Crustaceans have 2 pairs of antennae. Biology: external and internal structure of crayfish. Lesson learned exercises

Laboratory work.

Topic: External and internal structure of crustaceans on the example of crayfish

Target: to study the structural and functional features of crayfish as a representative of higher crayfish in connection with its habitat.

1. Systematics

2. Theoretical information

Subtype Gillbreathers (Branchiata)

Gill breathing, the body is divided into three sections: head, chest and abdomen. There are two pairs of antennae on the head: antennules - homologues of the palps located on the acron, and antennae - modified limbs of the first head segment. There are three pairs of jaws on the head. The number of thoracic and abdominal segments varies greatly. The limbs, except for the antenna, are biramous.

The subtype unites one class Crustaceans (Crustacea).

Class Crustaceans (Crustacea)

Crustaceans play an important role in the biological cycle of water

ecosystems. Small planktonic crustaceans feed on algae, while they themselves, in turn, serve as food for fish in all reservoirs. Even large marine animals - toothless whales, feed on small crustaceans, straining them out of the water in large quantities.

There are many biofilter feeders and detritivores among crustaceans; water cleaners. Many crustaceans are fished and eaten by humans. In many countries, fishing for crayfish, crabs, and shrimp is developed. At hatcheries, small crustaceans are bred as food, especially for fish fry.

External building. The body shape and size of crustaceans are varied.

The body of crustaceans consists of three sections: head, thorax and abdomen. In primitive forms, one department passes imperceptibly into another; in more organized forms, the departments are more differentiated.

Various paired and unpaired appendages are located on the segments of the head. Paired - these are antennules that carry the function of touch and smell. Antennae - modified legs, these limbs are longer, and perform the function of touch, or help to defend themselves. For example, in lobsters they are long flagellated. In copepods, the antennae perform a locomotor function.

The remaining three segments of the head also bear limbs, which are modified into mouth parts, and serve for the primary processing of food. The extremities of the second segment of the head are transformed into the upper jaws of the mandibles or mandibles - these are strongly chitinized serrated plates. On the third and fourth segments of the head are the first and second pair of lower jaws - maxilla.

The thoracic and abdominal sections of crustaceans can combine a different number of segments (from 5-58 to 50). The limbs of the thoracic region are also specialized.

In many (lower) limbs of the thoracic region, they are multifunctional and perform swimming, respiratory functions and filtration (getting food). In others (higher crayfish), for example, in crayfish, the first pairs of pectoral legs - biramous mandibles - serve to hold and pinch off food. The fourth pair of thoracic legs is the most powerful, ending with a claw, with which the crayfish captures prey. The remaining four pairs of thoracic legs are walking, although they can also carry small pincers. Gills are located at the base of the pectoral legs of cancer, and therefore they carry additional function breathing.

The abdominal region of crustaceans consists of a variable number of segments and a telson. Many crayfish in the abdominal region do not have limbs, most of the higher crayfish have biramous legs on the abdominal region, which do not specialize in movement, but serve for respiration or reproduction.

The limbs of crustaceans, no matter how they are arranged, all originate

from the primitive typical two-branched limb of crustaceans, which consists of the basal part of the protopodite, two branches depart from it: the outer one is the exopodite, and the inner one is the endopodite. The protopodite, in turn, consists of two segments: the coxopodite and the basipodite; the coxopodite bears a gill appendage, the epipodite.

The integument of the body of crustaceans is a chitinized cuticle, which is often impregnated with calcium carbonate salts, which provides greater strength.

Digestive system crustaceans is well developed and consists of a tubular digestive tract, into which the ducts of the digestive glands flow. The intestine is divided into the anterior middle and hindgut. The foregut is differentiated into the esophagus and stomach, is of ectodermal origin, and is lined with cuticle throughout. The cuticular lining in some sections of the foregut forms thickenings that serve to grind food.

The midgut of endodermal origin, in primitive forms (copepods) does not form outgrowths, but in most crayfish it forms lateral glandular outgrowths, which are called hepatic or simply liver.

The hindgut looks like a straight tube, stretches through the entire abdomen and ends with an anus. In crayfish, its length is more than ten times the length of the midgut. Like the foregut, it is lined with cuticle, which peels off during molting and exits in the form of a tube through the anus.

Respiratory system in small crustaceans it is absent, and their gas exchange occurs through the entire surface of the body. In most crustaceans, the respiratory organs are skin gills in the form of feathery or lamellar outgrowths.

Circulatory system crustaceans is not closed, and different types Very

varies greatly. The movement of hemolymph through the vessels occurs due to the contraction of the heart located on the dorsal side, in the thoracic or spinal region.

excretory system represented by paired kidneys - modified coelomoducts located in the head. Each kidney is the remains of a coelom in the form of a small sac, from which a convoluted excretory canal with glandular walls departs.

Nervous system consists of a paired supraesophageal ganglion and peripharyngeal ring and ventral nerve cord. Primitive crustaceans (gill legs) have a ladder type nervous system. In crustaceans, like other arthropods, there is a process of oligomerization and fusion of ganglia of different segments.

sense organs - represented by the eyes, the organs of touch, the chemical sense, and the organs of balance.

The eyes of crustaceans can be complex or simple. Some of the crustaceans have only simple eyes (cyclops), most of the higher ones have compound eyes, some have both (carpoid crustaceans). Many decapod crayfish have balance organs located at the base of the antennula - the statocyst.

reproductive system. Crustaceans are mostly dioecious animals. Hermaphrodites are found only among sessile forms. In dioecious, sexual dimorphism is often expressed: females are larger, a different number of legs in females and males. The sex glands are embryonicly laid as paired formations, but in the course of development both rudiments merge and form an unpaired gonad. And only some primitive crayfish retain paired glands.

Development crustaceans. Embryonic development is similar to that of annelids: helical non-uniform deterministic cleavage, teloblastic way of laying the mesoderm. Postembryonic development with metamorphosis. In crustaceans, a larva emerges from an egg - a nauplius, which leads a planktonic lifestyle. Nauplius is a characteristic larva for crustaceans. Nauplius then turns into the next larva - metanauplius. Metanauplius molts several times, and all segments, limbs, internal organs characteristic of adult animals gradually differentiate.

In higher crayfish, the nauplius develops into a larva - a zoea.

Classification. The crustacean class is divided into five subclasses: the subclass

Branchiopods (Branchiopoda), subclass Cephalocarida (Cephalocarida), subclass Maxillopods (Maxillopoda), subclass Shellfish (Ostracoda), subclass Higher crayfish (Malacostraca).

Subclass Branchiopoda

The most primitive crustaceans with homonomous body segmentation. The body ends with a furka, the limbs of the thoracic region are leaf-shaped, perform the following functions: movement, respiration, food capture. The head is free, it has compound eyes and an unpaired nauplis eye. Excretory organs are maxillary kidneys.

Development goes with metamorphosis: larvae - nauplius and metanauplius. For some, development may be direct. The subclass of branchiopods includes two orders - Gills (Anostraca) and Leaf-legs (Phyllopoda).

Subclass Cephalocarida (Cephalocarida)

Crustaceans of this subclass have become known since 1957 and were described by the American scientist Sanders. Small crustaceans live in sea silt. In their structure there are features intermediate between other subclasses. The head is free, bears antennae, antennules, mandibles, and two pairs of legs similar to pectorals. The segments of the thorax are the same, as are the legs of this section, which brings them closer to the branchiopods. The front pectoral legs are not modified into mandibles. The abdomen consists of nine segments and a telson, i.e. chest and abdomen are in proportion. Cephalocarids share many features of the ancestral forms of crustaceans.

Subclass Maxillopoda (Maxillopoda)

Subclass Shellfish (Ostracoda)

The name of the subclass is due to the fact that their body has a bivalve shell (a modified carapace) made of chitin, impregnated with calcium carbonate. Like bivalves, they have a mantle, an elastic ligament, a muscle-connector and a lock. This convergent similarity is associated with a burrowing lifestyle. Most barnacles lead a benthic lifestyle, crawling or burrowing into the silt in the seas and fresh waters. The presence of the shell significantly affected the appearance. Their body is practically not segmented, only the number of legs speaks of segmentation. The head may sometimes have compound eyes or only unpaired nauplis eyes. The part of the shell in front of the eye is transparent, so the cancer sees through the shell.

Reproduction is primarily sexual, although some may have parthenogenesis. Nauplius larvae have a thin bivalve shell, which is changed during growth. Adult shellfish, unlike other crustaceans, do not molt.

Subclass Higher crayfish (Malacostraca)

They differ from all other cancers in a certain number of body segments. Their head, like everyone else, consists of an acron and four segments, a chest of eight, and an abdomen of 6-7 segments and a telson. The head can be arranged in different ways: the gnathocephalon can merge with the thoracic segments, forming the jaw-thorax, or all the head segments are fused with each other and also with the 1st or 2nd thoracic segments, i.e. forming a head. In some, the abdominal segments and telson are fused. All higher crayfish have limbs on their abdomens, but no furca! (except for Nebalia).

The genital openings open on the segments of the chest - male on the eighth, female on the sixth. All have developed respiratory and circulatory systems. The excretory organs of adults are the antennal glands (kidneys), in the larvae they are maxillary. Only adult Nebalia have two pairs of kidneys. Development with metamorphosis and direct. Zoea larva is typical. Although the higher cancers are more organized than all the others, they still have a number of archaic features, for example, the presence of appendages (limbs) on all segments of the body, some have two pairs of kidneys. This suggests that they evolved independently of other subclasses.

3. Tasks

Exercise 1 . On the liquid preparation, consider the external structure of crayfish (Astacus astacus). Pay attention to the anterior part of the carapace - rostrum, compound eyes, antennules, antennae, mouth opening. Locate the occipital sulcus and the border between the protocephalon and the maxillofacial region.

Rice. 1. Crayfish (view from the dorsal side):

1 - rostrum; 2 – antennula; 3 - antenna; 4 – compound eye; 5 - claw of the first pair of walking legs; 6 – walking legs; 7 - carapace; 8 – occipital furrow; 9 – gill-heart grooves; 10 – abdomen; 11 - swimming plates; 12 – telson

Task 2 . On a fixed preparation, define the boundaries of the connection between the cephalothorax and the abdomen. Locate the gill-cordial grooves. Study the structure and functions of the metamerically located thoracic limbs corresponding to the segmental arrangement of parapodia in polychaetes. Study the structure of the abdomen and abdominal jointed legs. Pay attention to the last pair of limbs - uropods and the final part of the abdomen - the telson. Study their structure and functions (Fig. 2).

Rice. 2. Crayfish (view from the ventral side):

A - male; B - female: 1 - a tubercle with an excretory opening; 2 – genital opening; 3 – limbs of the first and second abdominal segments of the male; 4 – limbs of the third - fifth abdominal segments of the male; 5 – rudimentary limb of the first abdominal segment of the female; 6 – limbs of the second - fifth abdominal segments of the female with eggs; 7 - anus; 8 – the border between the protocephalon and the jawbone; 9 – mouth opening (covered by the upper jaws); 10 – third pair of mandibles; 11 – telson

Rice. 3. Limbs of a male crayfish:
1 - antennula; 2 – antenna; 3 - mandible; 4 - maxilla I; 5 – maxilla II; 6- 8 – mandibles; 9- 13 – walking legs; 14, 15 – copulatory apparatus; 16- 18 – biramous abdominal legs; 19 – uropod

Task 3 . Dissect the limbs of the crayfish, secure them sequentially with glue or thread on paper. Sign their names and indicate which body segment they belong to. Sketch the antennules and limbs of the crayfish (Zelikman, Fig. 100).

To get acquainted with the structure of the crayfish limbs, it is necessary to dissect them. To do this, turn the crayfish upside down. Starting from the last segment of the abdomen, the limbs should be torn off with tweezers, capturing them at the very base. Arrange the dissected limbs on a clean sheet of paper in strict sequence (Fig. 3). Pay special attention to the preparation of limbs in the area of the mouth opening. Carefully fix the prepared limbs on paper using threads, glue or other materials. Label each limb.

Task 4. Consider the general arrangement of the internal organs of a crayfish (Actacus astacus) dissected from the dorsal side. Examine the location of the heart and blood vessels extending from it. Digestive system. reproductive system- gonads, their ducts.

Draw a diagram of the circulatory system of crayfish (Zelikman, fig. 111, 114).

Task 5. Study the structure and location of the organs of the digestive system of crayfish - a two-lobed liver, esophagus, complex stomach, hindgut.

Task 6. Sketch the structure of the green gland of crayfish (Zelikman, Fig. 118).

4. Terminology

Carapace - _______________________________________________________________

Rostrum - ________________________________________________________________

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Compound eyes - _________________________________________________

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Abdomen - _________________________________________________________

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Thorax - ________________________________________________________________

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Cephalon - _______________________________________________________________

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Pleopods - ____________________________________________________________

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Uropods - ___________________________________________________________

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Maxillopods - ____________________________________________________

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Mandibles - ________________________________________________________

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Mandibles - ___________________________________________________________

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Telson - ________________________________________________________________

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Autotomy - _____________________________________________________________

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Gnathothorax - ____________________________________________________________

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Protocephalon - ____________________________________________________

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5. Issues for discussion

1. What are the common features of arthropods?

2. What are the common features of crustaceans?

3. What substances does the cancer shell consist of? What is its meaning?

4. What departments does the body of crayfish consist of?

5. How does molting occur in crayfish?

6. How to determine the number of segments that form the head of a crayfish? What is the function of the appendages of the head?

7. How many segments are included in the crayfish breast? What are the structural features and functions of the thoracic limbs?

8. How many segments form the abdomen of crayfish? What are the structural features and functions of the abdominal limbs?

9. What are the structural features of a typical two-branched limb of crustaceans?

10. What is the origin of antennas and antennules? What functions do they perform and where are they located?

11. What are the features of the structure and location of the organs of vision of crayfish?

12. What are the features of the movement of crayfish in various conditions environment?

13. How are the limbs of crayfish modified in connection with the functions they perform?

14. What determines the mobility of body segments and segments of the extremities of crayfish?

15. Why is segmentation of the crayfish body called heteronomous?

16. How is sexual dimorphism manifested in crayfish?

18. What explains the small number of eggs in the clutch of a female crayfish?

19. What are the features of the external structure of crayfish larvae?

20. Why are crayfish not found in water bodies with decaying organic matter or suspended particles of silt?

21. What is the biogeocenotic and commercial value of crustaceans?

associated with its environment. This is a typical inhabitant of fresh water, which breathes with the help of gills. In this article, crayfish will be considered. The structure, pictures, habitat and features of life are presented to your attention below.

Characteristic features of crustaceans

Like all arthropods, the structure of the crayfish (the figure below shows it) is represented by a segmented body and limbs. These are the head, thorax and abdomen. Body segments bear paired limbs, consisting of individual segments. They are able to perform quite complex movements. The jointed limbs are usually attached to the thoracic segments of the body. The external structure of crayfish is fully consistent characteristic features arthropods.

Habitat

Crayfish can be found in fresh water. Moreover, their presence can be considered an indicator of its environmental safety. After all, these animals prefer reservoirs with clean water and high oxygen content. The structure of crayfish determines their ability to crawl with walking legs or swim. During the day they are in natural shelters. At night they crawl out of holes, from under stones and logs. At this time they are looking for food. In this regard, crayfish are not picky. They are generally omnivores. Worms, fry, tadpoles, mollusks, algae - all of them will appeal to crayfish. They do not disdain dead organics either. If you decide to have this animal in your home aquarium, not only special food is suitable for eating, but also meat, vegetables and bread. True, while maintaining the purity of water will be quite difficult.

The external structure of crayfish

The body of crayfish consists of two parts. This is the cephalothorax and abdomen. The front part is covered with the so-called shell. And the abdomen consists of separate segments, on top of which there are small shields. The cephalothorax also bears two pairs of antennae, mouthparts, and five pairs of walking legs. Each of them performs certain functions. For example, the first pair ends with powerful claws that serve to capture food, tear it into pieces and protect it from enemies.

Six pairs of limbs are attached to the abdomen. The last pair of legs is widened and together with the anal plate forms the caudal fin. By appearance it resembles a fan. With the help of the anal fin, crayfish swim fast enough with the back end forward. Together they have 19 limbs.

body integuments

Structural features of crayfish are primarily determined by its cover. Like all arthropods, it is represented by a cuticle, which forms a powerful external skeleton. Additional hardness is given to it by calcium carbonate, with which it is impregnated.

Since the cuticle is not capable of stretching, the growth of crayfish is accompanied by periodic molts. This period is critical in the life of these aquatic inhabitants. A few days before this, crayfish become restless, stop feeding, and spend all their time looking for shelter. With the help of intense movements of the body and limbs, they get rid of the old cover, from which they crawl out through a gap at the border of the cephalothorax and abdomen. Crayfish remain in their safe hiding place for up to ten days, until the new cuticle hardens.

The internal structure of crayfish

During embryonic development, a secondary body cavity is laid in all arthropods. But it does not persist throughout the life of the animal. In the process of individual development, this structure is destroyed, merging with the remains of the primary and forming a mixed cavity. The spaces between the organs are filled in crustaceans with a fatty body. This is a kind of loose connective tissue, which performs important functions: the storage of nutrients, the formation of blood cells, protection from mechanical damage.

Musculoskeletal system

The structure of crayfish differs significantly from that of the coelenterates. In particular, they do not have a skin-muscular sac. Under the strong are bundles of striated muscles that can contract quickly.

Major organ systems

The internal structure of crayfish is represented by a rather complex digestive - through type, with the presence of the liver and salivary glands, which secrete enzymes that break down nutrients. The end products of metabolism are removed from the body with the help of the Malpighian vessels.

Crayfish are due to the aquatic environment of its habitat, these are gills. They are closely related to the circulatory system. She is unclosed. Blood vessels open into the body cavity, mix with its fluid, forming hemolymph. It transports oxygen, carbon dioxide, nutrients, metabolic products.

The most important function of hemolymph is protective. It contains specialized cells that carry out amoeboid movements, capture pathogens with pseudopods and digest them. The movement of hemolymph throughout the body is provided by a pulsating thickened vessel - the heart. Since the blood mixes with the abdominal fluid and is not divided into arterial and venous, crayfish is a cold-blooded animal. This means that the temperature of his body decreases with cooling in the environment.

In late autumn, crayfish begin to breed. These are dioecious animals with direct development and external fertilization. The male has a testis and two vas deferens, the female has an ovary and paired oviducts. After fertilization, the eggs are on the ventral legs of the female. So she shows maternal instinct, taking care of future offspring. In late spring or early summer, young crustaceans emerge from them, which are an exact copy of adults.

Quite complex and nervous system. It consists of differentiated sections: anterior, middle and posterior. The first regulates the work of the eyes, provides complex behavioral reactions of these animals, the rest innervates the antennae. The brain is anatomically connected to the ventral nerve cord, from which individual nerve fibers extend throughout the body.

Significance in nature and human life

Young individuals of crayfish form plankton of fresh water bodies - an important link in the food chain. Using dead animals for food, they clean up the habitat. Recently, due to the negative influence of man, the number of crayfish populations has declined sharply. In dirty water, the offspring of crayfish will inevitably die. This is also due to the important commercial value of this representative of arthropods. After all, crayfish meat is valuable food product and in some regions even a delicacy. It is rich in proteins, vitamins and trace elements. Crayfish is the largest representative of the class that lives in fresh water. To save this species in nature, until mid-summer, their fishing is officially prohibited.

The structure of crayfish is largely determined by its habitat and determines its importance in nature and human life.

Crustaceans are aquatic arthropods that breathe with gills. The body is divided into segments and consists of several sections: from the head, chest and abdomen or from the cephalothorax and abdomen. There are two pairs of antennae. The integuments of the body contain a special solid substance - chitin, and in some, they are also reinforced (impregnated) with calcium carbonate.

About 40 thousand species of crustaceans are known (Fig. 85). Their sizes are varied - from fractions of a millimeter to 80 cm. Crustaceans are widespread in the seas and fresh water, a few, such as wood lice, palm thief, have switched to a terrestrial lifestyle.

Rice. 85. Various crustaceans: 1 - crab; 2 - hermit crab; 3 - shrimp; 4 - wood lice; 5 - amphipod; 6 - sea duck; 7 - shield

Features of the structure and vital activity of crustaceans can be considered on the example of crayfish.

Lifestyle and external structure. Crayfish lives in various fresh water bodies with clean water: river backwaters, lakes, large ponds. During the day, crayfish hide under stones, snags, roots of coastal trees, in minks dug by themselves in the soft bottom. In search of food, they leave their shelters mainly at night.

Crayfish is a fairly large representative of arthropods, sometimes specimens over 15 cm long come across. The color of crayfish is greenish-black. The whole body is covered with a strong and dense chitinous shell, impregnated with calcium carbonate.

covers crayfish serve as an external skeleton. Bundles of striated muscles are attached to it from the inside. The hard shell of cancer prevents the animal from growing. Therefore, cancer periodically (2-3 times a year) sheds - sheds old integuments and acquires new ones. During molting, until the new shell gets stronger (it takes about a week and a half), the crayfish is defenseless and cannot eat. At this time, he hides in shelters. The body of crayfish consists of two sections - the cephalothorax and abdomen (Fig. 86). At the front end of the cephalothorax there is a pair of long and a pair of short antennae - these are the organs of touch and smell. Globular eyes sit on long stalks. Therefore, cancer can simultaneously look in different directions. In case of danger, he hides his eyes in the recesses of the shell.

Rice. 86. External structure of crayfish: 1 - long antennae; 2 - short antennae; 3 - claw; 4 - walking legs; 5 - eye; 6" - cephalothorax; 7 - abdomen; 8 - caudal fin

Cancer eyes are complex. Each eye consists of many very small eyes, facets, directed in different directions (Fig. 87, B). The image of an object in a complex (faceted) eye is made up of its individual parts, resembling mosaic pictures.

Rice. 87. Internal structure of crayfish (female): A - general plan of body structure: 1 - stomach; 2 - liver; 3 - heart; 4 - blood vessels; 5 - ovary; 6 - gut; B - diagram of the structure of the compound eye

Limbs are located on the cephalothorax of crayfish. If it is turned on its back, then three pairs of jaws can be found at the front end of the body: a pair of upper jaws and two pairs of lower jaws. With them, cancer breaks prey into small pieces. The jaws are followed by three pairs of short mandibles. They serve to bring food to the mouth. Both jaws and mandibles are modified legs. Behind the mandibles are five pairs of walking legs. With the help of four pairs of these legs, the crayfish moves along the bottom of the reservoirs. And the first pair of walking legs in cancer is turned into large claws. With them, cancer seizes prey, tears off large parts from it. With the same claws he defends himself.

And on the abdomen, the cancer has short limbs (legs), the female has four of them, the male has five pairs. At the very end of the abdomen there is a flat segment, on the sides of which modified, strongly flattened legs are developed. Together they form the tail fin. Sharply bending the abdomen, the crayfish is repelled from the water by its caudal fin, like an oar, and in case of danger it can quickly swim backwards.

Digestive system(Fig. 87, A) begins with the mouth opening. From the mouth, food enters the stomach, which consists of two sections. In the first section there are chitinous formations impregnated with calcium carbonate - millstones, with the help of which food is crushed. Then it ends up in the second section of the stomach, where it is filtered. Large food particles are retained and returned to the first section, while small ones enter the intestine. The ducts of the liver flow into the middle intestine. Digestion of food and absorption of nutrients occur in the intestines and liver. The digestive system ends with an anus located on the caudal segment of the abdomen. Crayfish feed on mollusks, insect larvae living in water, decaying animal corpses, and plants.

Respiratory organs crayfish have gills. They contain blood capillaries and gas exchange takes place. The gills look like thin feathery outgrowths and are located on the processes of the mandibles and walking legs. In the cephalothorax, the gills lie in a special cavity. The movement of water in this cavity is carried out due to very rapid vibrations of special processes of the second pair of jaws.

Circulatory system open.

In crustaceans, the body cavity is mixed; in the vessels and intercellular cavities of crustaceans (as in other arthropods), it is not blood that circulates, but a colorless or greenish liquid - hemolymph. It performs the same functions as blood and lymph in animals with a closed circulatory system.

The heart is located on the dorsal side of the cephalothorax. Hemolymph flows through the vessels, and then enters the cavities located in various organs. Here, the hemolymph gives off nutrients and oxygen, and receives waste products and carbon dioxide. Then the hemolymph enters the gills through the vessels, and from there to the heart.

excretory system represented by a pair of green glands located in front of the cephalothorax. They open outward at the base of the long antennae. Through these holes, harmful products that are formed in the process of life are removed.

Nervous system. Cancer has a central nervous system - a peripharyngeal nerve ring and an abdominal nerve chain and a peripheral nervous system - nerves extending from the central nervous system.

Sense organs. In addition to the organs of touch, smell and vision, crayfish also have organs of balance. They represent a recess in the main segment of the short antennae, where a grain of sand is placed. The grain of sand presses on the thin, sensitive hairs surrounding it, which helps the cancer to assess the position of its body in space.

Reproduction. Crayfish are characterized by sexual reproduction. Fertilization is internal. Fertilized eggs laid by the female (from 60 to 200 pieces) are attached to her ventral legs. Egg laying occurs in winter, and young crustaceans appear in spring. Having hatched from the eggs, they continue to hold on to the mother's abdominal legs (Fig. 88), and then leave her and begin an independent life. Young crustaceans eat only plant foods.

Rice. 88. Young crustaceans on the ventral legs of a female

Decapods include crayfish, large sea crayfish - lobsters (up to 60 cm long and weighing up to 15 kg) and lobsters (they do not have claws), small crustaceans - shrimp. Some of them move along the bottom, others actively swim in the water column with the help of abdominal legs. Hermit crabs belong to this group. They have a soft, unsegmented abdomen. Hermit crabs hide from enemies in the empty shells of sea snails, all the time carrying the shell with them, and in case of danger, completely hiding in it, covering the entrance with a highly developed claw. Crabs are decapods. They have a wide but short cephalothorax, very short antennae, and a short abdomen tucked under the cephalothorax. Crabs usually move sideways.

Small crustaceans, well-known to aquarists, belong to the Leaf-legged - daphnia 3-5 mm long (Fig. 89, 1). They live in small fresh water bodies. The entire body (with the exception of the head) of Daphnia is enclosed in a transparent chitinous shell-shell. Through the chitinous covers, a large complex eye and constantly working pectoral legs are visible, which ensure the flow of water under the shell. Daphnia has large, branched antennae. By waving them, she jumps in the water, which is why daphnia is sometimes called "water fleas." Daphnia feed on protozoa, bacteria, unicellular algae located in the water column.

Rice. 89. Crustaceans: 1 - daphnia: 2 - cyclops

A small crustacean, vaguely resembling a wood lice, lives in fresh water bodies - a water donkey. Amphipods are small (up to several centimeters) crustaceans swimming on their side, for which they are called amphipods. Using different legs, crustaceans can swim, walk along the bottom of reservoirs, along the wet soil of the banks, and also jump. Barnacles are small crustaceans that lead an attached lifestyle as adults, such as sea acorns. They live in the sea. Their whole body is covered with a calcareous shell-house. Most often, the shell is attached to stones, crab shells, the bottoms of ships, and whale skin. Barnacles catch their prey (planktonic organisms) with the help of long movable pectoral legs.

Crustaceans are primary aquatic arthropods with a hard and durable chitinous shell impregnated with calcium carbonate, articulated limbs located on the thoracic and abdominal regions. Crustaceans breathe with gills.

Lesson learned exercises

Find out, using Figure 86, what features arthropods have in their external structure. Name the features of their similarity with annelids.
What is the difference between the internal structure of crustaceans and representatives of other classes of arthropods? Explain with the example of crayfish.
What are the features of the structure of the sense organs in crayfish?
Use several examples and drawings to show the diversity of the class. Describe the habitats of crustaceans.
What is the role of crustaceans in nature?

Latin name Crustacea

Characteristics of crustaceans

The subphylum Gillbreathers contains one class of crustaceans (Crustacea), richly represented in the modern fauna. They are characterized by the presence of two pairs of head antennae: antennules and antennae.

Dimensions crustaceans range from fractions of a millimeter in microscopic planktonic forms to 80 cm in higher crustaceans. Many crustaceans, especially planktonic forms, serve as food for commercial animals - fish and whales. Other crustaceans themselves serve as the subject of fishing.

Body dismemberment

The body of crustaceans is segmented, but, unlike annelids, their segmentation is heteronomous. Similar segments that perform the same function are combined into departments. In crustaceans, the body is divided into three sections: the head (cephalon), chest (thorax) and abdomen (abdomen). The head of crustaceans is formed by an acron corresponding to the head lobe - the prostomium of annelids, and four trunk segments merged with it. Accordingly, the head section bears five pairs of head appendages, namely: 1) antennules - single-branched tactile antennae innervated from the brain (homologous to the palps of the annulus); 2) antennae, or second antennae, originating from the first pair of biramous limbs of the parapodial type; 3) mandibles, or mandibles, - upper jaws; 4) first maxillae, or first pair of lower jaws; 5) second maxillae, or second pair of lower jaws.

However, not all crustaceans have acron and the four segments that form the head are fused together. In some lower crustaceans, the acron is fused with the antennal segment, but does not merge with the independent mandibular segment, but both maxillary segments are fused together. The anterior part of the head, formed by the acron and the segment of the antennae, is called the primary head, the protocephalon. In many crustaceans (except for the formation of the primary head - the protocephalon), all the jaw segments (the mandibular and both maxillary) also merge to form the jaw section - the gnathocephalon. This section fuses with a greater or lesser number of thoracic segments (in crayfish with three thoracic segments), forming the jaw-thorax - gnathothorax.

In many, the head consists of five completely merged parts: an acron and four body segments (shields, cladocerans, some amphipods, and isopods), and in some, the head segments merge with one or two more thoracic segments (copepods, isopods, amphipods).

In many, the dorsal integuments of the head form an outgrowth at the back, more or less covering the thoracic region, and sometimes the entire body. This is how the cephalothoracic shield, or carapace, of crayfish and other decapods is formed, and the transverse groove on this shell indicates the border between the merged jaw and thoracic parts of the body. The carapace grows to the thoracic segments. Sometimes it can be compressed from the sides, forming a gable shell that hides the entire body (shellfish).

The thoracic segments, as indicated, can grow together with the head (1-3, even 4 segments), forming a cephalothorax. All thoracic segments bear limbs whose functions are not limited to motor and respiratory. So, in crayfish 3, the first pairs of thoracic limbs turn into mandibles, which provide food to the mouth.

The abdominal segments are usually movably connected to each other. Only the higher crustaceans have limbs on the abdominal segments; the rest of the abdomen is devoid of them. The abdominal region ends in a telson, which does not bear limbs and is homologous to the pygidium of polychaetes.

While in all crustaceans the number of head segments is the same (5), the number of thoracic and abdominal segments is very different. Only in higher crayfish (decapods, isopods, etc.) their number is constant: thoracic - 8, abdominal - 6 (rarely 7). In the rest, the number of thoracic and abdominal segments ranges from 2 (shellfish) to 50 or more (shields).

limbs

The limbs of the head are represented by five pairs. Antennules corresponding to the palps of the annulus retain in crustaceans mainly the functions of the sense organs - touch and smell. Antennules of crayfish consist of main segments and two segmented branches.

The antennae are the first pair of limbs of parapodial origin. In the larvae of many crustaceans, they are biramous, while in most adult crayfish they become single-branched or retain only a rudiment of the second branch (exopod). Antennas perform mainly a tactile function.

The mandibles make up the upper jaws. They correspond in origin to the second pair of limbs. In most crayfish, the mandibles are turned into hard jagged chewing plates (mandibles) and have completely lost their biramous character. It is believed that the chewing plate corresponds to the main part of the limb - the protopodite. In crayfish (and some others), a small three-segmented palp sits on the chewing plate - the remnant of one of the branches of the limb.

The first and second maxillae, or the first and second pairs of mandibles, are usually less reduced limbs than the mandibles. In decapods, the maxillae consist of two main segments, forming a protopodite, and a short, unbranched palp. With the help of the chewing plate of the protopodite, the maxillae perform the chewing function.

The thoracic limbs of representatives of different orders are arranged differently. In crayfish, the first three pairs of thoracic limbs are transformed into the so-called mandibles or maxillopods. The maxillae of the crayfish, especially the second and third pairs, retain a fairly strong biramous structure (endopodite and exopodite). The second and third pairs also bear gills, and their movement causes currents of water through the gill cavity. Therefore, they perform a respiratory function. However, their main function is to hold food and move it to the mouth. Finally, the endopod of the third pair serves as a kind of toilet device, with the help of which the antennules and eyes are cleaned of foreign particles adhering to them.

However, in many other crustaceans, the first three pairs of thoracic limbs perform a predominantly locomotor function.

A peculiar change in the thoracic limbs is their adaptation to grasping, for example, the claws of decapod crayfish. The claw is formed by two limb segments: the penultimate segment, which has a long outgrowth, and the last segment articulated with it, forming the other side of the claw. Fifth - eighth pairs of thoracic limbs of crayfish (and other decapods) are typical walking legs. They are single-branched, and their basal part (protopodite) and endopodite are preserved. The exopod is completely reduced. Two-branching of the thoracic limbs is observed much more often in lower crustaceans.

Abdominal limbs, as already mentioned, are absent in many groups of crustaceans. In higher crustaceans, they are usually less developed than thoracic ones, but more often they remain biramous; in many crayfish they are equipped with gills, simultaneously performing a respiratory function. In crayfish, the abdominal legs - pleopods - are changed in males. Their first and second pairs represent the copulatory apparatus. In females, the first pair is rudimentary. The second - the fifth pair of abdominal legs in females and the third - the fifth pair in males of the swimming type. They are biramous and consist of few segments, abundantly covered with hairs. The eggs laid by the crayfish females are attached to these legs, which they bear, and then the hatched crustaceans hold on to the legs of the female for some time.

The last, sixth pair of abdominal legs - uropods - is peculiarly changed in crayfish and in some other crayfish. Both branches of each leg are turned into flat swimming lobes, which, together with the flat last segment of the abdomen - the telson - form a fan-shaped swimming apparatus.

In crabs, an interesting protective adaptation is often observed - spontaneous discarding of limbs, sometimes occurring even with very little irritation. This autotomy (self-mutilation) is associated with a strong ability to regenerate. A new limb develops in place of the lost limb.

Skeleton and muscles

The chitinized cover is impregnated with calcium carbonate. This gives greater rigidity to the skeleton.

The mobility of the body and limbs in the presence of a hard cover is ensured by the fact that chitin covers the body and limbs with a layer of unequal thickness and hardness. Each segment of the crayfish abdomen is covered with hard plates of chitin on the dorsal and ventral sides. The dorsal shield is called tergite, the abdominal shield is called sternite. On the boundaries between the segments, the boggy and soft chitin forms folds, which straighten when the body is bent in the opposite direction. A similar adaptation is observed on the joints of the limbs.

The internal skeleton of the cancer serves as an attachment site for various muscles. In many places, especially on the ventral side of the thoracic region, the skeleton forms a complex system of crossbars that grow inside the body and form the so-called endophragmal skeleton, which also serves as a site of muscle attachment.

All sorts of bristles, hairs covering the body of the cancer, and especially its limbs, are outgrowths of the chitinous cover.

Digestive system

The digestive system is represented by the intestine, which consists of three main sections: the anterior, middle and hindgut. The anterior and posterior intestines are of ectodermic origin and are internally lined with a chitinous cuticle. Crustaceans are characterized by the presence of a paired digestive gland, usually called the liver. The digestive system reaches its greatest complexity in decapod crayfish.

The anterior intestine of crayfish is represented by the esophagus and stomach. The mouth is located on the ventral side, a short esophagus extends upwards from it to the dorsal side. The latter leads to the stomach, which consists of two sections - cardiac and pyloric. The cardial, or chewing, section of the stomach is lined from the inside with chitin, which forms a complex system of crossbars and protrusions equipped with teeth in its back. This formation is called the "gastric mill", it provides the final grinding of food. In front of the cardial section, white rounded limestone formations are placed - millstones. Calcium carbonate, which accumulates in them, is used during molting to impregnate the new chitinous cover with it. Food crushed in the cardial part of the stomach enters through a narrow passage into the second, pyloric part of the stomach, in which the food particles are pressed and filtered. This part of the stomach ensures that only highly crushed food enters the middle intestine and the digestive gland. It must be borne in mind that not only mechanical grinding of food takes place in the stomach, but also partly its digestion, since the secret of the digestive gland penetrates into the stomach. The remaining unground larger food particles, due to the special structure of the pyloric part of the stomach, pass directly into the hindgut, bypassing the midgut, and are brought out.

The midgut of crayfish is very short. It is approximately 1/20 of the entire length of the intestine. Digestion and absorption of food takes place in the midgut. Most of the liquid food from the stomach goes directly to the digestive gland (liver), which opens with two openings at the border of the midgut and the pyloric part of the stomach. Digestive enzymes that digest proteins, fats, and carbohydrates are not only excreted into the midgut and stomach, but are also used in the liver tubules themselves. Liquid food penetrates into these tubes, and here its final digestion and absorption takes place.

In many crustaceans, the digestive gland is much less developed (for example, in daphnia), and in some it is completely absent (in cyclops). In such crustaceans, the midgut is relatively longer.

The hindgut is a straight tube lined with chitin from the inside and opens with an anus on the ventral side of the telson.

Respiratory system

Most crustaceans have special respiratory organs - gills. By origin, the gills develop from the epipodites of the limbs and, as a rule, are located on the protopodites of the thoracic, less often ventral, legs. In a simpler case, the gills are plates sitting on the protopodite (amphibians, etc.); in a more perfect form, the gills are a rod seated with thin gill filaments. The lacunae of the body cavity - the mixocoel - go inside the gills. Here they form two channels, separated by a thin partition: one - bringing, the other - taking out.

In decapods, including crayfish, the gills are placed in special gill cavities formed by the lateral folds of the cephalothoracic shield. In crayfish, the gills are arranged in three rows: the lower row is located on the protopodites of all thoracic limbs, the middle row is located at the places where the limbs are attached to the cephalothorax, and the upper row is located on the side wall of the body. In crayfish, 3 pairs of mandibles and 5 pairs of walking legs are equipped with gills. Water constantly circulates in the gill cavities, getting there through holes at the base of the limbs, in places where the folds of the cephalothoracic shield do not fit tightly to them, and exits at its front edge. The movement of water is due to the rapid oscillatory movements of the second maxillae and partly of the first pair of maxillae.

Crustaceans that have moved to terrestrial existence have special adaptations that provide breathing atmospheric air. In land crabs, these are modified gill cavities, in wood lice - limbs pierced by a system of air tubes.

Many small forms (copepods, etc.) do not have gills and respiration is carried out through the integument of the body.

Circulatory system

Due to the presence of a mixed body cavity - mixocele - the circulatory system is open and blood circulates not only through blood vessels, but also in the sinuses, which are sections of the body cavity. The degree of development of the circulatory system varies and depends on the development of the respiratory system. It is most developed in higher crustaceans, especially in decapods, which, in addition to the heart, have a rather complex system of arterial vessels. In other crustaceans, the vascular system is much less developed. Daphnia has no arterial vessels at all and the circulatory system is represented only by the heart in the form of a bubble. Finally, copepods and barnacles also lack a heart.

The heart of crustaceans, tubular or sac-shaped, is placed on the dorsal side of the body in the pericardial cavity - the pericardium (the crustacean pericardium is not connected with the coelom, but is a section of the mixocoel). Blood enters the pericardium from the gills, sufficiently enriched with oxygen. The heart communicates with the pericardium by paired slit-like openings equipped with valves - ostia. Crayfish have 3 pairs of ostia, crayfish with a tubular heart can have many pairs. With the expansion (diastole) of the heart, blood enters it through the ostia from the pericardium. With the contraction (systole) of the heart, the valves of the ostia close and blood is driven from the heart through the arterial vessels to various parts of the body. Thus, the pericardial region of the mixocoel performs the function of the atrium.

In crayfish, the system of arterial vessels is quite strongly developed. Three vessels extend forward from the heart to the head and to the antennae. Back from the heart there is one vessel that carries blood to the abdomen, and two arteries that flow into the lower abdominal vessels. These vessels branch into smaller ones, and eventually the blood enters the sinuses of the mixocoel. Having given oxygen to the tissues and receiving carbon dioxide, the blood is collected in the abdominal venous sinus, from where it is sent through the afferent vessels to the gills, and from the gills through the efferent vessels to the pericardial region of the mixocel.

excretory system

The excretory organs of crustaceans are altered metanephridia. In crayfish and other higher crustaceans, the excretory organs are represented by one pair of glands located in the head part of the body and opening outward through holes at the base of the antennae. They are called antennal glands. The gland is a complexly twisted drip with glandular walls, consisting of three sections: white, transparent and green. At one end, the canal closes with a small coelomic sac, which is the remnant of the coelom. At the other end, the canal expands into the bladder and then opens with an opening to the outside. The excretory glands of crayfish are also called green glands due to their greenish color. Substances released from the blood diffuse into the walls of the canal, accumulate in bladder and are released to the outside.

The rest of the crustaceans also have one pair of excretory glands of a similar structure, but they open outward not at the base of the antennae, but at the base of the second pair of maxillae. Therefore, they are called maxillary glands. In crustacean larvae developing with metamorphosis, the location of the excretory organs is reversed, namely: the larvae of higher crustaceans have maxillary glands, while the larvae of the rest have antennal glands. Apparently, this is due to the fact that the ancestors of crustaceans initially had two pairs of excretory organs - both antennal and maxillary. Subsequently, the evolution of crayfish followed different paths and led to the fact that in higher crustaceans only the antennal glands were preserved, and in the rest only the maxillary glands. Proof of the correctness of this point of view is the presence of two pairs of excretory glands in some crustaceans, namely in marine crayfish, from primitive higher crustaceans, and also in shellfish from lower crayfish.

Nervous system

The central nervous system of most crustaceans is represented by the ventral nerve cord and is very close to the nervous system of annelids. It consists of the supraesophageal ganglion (paired in origin), which forms the brain, connected with the subesophageal ganglion by peripharyngeal connectives. From the subesophageal ganglion comes a double ventral nerve trunk, forming a pair of contiguous ganglia in each segment.

In higher crustaceans, the nervous system reaches a relatively high level of development (the structure of the brain), while in other groups of crustaceans it has a more primitive character. An example of the most primitive structure is the nervous system of branchiopods, which have a head ganglion, near-pharyngeal connectives, and two relatively far-spaced nerve trunks extending from them. On the trunks in each segment there are small ganglionic thickenings, connected by double transverse commissures. In other words, the nervous system of these crayfish is built according to the ladder type.

In most crustaceans, there is a convergence of longitudinal nerve trunks, the paired ganglia of which merge together. In addition, as a result of the fusion of segments and the formation of body parts, their ganglia merge.

This process is associated primarily with the formation of the head (cephalization). So, the brain of crayfish (and other decapods) is formed by the head ganglion itself with two sections - the antennal and the antennal attached to it (the first pair of ganglia of the abdominal nerve chain that innervates the antennae). The subpharyngeal ganglion was formed by the fusion of the following 6 pairs of ganglia of the ventral nerve chain: ganglia innervating the mandibles, two pairs of maxillae, and three pairs of mandibles. This is followed by 11 pairs of ganglia of the abdominal chain - 5 thoracic and 6 abdominal.

On the other hand, fusion of ganglia may also take place due to the shortening of the body or small size in one or another group of crustaceans. Particularly interesting in this respect is the fusion of all the ganglia of the abdominal chain into one large knot observed in crabs.

sense organs

Crustaceans have organs of touch, organs of chemical sense (smell), organs of balance and organs of vision.

reproduction

With rare exceptions (barnacles), all crustaceans have separate sexes, and many have quite pronounced sexual dimorphism. Thus, the female crayfish differs in a noticeably wider abdomen and, as we know, in 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 (shieldmen, cladocerans, shellfish) parthenogenesis and alternation of parthenogenetic and bisexual generations take place.

Crayfish belong to a number of decapod crustaceans. This series is considered a highly organized species among crustaceans. It got its name from the number of limbs. This article will help to understand in more detail the external structure of crayfish.

The external structure of crayfish

The body of cancer is formed by eighteen segments, some of which merge with each other, making up two main sections:

cephalothorax;
abdomen.

The cephalothorax arose by the fusion of the head and the thoracic part of the body. Antennae, antennules and compound eyes are located at the base of the head part. Antennules in their structure are single-branched processes that serve for smell. Antennae - two-branched processes that extend from the first head segment and perform the functions of the organs of touch.

The thoracic region is formed from three head and eight thoracic segments and bears eleven pairs of limbs.

The limbs of crayfish include:

Three pairs of jaws;
Three pairs of fused mandibles;
Five pairs of walking legs.

Fig.1. The structure of crayfish

The structure of the limbs

Three pairs of jaws depart from the chest, one of them forms the upper jaw, the other two pairs form the lower jaw.

What have we learned?

The body of cancer consists of two sections, each of which has a special structure of limbs that perform separate functions. Their coordinated work ensures the movement of the animal different ways(walking, swimming). Also, the limbs play an important role for digestion, help work respiratory system and reproduction.

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