Crustaceans - description, characteristics, nutrition, reproduction and classification. General characteristics of the crustacean class - Crustacea Divisions of crustaceans

22.07.2010

Arthropods are distinguished by a hard cover, which includes a special organic substance - chitin. The chitinous cover has a protective function and is the exoskeleton. The body of arthropods, like that of annelids, consists of separate segments, but unequal in structure. There are jointed limbs. Their appearance in the process of evolution is associated with the development of a strong exoskeleton.

Due to the fact that the cover is inextensible, the growth of arthropods is accompanied by molting. Body cavity mixed, i.e., along with the remains of the primary cavity, there are rudiments of the secondary cavity. Circulatory system open. Land insects breathe using tracheas, while aquatic insects also have gills.

CLASS CRUSTACEANS

Crustaceans are characterized by the presence of two pairs of whiskers, complex ( faceted) eyes consisting of many individual simple ocelli. The body is divided into a cephalothorax and a segmented abdomen. Crustaceans breathe with the help of gills with oxygen dissolved in water. Few of them have adapted to life on land, but even in this case they breathe using gills. About 25,000 species of this class are known.

Crayfish live in fresh running waters. Leads a nocturnal lifestyle. Its body consists of two sections - the cephalothorax and abdomen (28). The cephalothorax is formed from fused segments of the head and chest. The anterior part of the cephalothorax is elongated and pointed and ends with a sharp spine. At its base, two compound eyes are located on stalks, thanks to which the cancer can turn them in different directions and expand the field of vision. Compound, or compound, eyes consist of many small ocelli (up to 3000). There are two pairs of antennae on the cephalothorax of the crayfish. Long antennae serve as organs of touch, and short antennae serve as organs of smell and touch. Below the antennae are the mouthparts. Mouthparts are modified limbs. The first pair forms the upper jaws, the second and third form the lower jaws, the remaining three pairs form the maxillae. There are five pairs of walking legs on the cephalothorax; on the three front pairs there are claws, which are organs of attack and defense. In addition, with the help of claws, the crayfish captures, tears apart prey and brings it to its mouth. The segmented abdomen bears abdominal legs, on which females carry eggs. Crayfish are omnivores. Food crushed by the oral organs through the pharynx and esophagus enters the stomach, which consists of two sections - chewing and filtering. With the help of chitinous teeth located on the inner walls of the chewing section of the stomach, food is ground. Entering the filter section of the stomach, food is filtered and enters the intestine, and then into the digestive gland, where it is digested under the action of digestive juice and absorbed.

The respiratory organs - gills, skin outgrowths - are located on the sides of the cephalothorax. Oxygen penetrates into the blood flowing through the gill vessels, and carbon dioxide is released from the blood.

The circulatory system of cancer is not closed and consists of a sac-shaped heart lying on the dorsal side of the body and vessels extending from it.

The nervous system of cancer consists of large suprapharyngeal and subpharyngeal nerve nodes and the abdominal nerve cord. The excretory organs of cancer are a pair of round green glands. An excretory channel extends from each of them, opening outward at the base of the antennae. Through the green glands, harmful waste products dissolved in the blood are removed from the cancer body. Cancers are dioecious. In winter, the female lays eggs, each egg being glued to her abdominal legs. At the beginning of summer, crustaceans emerge from the eggs, which the female carries on her legs for a long time. Periodically, the old cover becomes tight for the growing organism. A new cover is formed under it. Molting occurs: the old cover bursts, and a cancer comes out of it, covered with soft and colorless chitin. The cancer grows quickly, and the chitin cells become saturated with lime, harden, and growth stops until a new molt occurs.

The significance of crustaceans in nature and the human economy is diverse. Crabs, crayfish, and shrimp are used as food. Small crustaceans are part of plankton (mass of floating food for aquatic animals, fish, whales, etc.). By eating animal carcasses, crayfish play an important role in cleaning water bodies. Daphnia, cyclops and other small crustaceans are good food for fish.

The class Crustaceans includes about 25 thousand species of animals that live mainly in marine and fresh waters. A typical representative of this class is crayfish.

External structure

The body of the cancer has a hard chitinous cover, under which there is a layer of epithelial cells. In crustaceans, the head and thorax are usually fused to form the cephalothorax. A characteristic feature of crustaceans is the transformation of the anterior body segments into the head.

On each segment, except the last, as a rule, there is a pair of limbs. Due to various functions, the shape of the limbs of crustaceans is very diverse. The limbs of the head segments usually lose motor function, turning either into part of the oral apparatus or into sensory organs.

On the front part of the cephalothorax there are 5 pairs of limbs, some of which have turned into long and short antennae that serve as organs of touch, hearing, smell, balance or chemical sense, while others are used for grinding food and chewing it. Each chest segment has a pair of legs. The 3 anterior pairs are transformed into jaws, which take part in capturing, retaining food particles and transferring them to the mouth. The other 5 pairs of thoracic legs are used for crawling (locomotor legs, also known as walking legs).

The front legs are also used to capture food, defend and attack, which is why they have claws. In hermit crabs, crabs and other related species, claws were formed only on the front pair of walking legs, in many species of shrimp - on the two front pairs of limbs, and in lobsters, crayfish and others - on the three front pairs, but on the first pair of claws significantly larger than the others. With the help of walking legs, the crayfish moves along the bottom with its head forward, and swims forward with its tail end.

Nervous system and sensory organs

The sense organs are well developed. Eyes are of two types: one simple eye in the larva, which is absent in adult higher crayfish, and a pair of complex compound eyes in adult higher crayfish. A compound eye differs from a simple eye in that it consists of individual eyes, identical in structure and consisting of the cornea, lens, pigment cells, retina, etc. It is believed that each eye sees only part of the object (mosaic vision).

Cancer's organs of touch are long antennae. There are many bristle-like appendages on the cephalothorax, which apparently perform the function of organs of chemical sense and touch. The organs of balance and hearing are located at the base of the short antennae. The organ of balance looks like a pit or sac with sensitive bristles on which grains of sand press.

Like annelids, the nervous system of crustaceans is represented by a peripharyngeal nerve ring and a ventral nerve cord with a paired ganglion in each segment. From the suprapharyngeal ganglion, nerves extend to the eyes and antennae, from the subpharyngeal ganglion to the oral organs, and from the abdominal nerve cord to all limbs and internal organs.

Digestive and excretory systems

Crayfish feed on both live and dead prey. Their digestive system begins with a mouth opening, surrounded by modified limbs (the upper jaws were formed from the first pair of legs, the lower ones - from the second and third, the maxillae - from the fourth to sixth). The crayfish grabs its prey with its claws, tears them apart, and brings pieces of it to its mouth. Then, through the pharynx and esophagus, food enters the stomach, which consists of two sections: chewing and filtering.

On the inner walls of the larger chewing section there are chitinous teeth, thanks to which food is easily ground. In the filtering section of the stomach there are plates with hairs. Through them, the crushed food is filtered and enters the intestine. Here food is digested under the influence of the secretion of the digestive gland (liver). Digestion and absorption of food can occur in the outgrowths of the liver. In addition, the liver has phagocytic cells that capture small particles of food and are digested intracellularly. The intestine ends with the anus, located on the middle blade of the caudal fin.

In spring and summer, white pebbles (millstones) consisting of lime are often found in the stomach of crayfish. Its reserves are used to soak the soft skin of crayfish after molting.

The excretory system in cancer is represented by a pair of green glands located in the head section. The excretory canals open through holes at the base of the long antennae.

Circulatory and respiratory systems

The class Crustaceans has an open circulatory system. On the dorsal side of the body there is a pentagonal heart. From the heart, blood flows into the body cavities, supplying the organs with oxygen and nutrients, then through the vessels it enters the gills and, enriched with oxygen, returns to the heart.

Crustaceans breathe using gills. They are even found in terrestrial crustaceans - wood lice, living in cellars, under stones and in other damp and shaded places.

Reproduction of crustaceans

Most crustaceans are dioecious. The gonads in both sexes are paired and located in the chest cavity. The female crayfish is noticeably different from the male; her abdomen is wider than the cephalothorax, while the male’s is narrower.

The female spawns eggs on the abdomen at the end of winter. The crustaceans hatch in early summer. From 10 to 12 days they remain under the mother’s abdomen, and then begin to lead an independent lifestyle. Since the female lays a small number of eggs, such care for the offspring contributes to the preservation of the species. The class of crustaceans is divided into 5 subclasses: cephalocarids, maxillopods, branchiopods, shellfish and higher crustaceans.

Meaning in nature

Higher crustaceans are inhabitants of marine and fresh waters. Only certain species from this class live on land (woodlouse, etc.).

Crayfish, crabs, shrimp, lobsters and others are used as food by humans. In addition, many crayfish have sanitary significance, as they clear water bodies of animal corpses.

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, 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 as intermediate hosts of helminths (Cyclops and Diaptomus).

Until recently, the Crustacean class was divided into two subclasses - lower and higher crustaceans. The subclass of lower crayfish included phyllopods, jawed crayfish and shell crayfish. It is now recognized that such a unification is impossible, since these groups of crayfish are different in origin.

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). The abdomen of higher crayfish also bears 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. The most important are copepods (Copepoda), which serve as intermediate hosts of human helminths (diphyllobothriids and guinea worms). 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 with a rather hard shell, which makes it reluctant to be 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 of the Far East, a gigantic Pacific crab is caught, whose walking legs 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 a large size, 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 the chemical substances that give color to the shell are destroyed, but one of them - the red pigment astaxanthin - does not decompose at 100 °C, which determines the red color of the 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; the sixth pair of limbs are wide, lamellar, part of the caudal fin (it, together with the caudal blade, plays an 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 includes 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 large amounts 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 pond fish farms and 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 fishery consists of 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, Kamchatka crab is harvested in the waters of the Bering, Okhotsk and Japan seas. Commercial fishing for crayfish is carried out in fresh water bodies, mainly in Ukraine.

Class Crustacea (crustaceans)

Reproduction

With rare exceptions (barnacles), all crustaceans are dioecious, and many have quite pronounced sexual dimorphism. Thus, the female crayfish is distinguished by a noticeably wider abdomen and, as we know, by the structure of the first and second pairs of abdominal legs. In many lower crustaceans, males are significantly smaller than females.

Crustaceans reproduce exclusively sexually. In a number of groups of lower crustaceans (scutellites, cladocerans, shellfishes) parthenogenesis and alternation of parthenogenetic and bisexual generations take place.

The structure of the male and female genital organs is simple. They consist in most cases of paired gonads and excretory ducts extending from them - oviducts or vas deferens, opening outwards with the genital openings. Gonads have a cavity inside.

In crayfish, for example, the male gonad (testis), although not paired, is bifurcated in the anterior part, which indicates its formation from two glands. Paired vas deferens, rather long and convoluted, extend from the testis. Parts of the vas deferens located closer to the outlet have thicker muscular walls (ejaculatory duct). The male genital openings are located on the coxopodites of the fifth pair of thoracic legs (eighth thoracic segment).

The female gonad (ovary) of crayfish in its immature state has an external resemblance to the testes, but later rather large egg cells are clearly visible in it. The oviducts are short tubes. The female genital openings in crayfish are located on the coxopodites of the third pair of thoracic legs (sixth thoracic segment).

Fertilization in crustaceans occurs differently. In some, sperm is introduced into the female’s genital tract during mating; in such cases, females have spermatheca. In other crustaceans, sperm in the form of lumps of various shapes (spermatophores) is glued to the integument of the female near the female genital opening. In crayfish, this is accomplished with the help of the first and second pairs of abdominal legs of the male. The female then lays eggs. At the same time, she bends the abdomen to the lower part of the cephalothorax. The secretion released along with the eggs dissolves the spermatophores, and the sperm fertilize the eggs, which adhere to the abdominal legs of the female.

Crustacean development

The development of fertilized eggs occurs differently and depends on the degree of richness of their yolk. The development of yolk-poor eggs (copepods, etc.) is characterized by complete crushing. In some groups, individual elements of spiral fragmentation are observed (similar to the fragmentation of annelids). In cases of complete fragmentation, development usually proceeds with metamorphosis. A small nauplius larva emerges from the egg, the structure of which is very primitive, and the formation of adult cancer organs occurs already in the postembryonic period.

Crustacean eggs, rich in yolk, are characterized by superficial crushing. In this case, the larval stages of development take place completely in the egg shells due to the use of the yolk, and a fully formed crustacean emerges from the egg. Thus, in a crayfish, the eggs of which are overloaded with yolk, a young crustacean emerges from the egg, very similar to an adult (all body segments are formed), only very small. This development is called direct. Direct development is characteristic of some daphnia, mysids and crayfish.

Metamorphosis of different groups of crustaceans does not proceed in the same way. The main differences are in the number of larval stages and the structure of the larvae hatching from the eggs.

In many groups of crustaceans (branchopods, copepods, barnacles, barnacles and some shrimp), an externally unsegmented larva, the nauplius, emerges from the egg. It has three pairs of limbs: single-branched antennules, double-branched antennae and imandibles. The antennae are located behind the mouth, and not in front of it, as in adult forms. Nauplius has an unpaired nauplial eye. At the posterior end of the body of the nauplius there is an anal opening, in front of which a growth zone is separated, where the formation of new segments with their limbs occurs, in order from front to back: maxillary, then thoracic and abdominal. From the time of separation of the maxillary and part of the thoracic segments, the larva is called a metanauplius. Metanauplius develops into an adult crustacean. This process occurs somewhat differently in different species of crustaceans. The development of the Cyclops is also characterized by the third stage of development - the copepoid larval stage.

In higher crustaceans that develop with the most fully expressed metamorphosis, for example in shrimp, the first larval stages are the nauplinus and metaplinus stages. But higher crayfish are characterized by the presence of special larval stages - protozoea and zoea.

The protozoan stage is characterized by: 1) the appearance of complex compound eyes, 2) the development of jaws, 3) a clear differentiation of the body into the cephalothorax and abdomen.

The zoea stage differs from the protozoa in the appearance of the primordia of the following thoracic limbs and an even greater differentiation of the cephalothorax and abdomen. It is characteristic that the rudiments of the thoracic limbs in zoea and protozoea are bibranched. The zoea stages of different forms do not look the same, differing in the presence of various outgrowths and spines, etc., but these differences are of a secondary nature and are associated with the adaptation of the larvae to a planktonic lifestyle.

After the zoea stage in shrimp there is a mysid stage. This larva already more closely resembles a fully formed crustacean in shape and body dissection. The most characteristic feature of the mysid stage is the presence of abdominal limbs, while the pectoral limbs still retain their bibranched character. After several molts, the mysid stage develops into a young shrimp.

In other higher cancers, development proceeds somewhat differently, in a more or less shortened way. Thus, in crabs, a zoea larva emerges from the egg and has a very characteristic appearance - with a large spike on the cephalothorax. In further development, the crab is characterized by a special larval stage - megalopa, which already leads a bottom lifestyle. Megalopa strongly resembles a crab, but still has a fairly developed abdomen and abdominal limbs.

Characteristics of crustaceans

From a medical point of view, some species of planktonic crustaceans are of interest as intermediate hosts of helminths (Cyclops and Diaptomus).

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

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.