of “winter” or “resting” eggs, which, due to their insignificant size, can be transported over long distances by the wind along with dust. Cladocera reproduces several and sometimes many times during the year, and it is remarkable that it can for a long time move without the participation of males (parthenogenetically), but in this case only ordinary “summer” eggs are formed; with the deterioration of living conditions, males appear, fertilize females, who then lay “winter” eggs.

Cladocera are one of the main components of the plankton of fresh water bodies, and also inhabit the coastal zone and thickets in large numbers. They are an important, and sometimes the main food item for various commercial and non-commercial “plank-eating” fish (herring, sprat, bleak, etc.) and juveniles of most fish that feed on bottom fauna as adults. When dried, Cladocera serves as a universal food for aquarium fish. This food is called Daphnia, although in reality Daphnia is only one of the very numerous forms of Cladocera.

In reservoirs of the Rostov region. Cladocera are represented as richly and diversely as in all water bodies of temperate and southern latitudes(at least 40 species of them have been found in the Don basin). Among the planktonic forms often found in the Don River, the above-mentioned daphnia (Daphnia longispina) can be mentioned. This is a transparent crustacean 1-2 long mm, the shell of which is equipped with a long needle, and the head bears a pointed helmet (Fig. 3). Even more common than Daphnia are its close relatives - Moina and Diaphanosoma, distinguished by the absence of a helmet and a needle. Of the other Cladocera of the Don plankton, the most numerous are Bosmina longiros tris, very small (no more than 1/2 mm) a rounded crustacean with a long beak, and Chydorus sphaericus, also completely round, but without a beak. In the thickets of the coastal strip and near the bottom there live many other related cladocerans from the family Chydoridae.

In the salty reservoirs of Manychi, most Cladocera, generally adapted to fresh water, cannot exist. Only the most resistant to salinity, Moina and Diaphanosoma, remain, but they reproduce in large quantities.

Among Cladocera, Leptodora kindtii, which lives in the plankton of the Don and generally large reservoirs, stands out. It is comparatively very large - about 1 cm- a crustacean whose elongated body is almost free of a shell (covering only the “brood pouch” with eggs) (Fig. 4). Leptodora, in contrast to most other Cladocera, leads predatory image life and is characterized by extraordinary transparency. When alive, it is almost impossible to distinguish it in water, and only when killed with formaldehyde or alcohol does it turn white and become clearly visible.

Free-living copepods (Euco-pepoda) have a clearly dissected body, subdivided into a wide cephalothorax, equipped with 4 pairs of two-branched swimming legs and a narrow abdomen ending in a two-branched fork with setae ("furka"). The cephalothorax bears one small ocellus in front and a pair of very long antennae used for swimming.

Like Cladocera, all copepods are very small, often semi-microscopic forms, extremely widespread in all kinds of water bodies. They also form resting eggs and are part of plankton, representing an important food source for fish fry and adult planktivorous fish.

The lifestyle of copepods is similar to the lifestyle of cladocerans; It should be noted, however, that in contrast to Cladocera, which reproduce only after the water has completely warmed up and quickly disappear with cold weather, copepods are much more tolerant of low temperatures and appear in masses in very early spring, and many live throughout the winter, under the ice.

The most common copepods are the Cyclops, which belong to the genus Cyclops (this genus is now divided into several others). Cyclops have an oval cephalothorax, an elongated abdomen with long tail setae, and relatively short swimming antennae. Females carry eggs in two egg sacs on the sides of the abdomen (Fig. 5). Cyclops are small crustaceans (no more than 2-3 mm in length), found in all water bodies, with the exception of heavily polluted ones, and usually leading a planktonic lifestyle. Among the numerous species of this genus (at least 20 species of Cyclops are known for the Rostov region), Cyclops strenuus, C. vernalis, and C. oithonoides are most often found in the plankton of the Don.

Along with cyclops, especially in shallow spring reservoirs, representatives of the genus Diaptomus are often found, distinguished by their slightly larger sizes (up to 5 mm), longer antennae and cephalothorax and short abdomen. Many of them are red or blue in color. Among the numerous (about 15 found in the Rostov region) species of Diaptomus, interesting are D. salinus and D. (Paradiaptomus) asiatlcus, which develop in large quantities in the salty reservoirs of Manychi. Other copepods (Heterocope, Calanipeda, Eurytemora) are also found in the plankton of the Don.

Copepods belonging to the group Harpacticidae live in the coastal zone and at the bottom of reservoirs. These are extremely small crustaceans with a long body and poorly developed swimming antennae, running along the bottom and, due to their scarcity and small size, usually eluding observation.

A significant part of the plankton of most water bodies is played by peculiar copepod larvae - nauplii. These are very microscopic animals with three pairs of legs and one red eye, often, especially in the spring, inhabiting water in countless numbers. All copepods in their development pass through this larval stage, which after a few weeks turns into an adult form through a series of successive molts.

Very close to copepods (but now distinguished among special squad branchiura) also “fish or carp lice” (flrgulus). These are small (no more than 1/2 cm) crustaceans with a flat body, two compound eyes and two suckers with which they attach to the skin of fish. They suck blood from fish, but often separate from their prey and swim freely in the water for a while. One of the species of this genus, Argulus foliaceus, is often found in the Don.

4. Barnacle crustaceans (Ostracoda). Shell crustaceans are small crustaceans that live in oval bivalve shells. The presence of a shell brings them closer together, but shell shells differ from the latter only in their smaller sizes (usually no more than 5-7 mm) and an undifferentiated body with only three pairs of legs, used not for swimming, but for running (Fig. 7). In addition, their lime-impregnated shells are usually very durable and preserved in fossil form, making Ostracoda important in paleontology.

Most barnacle crustaceans live among thickets and at the bottom of various bodies of water. Although they do not have special “winter” eggs, their eggs, and often the adult crustaceans themselves, are also able to withstand drying out and freezing without harm.

In fresh water bodies they usually do not reproduce in large numbers and can easily go unnoticed by the untrained eye.

In the Rostov region. barnacle crustaceans have hardly been studied. Only a few widespread species inhabiting small floodplain lakes and puddles can be noted: Candona, one of the largest forms with a white shell; Cyclocypris, smaller, rounded; Limnicythere - with an elongated shell equipped with several large swellings.

1) breathing using gills;

2) fusion of the head and thoracic regions to form the cephalothorax;

3) the presence of two pairs of antennae that perform tactile and olfactory functions, a pair of compound, or facet, eyes, and three pairs of oral limbs (a pair of upper and two pairs of lower jaws that capture and grind food);

4) the varied structure of the thoracic limbs, which perform the functions of holding and moving food to the mouth, body movement, and breathing;

5) the abdominal limbs are used for swimming, and in females, for attaching fertilized eggs;

6) crustaceans of all age groups molt, but juveniles molt more often than adults.

Features of the structure and life processes. Crayfish is a characteristic representative of the class Crustaceans. Lives in fresh, low-flowing water bodies. Active during twilight and night time. Crayfish are omnivores: they eat plant foods, living and dead prey. Reaching significant sizes (15 cm or more) and having good taste qualities, crayfish is a valuable commercial object.

The body of the crayfish consists of 18 segments, combined into the cephalothorax and abdomen. It is covered with a thick layer of chitinous cuticle, strengthened by lime deposits. The uppermost wax-like layer of the cuticle, which prevents the evaporation of water from the body in terrestrial arthropods, is absent in crustaceans, which explains their existence exclusively in an aquatic or near-aquatic environment.

The head consists of a head lobe bearing a pair of antennae - antennules (first antennae), and four segments, each of which has paired transformed limbs: antennae (second antennae), upper jaws and first and second lower jaws. The thoracic region is formed by eight segments bearing three pairs of jaws and five pairs of walking limbs. The segmented, movable abdomen has six segments, each of which contains a pair of swimming limbs. In males, the first and second pair of abdominal limbs are long, groove-shaped and used as a copulatory organ. The female's first pair of limbs is greatly shortened. The abdomen ends with a caudal fin formed by a sixth pair of wide lamellar limbs and a caudal blade.

The gills of crayfish are thin-walled feathery outgrowths of the skin of the thoracic limbs and the side walls of the thoracic part of the body. They are located on the sides of the chest in the gill cavity, covered by the cephalothoracic carapace. The circulation of water in the gill cavity is ensured by the movement of a special process of the second pair of lower jaws (200 times per minute).

Digestive system begins with a mouth opening located on the underside of the head. Through it, food crushed by the oral limbs passes through the short pharynx and esophagus into the stomach, which consists of two sections - chewing and filtering. On the inner walls of the chewing section of the stomach there are chitinous teeth, with the help of which food is ground. The food gruel is filtered through the bristles of the straining section, and its liquid part enters the midgut and digestive gland (“liver”), where it is digested and absorbed. The hindgut, in the form of a straight tube, is located in the abdomen of the crayfish and opens with the anus at its end.

Circulatory system typical of all arthropods - unclosed with a compact heart in the form of a pentagonal sac on the dorsal side of the cephalothorax.

Metabolic products are removed through excretory organs - paired green glands that lie at the base of the head and open outward at the base of the antennae. In their structure, the glands resemble modified metanephridia, which remove metabolic products from the body cavity.

Cancer's eyes are complex. They consist of a large number of individual eyes, or facets, separated from each other by thin layers of pigment. Vision is mosaic, since each facet sees only part of the object. The eyes are located on movable stalks. The mobility of the eye compensates for the immobility of the head. The organs of touch are long mustache- antennae, and the olfactory organs - short whiskers - antennules. At the base of the short mustache is the organ of balance.

At the end of winter, females lay fertilized eggs on their abdominal limbs. At the beginning of summer, the eggs hatch into crayfish, which remain under the protection of the female for a long time, hiding on her abdomen on the underside. Young crayfish grow rapidly and molt several times a year; adults molt only once a year. The cancer then produces soft chitin. After some time, it becomes saturated with lime, hardens, and the growth of the cancer stops until the next molt.

The role of crustaceans in nature and their practical significance. Crustaceans are of great importance in nature and human economy. Countless crustaceans inhabiting sea and fresh waters serve as food for many species of fish, cetaceans and other animals. Daphnia, cyclops, diaptomus, bokogshava are excellent food for freshwater fish and their game. Many small crustaceans feed by filtration, that is, they strain out the food suspension with their thoracic limbs. Thanks to their nutritional activity, natural water is clarified and its quality is improved.

Many large crustaceans are commercial species, for example lobsters, crabs, lobsters, shrimps, crayfish. Medium-sized marine crustaceans are used by humans to prepare nutritious protein paste.

The most primitive crustaceans belong to the subclass Branchiopods. Daphnia are representatives of the order Listopods, suborder Cladocera. Quite often, Daphnia, inhabitants of the water column, are called water fleas, due to their spasmodic method and movement of small sizes. The body of the crustaceans reaches up to 6 mm in length, with a bivalve shell on top, flattened on the sides. On the head of the crustacean there is a large black spot- eye, in the trunk region the brownish-greenish intestine clogged with food is visible. Daphnias do not remain quiet for a minute. Swings of the long side antennas perform main role in move. Daphnia's legs are small, leaf-shaped, and do not take any part in movement, but they regularly serve for breathing and feeding. The legs are constantly working, making up to 500 strokes per minute. In a similar way, they create a current of water that carries bacteria, algae, yeast and oxygen.- "copepods". Diaptomuses are also quite peaceful animals. Diaptomus hover smoothly, balancing with outstretched antennae, the length of which is almost equal to the entire length of the body. Having dropped down, Diaptomus makes a sharp stroke with its legs and small abdomen and “jumps” up. The elongated body of the crustacean is colorless and translucent; they need to remain invisible to predators. Females often carry a small pouch under their abdomen. Males can be recognized by the right antenna with a node in the middle and the complex last pair of legs, with long hooked outgrowths. More often in fresh waters you can find cyclops, named after the one-eyed hero of myths Ancient Greece. There is only one eye on the head of these crustaceans! Cyclops have short antennae.

This type

characterized by fussy, seemingly chaotic movement. They often "jump" and periodically tumble in the water. The chaotic and fast movement of cyclops pursues two main goals: not to get caught in the mouth of a fish, and to have time to grab something edible. Cyclops are not vegetarians. They can also eat large algae, but they still prefer the juveniles of their copepods and cladocerans, as well as other aquatic small creatures, for example, rotifers and ciliates.

Crustaceans are ancient aquatic animals with a complex body structure covered with a chitinous shell, with the exception of woodlice that live on land. They have up to 19 pairs of jointed legs that perform various functions: capturing and grinding food, movement, protection, mating, and bearing young. These animals feed on worms, mollusks, lower crustaceans, fish, plants, and crayfish also eat dead prey - the corpses of fish, frogs and other animals, acting as orderlies of reservoirs, especially since they prefer very clean fresh water.

Lower crustaceans - daphnia and cyclops, representatives of zooplankton - serve as food for fish, their fry, and toothless whales. Many crustaceans (crabs, shrimp, lobsters, lobsters) are commercial or specially bred animals.

2 species of crustaceans are included in the Red Book of the USSR. general characteristics From a medical point of view, some species of planktonic crustaceans are of interest, such as

intermediate hosts

In this section, the Crustacean class will be considered according to the old classification.

The body of crustaceans is divided into the cephalothorax and abdomen. The cephalothorax consists of segments of the head and chest, merging into a common, usually undivided body section. The abdomen is often dissected.

All crustaceans have 5 pairs of head limbs. The first 2 pairs are represented by segmented antennae; These are the so-called antennules and antennae. They carry the organs of touch, smell and balance. The next 3 pairs - oral limbs - are used to capture and grind food. These include a pair of upper jaws, or mandibles, and 2 pairs of lower jaws - maxilla. Each chest segment carries a pair of legs. These include: jaws, which are involved in holding food, and locomotor limbs (walking legs). Abdomen higher crayfish also carries limbs - swimming legs. The lower ones don't have them.

Crustaceans are characterized by a bibranched limb structure. They distinguish between the base, external (dorsal) and internal (ventral) branches. This structure of the limbs and the presence of gill projections on them confirms the origin of crustaceans from polychaete annelids with bibranched parapodia.

In connection with the evolution in the aquatic environment, crustaceans have developed organs of aquatic respiration - gills. They often appear as outgrowths on the limbs. Oxygen is delivered by blood from the gills to the tissues. Lower crayfish have colorless blood called hemolymph. Higher crayfish have real blood containing pigments that bind oxygen. The blood pigment of crayfish - hemocyanin - contains copper atoms and gives the blood a blue color.

The excretory organs are one or two pairs of modified metanephridia. The first pair is localized in the anterior part of the cephalothorax; its duct opens at the base of the antennae (antennary glands). The duct of the second pair opens at the base of the maxillae (maxillary glands).

Crustaceans, with rare exceptions, are dioecious. They usually develop through metamorphosis. A nauplius larva emerges from the egg with an unsegmented body, 3 pairs of limbs and one unpaired eye.

• Subclass Entomostraca (lower crayfish).

  Lower crayfish live in both fresh waters and seas. They are important in the biosphere, being an essential part of the diet of many fish and cetaceans. Highest value have copepods (Copepoda), which serve as intermediate hosts for human helminths (diphyllobothriids and guinea guinea). They are found everywhere in ponds, lakes and other standing bodies of water, inhabiting the water column.

general characteristics

The body of the crustacean is divided into segments. The complex head bears one eye, two pairs of antennae, mouthparts, plus a pair of legs-jaws. One pair of antennas is much longer than the other. This pair of antennas is highly developed, their main function is movement. They also often serve to hold the female by the male during mating. Thorax of 5 segments, pectoral legs with swimming setae. Abdomen of 4 segments, at the end - a fork. At the base of the female's abdomen there are 1 or 2 egg sacs in which eggs develop. Nauplii larvae emerge from the eggs. The hatched nauplii look completely different from adult crustaceans. Development is accompanied by metamorphosis. Copepods feed on organic debris, tiny aquatic organisms: algae, ciliates, etc. They live in reservoirs all year round.

The most common genus is Diaptomus.

Diaptomus live in the open part of water bodies. The size of the crustacean is up to 5 mm. The body is covered quite hard shell in connection with which it is reluctantly eaten by fish. The color depends on the nutrient base of the reservoir. Diaptomuses have 11 pairs of limbs. The antennules are single-branched, the antennae and legs of the thoracic segments are biramous. The antennules reach especially great lengths; they are longer than the body. Scattering them widely, diaptomuses float in the water, the thoracic limbs cause the jerky movements of the crustaceans. The oral limbs are in constant oscillatory motion and drive particles suspended in water towards the mouth opening. In Diaptomus, both sexes take part in reproduction. Diaptomus females, unlike Cyclops females, have only one egg sac.

Species of the genus Cyclops (cyclops)

inhabit mainly coastal zones of water bodies. Their antennae are shorter than those of diaptomus and participate, along with the thoracic legs, in irregular movements. The color of cyclops depends on the type and color of the food they eat (gray, green, yellow, red, brown). Their size reaches 1-5.5 mm. Both sexes take part in reproduction. The female carries fertilized eggs in egg sacs (Cyclops have two), attached at the base of the abdomen.

In terms of their biochemical composition, copepods are in the top ten high-protein foods. In aquarium farming, “Cyclops” is most often used to feed grown juveniles and small-sized fish species.

Daphnia, or water fleas

move spasmodically. The body of daphnia, 1-2 mm long, is enclosed in a bivalve transparent chitinous shell. The head is extended into a beak-like outgrowth directed towards the ventral side. On the head there is one complex compound eye and in front of it a simple ocellus. The first pair of antennae is small and rod-shaped. The antennae of the second pair are highly developed, bibranched (with their help, daphnia swims). On the thoracic region there are five pairs of leaf-shaped legs, on which there are numerous feathery bristles. Together they form a filtration apparatus that serves to filter small organic residues, unicellular algae and bacteria from the water that daphnia feed on. At the base of the thoracic legs there are gill lobes in which gas exchange occurs. On the dorsal side of the body there is a barrel-shaped heart. There are no blood vessels. Through the transparent shell, the slightly curved tube-shaped intestine with food, the heart, and below it the brood chamber in which daphnia larvae develop are clearly visible.

• Subclass Malacostraca (higher crayfish). The structure is much more complex than that of lower crayfish. Along with small planktonic forms, relatively large species are found.

  Higher crayfish are inhabitants of marine and fresh water bodies. Only woodlice and some crayfish (palm crayfish) live on land from this class. Some species of higher crayfish serve as commercial fisheries. In the seas Far East A gigantic Pacific crab is harvested, the walking legs of which are used for food. IN Western Europe lobster and lobster are caught. In addition, crayfish have sanitary significance, because... clear water bodies of animal corpses. Freshwater crayfish and crabs in Eastern countries are intermediate hosts for the pulmonary fluke.

  A typical representative of higher crayfish is the river crayfish.

Crayfish live in flowing fresh water bodies (rivers, streams), feed mainly on plant foods, as well as dead and living animals. During the day, the crayfish hides in safe places: under stones, between the roots of coastal plants, or in burrows that it digs with its claws in steep banks. Only when night falls does he come out to look for food. For the winter, crayfish hide in their burrows.

Structure and reproduction of crayfish

External structure. The body of the crayfish is covered on the outside with a cuticle impregnated with calcium carbonate, which gives it strength, which is why the cuticle is called the shell. The shell protects the body of the crayfish from damage and serves as an exoskeleton. At a young age, during the growth period, crayfish change their shell. This process is called molting. Over time, when the crayfish reaches large sizes, it grows slowly and sheds rarely.

The color of the shell of a living crayfish depends on the color of the muddy bottom on which it lives. It can be greenish-brown, light green, dark green and even almost black. This coloring is protective and allows the cancer to become invisible. When caught crayfish are boiled, some of them are destroyed. chemical substances giving color to the shell, but one of them - the red pigment astaxanthin - does not disintegrate at 100 °C, which determines the red color of boiled crayfish.

The crayfish's body is divided into three sections: head, chest and abdomen. On the dorsal side, the head and thoracic sections are covered with a single cephalothoracic solid, strong chitinous shield, which bears a sharp spike in front; on its sides, in recesses on movable stalks, there are compound eyes, a pair of short and a pair of long thin antennae. The latter are a modified first pair of limbs.

On the sides and below the mouth opening of the crayfish there are six pairs of limbs: the upper jaws, two pairs of lower jaws and three pairs of maxillae. There are also five pairs of walking legs on the cephalothorax, and the three front pairs have claws. The first pair of walking legs is the largest, with the most well-developed claws, which are organs of defense and attack. The oral limbs, together with the claws, hold food, crush it and direct it into the mouth. The upper jaw is thick, jagged, and powerful muscles are attached to it from the inside.

The abdomen consists of six segments. The limbs of the first and second segments are modified in the male (they participate in copulation), while in the female they are reduced. On four segments there are two-branched segmented toes; sixth pair of limbs - wide, lamellar, part of the caudal fin (it plays together with the caudal blade important role when swimming backwards).

Movement of crayfish. Crayfish can crawl and swim forward and backward. It crawls along the bottom of the reservoir with the help of its pectoral walking legs. The crayfish swims forward slowly, moving its abdominal legs. To move backwards, it uses the caudal fin. By straightening it and tucking its abdomen, the crayfish makes a strong push and quickly swims back.

Digestive system begins with the mouth opening, then food enters the pharynx, short esophagus and stomach. The stomach is divided into two sections - chewing and filtration. On the dorsal and lateral walls of the chewing section, the cuticle forms three powerful chitinous chewing plates impregnated with lime with serrated free edges. In the filtering section, two plates with hairs act like a filter through which only highly crushed food passes. Next, the food enters the midgut, where the ducts of the large digestive gland open. Under the influence of digestive enzymes secreted by the gland, food is digested and absorbed through the walls of the midgut and gland (it is also called the liver, but its secretion breaks down not only fats, but also proteins and carbohydrates, i.e. functionally corresponds to the liver and pancreas of vertebrates). Undigested remains enter the hindgut and are excreted through the anus on the tail blade.

Respiratory system. Crayfish breathe using gills. Gills are feathery outgrowths of the thoracic limbs and lateral walls of the body. They are located on the sides of the cephalothorax shield inside a special gill cavity. The cephalothorax shield protects the gills from damage and rapid drying, so the crayfish can live out of water for some time. But as soon as the gills dry out a little, the cancer dies.

Circulatory organs. The circulatory system of crayfish is not closed. Blood circulation occurs due to the work of the heart. The heart is pentagonal in shape, located on the dorsal side of the cephalothorax under the shield. Blood vessels extend from the heart and open into the body cavity, where the blood gives oxygen to tissues and organs. The blood then flows into the gills. The circulation of water in the gill cavity is ensured by the movement of a special process of the second pair of lower jaws (it produces up to 200 flapping movements per minute). Gas exchange occurs through the thin cuticle of the gills. Oxygen-enriched blood is directed through the gill-cardiac canals into the pericardial sac, from where it enters the heart cavity through special openings. Cancer blood is colorless.

Excretory organs paired, they look like round green glands, which are located at the base of the head and open outward with a hole at the base of the second pair of antennae.

Nervous system consists of a paired suprapharyngeal node (brain), peripharyngeal connectives and a ventral nerve cord. From the brain, nerves go to the antennae and eyes, from the first node of the abdominal nerve chain, or subpharyngeal ganglion, to the oral organs, from the next thoracic and abdominal nodes of the chain, respectively, to the thoracic and abdominal limbs and internal organs.

Sense organs. The compound or compound eyes of crayfish are located in the front of the head on movable stalks. Each eye contains more than 3 thousand ocelli, or facets, separated from each other by thin layers of pigment. The photosensitive part of each facet perceives only a narrow beam of rays perpendicular to its surface. The whole image is made up of many small partial images (like a mosaic image in art, which is why arthropods are said to have mosaic vision).

The crayfish's antennae serve as organs of touch and smell. At the base of the short antennae there is an organ of equilibrium (statocyst, located in the main segment of the short antennae).

Reproduction and development. Crayfish have developed sexual dimorphism. In the male, the first and second pairs of abdominal legs are modified into a copulatory organ. In the female, the first pair of abdominal legs is rudimentary; on the remaining four pairs of abdominal legs, she bears eggs (fertilized eggs) and young crustaceans, which remain under the protection of the mother for some time, clinging to her abdominal limbs with their claws. This is how the female takes care of her offspring. Young crayfish grow rapidly and molt several times a year. Development in crayfish is direct. Crayfish reproduce quite quickly, despite the fact that they have relatively few eggs: the female lays from 60 to 150-200, rarely up to 300 eggs.

The meaning of crustaceans

Daphnia, Cyclops and other small crustaceans consume a large number of organic remains of dead small animals, bacteria and algae, thereby purifying the water. In turn, they represent an important source of food for larger invertebrate animals and juvenile fish, as well as for some valuable planktivorous fish (for example, whitefish). In ponds fish farms and in fish hatcheries, crustaceans are specially bred in large pools, where favorable conditions are created for their continuous reproduction. Daphnia and other crustaceans are fed to juvenile sturgeon, stellate sturgeon and other fish.

Many crustaceans are of commercial importance. About 70% of the world's crustacean fisheries are shrimp, and they are also bred in ponds created in the coastal lowlands and connected to the sea by a canal. Shrimp in ponds are fed with rice bran. There is a fishery for krill - planktonic marine crustaceans that form large aggregations and serve as food for whales, pinnipeds and fish. Food pastes, fat, and feed meal are obtained from krill. The fishing for lobsters and crabs is of less importance. In our country in the waters of the Bering, Okhotsk and Japanese seas mine Kamchatka crab. Commercial fishing for crayfish is carried out in fresh water bodies, mainly in Ukraine.

• Class Crustacea (crustaceans)

Lower Crustaceans

Subclass Gill-footed

The most primitive. These small crustaceans have leaf-shaped legs and are used equally for movement and breathing. They also create a current of water that carries food particles to the mouth. Their eggs easily tolerate drying out and wait in the soil for the new rainy season. Artemia is an interesting branchiopod: it can live in salt lakes with a salt concentration of up to 300 g/l, and in fresh water dies after 2-3 days.

Subclass Maxillopods (jaws)

Representatives of the barnacle order are amazing: sea acorns and barnacles. These sea crayfish switched to a sedentary lifestyle in houses made of limestone plates. The larva is a typical nauplius, sinks to the bottom and attaches itself with antennules. The antennules and the entire anterior part of the head turns into an organ of attachment (a long fleshy stalk in sea ducks, or a flat wide sole in sea acorns), the antennae and compound eyes atrophy, the thoracic legs extend into long two-branched “antennae”, driving food to the mouth.