The class Crustaceans are mainly aquatic animals that inhabit seas and fresh water bodies. Their body is divided into the cephalothorax and abdomen. They have two pairs of antennae and compound or compound eyes. They breathe with gills. Total number known species - 20 000.
A typical representative is crayfish. Lives in fresh running waters. During the day it hides under stones or in holes dug at the bottom, or under the roots of trees.
At night they crawl out of their hiding places in search of food. Crayfish are omnivores. The cephalothorax of the cancer is formed from fused segments of the head and chest: The anterior part of the cephalothorax is elongated, pointed and ends with a sharp spine. At its base there are two compound eyes located on stalks, thanks to which the cancer can turn them into different sides. Compound eyes consist of many small ocelli - up to 3,000 and are called faceted. The cephalothorax of the crayfish bears two pairs of antennae. The long ones serve as organs of touch, and the short ones serve as organs of smell. Below the antennae are the mouthparts, which are modified limbs. The first pair forms the upper, and the second and third - the lower jaws, the remaining three pairs - maxillary. On the cephalothorax there are five pairs of jointed walking legs. Of these, the front pair of limbs has the most powerful organ of attack and defense - claw. The claw also serves as a food grabber. The segmented abdomen bears abdominal legs on which females carry eggs.
Crayfish are omnivores. Shredded oral organs food through the pharynx and esophagus enters the stomach, which consists of two sections - chewable And filter. On the inner walls of the chewing section there are chitinous denticles, with the help of which food is ground. In the filtering section - it is filtered and enters the intestine, and then into the digestive gland, where it is digested and absorbed nutrients.
Respiratory organs cancer - gills located on the sides of the cephalothorax. Oxygen penetrates into the blood flowing through the gill vessels, and oxygen is released from the blood. carbon dioxide. Circulatory system cancer is not closed and consists of saccular heart, lying on the dorsal side of the body, and the vessels extending from it.
The nervous system of cancer consists of large suprapharyngeal and subpharyngeal nerve nodes, forming a peripharyngeal ring, and the abdominal nerve cord.
Organs of cancer excretion - pair of green glands located in the head of the body. Their excretory channels open outward at the base of the antennae. Through the green glands, cancer dissolved in the blood is removed from the body harmful products metabolism.
Crayfish are dioecious. In winter, the female lays eggs, gluing each one to her abdominal legs. At the beginning of summer, young crustaceans emerge from the eggs (eggs), which the female carries on her legs for a long time.
The class Crustaceans belongs to several orders. Among them: decapod crustaceans, isopods, Cladocera, copepods, carp eaters.
Order Decapods. This includes the one described above crayfish, and planktonic shrimp species, large-sized sea crayfish - lobster, lobsters, various crabs. All of them are valuable food items and are used as food for preparing all kinds of delicious dishes. This group includes Cancer hermit leading a unique lifestyle. Young crustaceans find gastropods with shells of appropriate sizes, kill and eat them, and hide their abdomen in the shell. After each molt, the size of the crayfish increases and they have to look for a new mollusk with large size shells, and everything repeats from the beginning.
Order Isopods. This includes both aquatic and terrestrial crustaceans, the abdominal and thoracic limbs of which differ little, as, for example, in woodlice. These are small (up to 10-15 mm) gray or whitish animals that live in damp places, in leaf litter, some are even found in deserts.
Order Cladocera, whose representative is daphnia. Because of its way of moving by jumping, it is popularly called the “water flea.”
Order copepods, which refers to Cyclops. These are planktonic crustaceans that feed on many species of marine and freshwater commercial fish, and even such large animals as the baleen whale.
In total, there are about 50,000 species of crustaceans.
Crustaceans
Crustaceans are characterized by the presence of two pairs of whiskers and complex (compounded) 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.
Crayfish live in fresh running waters. Leads a nocturnal lifestyle. Its body consists of two sections - the cephalothorax and abdomen. 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; the three front pairs have 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 enters the stomach through the pharynx and esophagus, 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 influence 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 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 emerges, 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.
Crustaceans are intermediate hosts for a number of helminths. Thus, of the lower crustaceans, Cyclops and Diaptomus are intermediate hosts of the guinea worm (Dracunculus medinensis) and the broad tapeworm (Diphyllobothrium latum). Of the higher crayfish, decapod crayfish (Decapoda) are of medical importance. The freshwater crab (Eriocheir japonicus) is an intermediate host of the lung fluke (Paragonimus westermani).
Many R. are consumed by humans (lobsters, lobsters, crabs, shrimp, crayfish). Some R., making up the bulk of plankton, serve as food for commercial fish. R. contribute to the purification of water bodies, as they eat the corpses of dead animals.
Arachnids
Arachnids are mainly terrestrial arthropods. Only a few of them switched to living in water. Arachnids include various spiders, mites, and scorpions. Animals of this class differ from other arthropods in that their body consists of a fused cephalothorax and a usually undivided abdomen. They do not have antennae or compound eyes; move on four pairs of walking legs. Currently, more than 35 thousand species of arachnids are known.
Habitats of spiders, features of their structure and life activity. In the second half of summer, in forests and gardens, under various canopies, you can often see networks of cross spiders and the owners of the web themselves. Cross spiders can be recognized by their smooth spherical abdomen, on the dorsal side of which a cross-shaped pattern is visible. The house spider is common in residential premises and sheds. His net looks like a hammock. The silverback spider lives in ponds and river backwaters. In the water, he makes a home out of a web in the form of a small bell, filling it with air bubbles.
Spiders have a small cephalothorax and a large, unsegmented abdomen. On the cephalothorax there are eight simple ocelli, claw-like jaws and tentacles (organs of touch), and four pairs of walking legs. The legs of spiders end in comb-like claws. With their help, they weave trapping nets from cobwebs, which are produced in the arachnoid glands located at the posterior end of the abdomen. The ability to secrete cobwebs has provided spiders with high survival rate in nature: thanks to webs made from cobwebs, they catch prey, make cocoons that protect eggs from adverse influences, and run quickly.
Spiders are predators. They feed mainly on insects and other small arthropods. The spider injects a poisonous liquid into the caught prey, which kills the victim and acts as digestive juice. After about an hour, the spider sucks out the entire contents of the prey using a sucking stomach.
Spiders breathe atmospheric air. They have pulmonary sacs and trachea. The circulatory, nervous and other organ systems of spiders are approximately the same as those of other arthropods. Other arachnids. Small arachnids - mites - live in the soil, in plant organs, on the bodies of animals and humans. Their body is usually fused. They breathe using tracheas or lungs. In hot areas (in Central Asia, in the Caucasus, in the Crimea) live quite large arachnids - scorpions.
Unlike spiders, they have a long, segmented abdomen. Scorpions catch and hold prey with their tentacles, on which claws are developed. On the last segment of the abdomen, scorpions have a sting with ducts coming from the poisonous glands. They sting their prey, inject poison into it, and then eat it. Meaning of arachnids. Most arachnids destroy flies, which is of great benefit to humans. Many species of soil mites are involved in soil formation. Many species of birds feed on spiders.
There are many arachnids that cause great damage to human health and the number of commercial domestic animals. Of the spiders, the karakurt, which lives in Central Asia, the Caucasus and Crimea, is especially dangerous. Horses and camels often die from its poison. Scorpion venom is also dangerous to humans. The bite site becomes red and swollen, nausea and cramps appear. Only a doctor can provide the necessary assistance to the victim.
Scabies cause great harm. They can get into the skin of animals and humans, gnawing passages in it. From the eggs laid by the female, young mites emerge, which come to the surface of the skin and gnaw new passages. In humans, they usually settle between the fingers.
The most dangerous disease, spread by blood-sucking ticks - taiga encephalitis. The carrier of its pathogens is the taiga tick. Digging into human skin, it introduces the blood of encephalitis pathogens, which then penetrate the brain. Here they multiply and infect him.
Crustaceans are aquatic arthropods that breathe with gills. The body is divided into segments and consists of several sections: the head, thorax and abdomen or the cephalothorax and abdomen. There are two pairs of antennae. The integument of the body contains a special solid substance - chitin, and in some they are also strengthened (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 bodies; a few, for example, woodlice, palm thief, have switched to a terrestrial lifestyle.
Rice. 85. Various crustaceans: 1 - crab; 2 - hermit crab; 3 - shrimp; 4 - woodlice; 5 - amphipod; 6 - sea duck; 7 - shield
The structural features and vital functions of crustaceans can be considered using the example of crayfish.
Lifestyle and external structure. Crayfish live in various fresh water bodies with clean water: river creeks, lakes, large ponds. During the day, crayfish hide under stones, snags, roots of coastal trees, and in holes dug by themselves in the soft bottom. In search of food, they leave their shelters mainly at night.
Crayfish - pretty major representative arthropods, sometimes specimens over 15 cm in length are found. The color of crayfish is greenish-black. The entire body is covered with a durable and dense chitinous shell, impregnated with calcium carbonate.
Veils crayfish serve as an exoskeleton. Bundles of striated muscles are attached to it from the inside. The hard shell of the crayfish prevents the animal from growing. Therefore, cancer periodically (2-3 times a year) molts - sheds its old covers and acquires new ones. During molting, until the new shell gets stronger (this takes about a week and a half), the cancer is defenseless and cannot feed. At this time, he hides in shelters. The body of the crayfish consists of two sections - the cephalothorax and abdomen (Fig. 86). At the anterior end of the cephalothorax there is a pair of long and a pair of short antennae - these are the organs of touch and smell. The spherical eyes sit on long stalks. Therefore, cancer can look in different directions at the same time. In case of danger, it hides its eyes in the recesses of its shell.
Rice. 86. External structure crayfish: 1 - long antennae; 2 - short antennae; 3 - claw; 4 - walking legs; 5 - eye; 6" - cephalothorax; 7 - abdomen; 8 - caudal fin
Cancer's eyes are complex. Each eye consists of many very small eyes directed in different directions - facets (Fig. 87, B). The image of an object in a compound (compound) eye is composed of its individual parts, resembling mosaic pictures.
Rice. 87. Internal structure crayfish (female): A - overall plan body structure: 1 - stomach; 2 - liver; 3 - heart; 4 - blood vessels; 5 - ovary; 6 - intestine; B - diagram of the structure of the compound eye
The crayfish has limbs on its cephalothorax. If you turn it over on its back, then at the front end of the body you can find three pairs of jaws: a pair of upper jaws and two pairs mandibles. The cancer uses them to tear its prey into small pieces. The jaws are followed by three pairs of short jaws. They serve to bring food to the mouth. Both the mandibles and the maxillomandibles are modified legs. Behind the jaws are five pairs of walking legs. With the help of four pairs of these legs, the crayfish moves along the bottom of reservoirs. And the first pair of walking legs of the crayfish are turned into large claws. With them, the cancer grabs prey and tears off large parts from it. He defends himself with these same claws.
And on the abdomen the crayfish has short limbs (legs), the female has four pairs, the male has five pairs. At the very end of the abdomen there is a flat segment, on the sides of which modified, highly flattened legs are developed. Together they form the caudal fin. By sharply bending its abdomen, the crayfish pushes off the water with its tail fin, like an oar, and in case of danger can quickly swim backwards.
Digestive system(Fig. 87, A) begins with the oral 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 particles of food are retained and returned to the first section, while small particles enter the intestine. The liver ducts empty into the mid-intestine. Digestion of food and absorption of nutrients occurs in the intestines and liver. Ends digestive system anal opening located on the caudal segment of the abdomen. Crayfish feed on mollusks, insect larvae living in water, decomposing animal corpses, and plants.
Respiratory organs crayfish have gills. They contain blood capillaries and gas exchange occurs. The gills have the appearance of thin feathery outgrowths and are located on the processes of the jaws 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 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 at various organs. Here the hemolymph gives off nutrients and oxygen, and receives waste products and carbon dioxide. Then the hemolymph travels through the vessels to the gills, and from there to the heart.
Excretory system represented by a pair of green glands located in the front of the cephalothorax. They open outward at the base of the long antennae. Through these holes, harmful products that are formed during life are removed.
Nervous system. Cancer has a central nervous system- peripharyngeal nerve ring and ventral nerve cord and 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 depression 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 assess the position of its body in space.
Reproduction. Common crayfish sexual reproduction. Fertilization is internal. Fertilized eggs laid by the female (from 60 to 200 pieces) are attached to her abdominal legs. Eggs are laid 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 feed only on plant foods.
Rice. 88. Young crustaceans on the abdominal legs of the female
Decapods include crayfish, large sea crayfish - lobsters (up to 60 cm long and weighing up to 15 kg) and lobsters (they lack claws), small crustaceans - shrimp. Some of them move along the bottom, others actively swim in the water column using their abdominal legs. Hermit crabs belong to the same group. They have a soft, unsegmented abdomen. Hermit crabs hide from enemies in the empty shells of sea snails, carrying the shell with them all the time, and in case of danger, completely hiding in it, covering the entrance with a highly developed claw. Decapods include crabs. They have a wide but short cephalothorax carapace, very short antennae, and a short abdomen tucked under the cephalothorax. Crabs most often move sideways.
Listopods include small crustaceans well known to aquarists - daphnia 3-5 mm long (Fig. 89, 1). They live in small fresh water bodies. The entire body (except for the head) of daphnia is enclosed in a transparent chitinous shell-shell. Through the chitinous covers one can see a large complex eye and constantly working chest legs, which ensure the flow of water under the shell. Daphnia has large branched antennae. By flapping them, she jumps in the water, which is why daphnia is sometimes called “water fleas.” Daphnia feed on protozoa, bacteria, and single-celled algae found in the water column.
Rice. 89. Crustaceans: 1 - daphnia: 2 - cyclops
Freshwater bodies of water are inhabited by a small crustacean that vaguely resembles a wood lice - the water donkey. Multipods are small (up to several centimeters) crustaceans that swim on their sides, for which they are called amphipods. Using different legs, crustaceans can swim, walk along the bottom of reservoirs, along the wet soil of banks, and also jump. Barnacles are small crustaceans that lead an attached lifestyle as adults, for example sea acorns. They live in the sea. Their entire body is covered with a calcareous shell-house. Most often, the shell is attached to stones, crab shells, the bottoms of ships, and the skin of whales. Barnacles catch their prey (planktonic organisms) using long, movable chest legs.
Crustaceans are primary aquatic arthropods with a hard and durable chitinous shell, impregnated with calcium carbonate, and articulated limbs located on the thoracic and abdominal regions. Crustaceans breathe using gills.
Exercises based on the material covered
- Find out, using Figure 86, what features arthropods have in their external structure. Name their similarities with annelids.
- What is the difference between the internal structure of crustaceans and representatives of other classes of arthropods? Explain using the example of crayfish.
- What are the structural features of the sensory organs of crayfish?
- Use a few examples and pictures to show the diversity of the class. Describe the habitats of crustaceans.
- What is the role of crustaceans in nature?
Crustaceans - primary water animals, therefore, as respiratory organs they have special outgrowths of the limbs - gills. Representatives of this class differ from all other arthropods by the presence on the head two pairs of antennae. The limbs of crustaceans often retain a primitive two-branched type of structure.
Crayfish. Let us consider the main morphophysiological features of this class using the example of good famous representative- river crayfish.
External structure and lifestyle. Crayfish lives in fresh waters: rivers, streams, lakes. The presence of crayfish in a pond indicates the purity of the water. Crayfish lead an active nocturnal lifestyle, and during the day they hide under stones, snags or in burrows. Crayfish are omnivores; they feed on both plants and animals, including their decaying remains. The size of an adult cancer reaches 20 cm or more.
On the outside, the cancer is covered with a hard chitinous shell, which serves reliable protection from enemies. The dark greenish-brown color of the shell makes the crayfish invisible at the bottom. Like all crustaceans, the body of the crayfish consists of the head, thoracic and abdominal sections. However, its structure has some peculiarities. The external structure and size of crustaceans can be very diverse. Thus, in some primitive forms the segmentation of sections is almost homonomic, and one part of the body imperceptibly passes into another. In more highly organized species, body parts are clearly differentiated. The head of a crayfish consists of a head lobe (acron), on which the first pair of antennae is located (antennas 1, or antennules, and 4 segments (Fig. 42).
Rice. 42. Limbs of a female crayfish: 1 - antenna, 2 - antenna 11, 3 - limbs of the head, 4 - limbs of the chest, 5 - limbs of the abdomen
The limbs of the first segment represent the second pair of antennae (antennas), much longer than the antennules. The antennae are mobile and serve for touch and smell. The remaining 3 segments of the head also bear modified 4 limbs: on the second segment - upper jaws (mandibles), on the third and fourth - two pairs of lower jaws (maxill). The jaws surround the mouth opening and form an oral apparatus that crushes food and delivers it to the mouth.
The chest consists of 8 segments. The first 3 segments are equipped with paired jaws, involved in grinding, sorting and transferring food particles to the oral apparatus. The next 5 segments are carried in pairs walking legs. Powerful claws on the first pair of walking legs are used to capture food, attack and defend. The cancer uses the remaining walking legs for movement.
The abdomen consists of six movably articulated flattened segments. The first two abdominal segments of males are equipped sexual legs, shaped like tubes. With their help, sperm is transferred to the female genital tract. In females these legs are vestigial. On the following segments there are small two-branched swimming legs. On the last, sixth segment of the abdomen, the swimming legs are greatly enlarged and, together with the wide anal lobe, form caudal fin.
It should be noted that the head of the crayfish consists of two articulated sections: the protocephalon and the gnathocephalon. The protocephalon is formed by the fusion of the head lobe and the first head segment, and the gnatocephalon by the fusion of the three subsequent head segments bearing the jaws. Moreover, the gnatocephalon fuses with thoracic region, forming the so-called jaw-thorax (gnathothorax), covered on top and sides by a strong solid shell - caropax. Thus, the body of the crayfish is divided into the following sections: head - progocephalon (acron and one segment), maxillary thorax - gnathothorax (three cephalic and eight thoracic segments) and abdomen (six segments and anal lobe). Quite often in textbooks they talk about dividing the cancer body into the cephalothorax and abdomen. As you can see, this is not entirely correct, because the head lobe and the first segment of the head section are not fused with the underlying segments.
In a calm state, the crayfish moves along the bottom on its walking legs, head first. At the moment of danger, the crayfish, straightening its tail fin, sharply and often bends its abdomen and quickly swims away backwards with jerks.
COVERS. In primitive crustaceans, the integument is relatively thin and is formed by cuticle plates that cover the body on all sides. However, in crayfish and other highly organized forms, the outer integument thickens and forms a hard shell. Outer layer The cuticles are impregnated with salts, which significantly increases the strength of the integument.
The shell reliably protects the animal's body, but does not allow it to grow. Therefore, the growth and development of crayfish occurs during periodic molts. Young crayfish grow quickly and therefore molt several times a year, adult crayfish molt much less frequently - once a year. After shedding the old cuticle, the new cuticle remains soft and easily stretchable for some time. At this moment, the crayfish become vulnerable to enemies and hide in shelters. Then the cuticle hardens, becomes saturated with lime, and the animal’s growth stops until the next molt.
Digestive system. The digestive system begins with the mouth opening, covered with outgrowths of the cuticle - the upper and lower lip. The foregut includes a short esophagus and stomach (Fig. 43). The cancer stomach consists of two sections: chewable And filtering (nuloric). The inner walls of the chewing section bear powerful chitinous plates, with the help of which food is finely ground. There are also white rounded calcareous thickenings - millstones. They accumulate calcium carbonate, which is necessary for the cancer to saturate the cuticle after molting. In the filtering section of the stomach, thin outgrowths of the cuticle form a sieve through which only highly crushed food is filtered. From the stomach, food is sent to the short midgut. In most crustaceans, the midgut has lateral glandular projections, not entirely correctly called liver. In crayfish, the liver is formed by two independent lobes (right and left), the ducts of which flow into the midgut. The liver produces digestive enzymes that enter the chewing stomach. Cavitary and intracellular digestion and absorption of nutrients coming from the midgut also occur in it. 
Rice. 43. Internal structure of crayfish (female):
1 - antennas II, 2 - antennas 1 (antennas), 3 - eye, 4 - stomach, 5 - digestive gland, 6 - arteries, 7 - ovary, 8 - heart, 9 - abdominal nerve cord, 10 - hindgut, 11 - gills
Since the liver of crustaceans combines the functions of the liver and pancreas, zoologists prefer to call this organ simply the digestive gland. Since the liver partially performs the functions of the midgut, in the class of crustaceans there is an inverse relationship between the development of the midgut and the liver. For example, daphnia have a small liver and a long midgut, while in crayfish the midgut is a short tube, the length of which is 10 times less than the hindgut.
Undigested food remains enter the long rectum, which passes through the abdomen and opens with an opening in the anal lobe.
Front and hindgut, having an ectodermal origin, are lined with a cuticle, which peels off during molting and comes out in the form of tubes. Therefore, during molting, crayfish do not feed.
Breath. Crayfish breathes with gills (see Fig. 43). They are located under the carapace in the gill chambers and are reliably protected from damage. Fresh water constantly enters the chambers due to the flow of water created by the limbs. The gills are delicate, numerous thread-like outgrowths of the thorax limbs, covered with a thin cuticup, into which the body cavity extends. Gas exchange occurs through the thin coverings of the gills. Hemolymph, passing along the gill filaments, is saturated with oxygen and releases carbon dioxide.
Many small crustaceans with a thin cuticle lack gills and breathe over the entire surface of the body. Land crustaceans have special respiratory organs. Thus, woodlice have deep branching invaginations of the integument on their abdominal legs, reminiscent of trachea, in which gas exchange occurs.
Circulatory system. Circulatory system open The heart of cancer is located on the dorsal side of the chest and is a pulsating pentagonal muscle pouch with three pairs of holes (osty)(see Fig. 43). When the heart contracts, hemolymph is pushed into the branching arteries and distributed throughout the body. From blood vessels it pours into the body cavity, washes the internal organs, gradually releases oxygen and goes to the gills. After being saturated with oxygen in the gills, the hemolymph enters the pericardium and from it through the ostia into the heart.
Excretory system. Excretory organs of crayfish - green glands, received this name for their color. They are located in the anterior part of the maxillary thorax. Interior The gland, which looks like a small sac, is a remnant of the coelom and opens into the body cavity. It is followed by a thin convoluted tubule consisting of several sections, the last of which expands into the bladder. From Bladder a short canal emerges, which opens outwards with an excretory opening at the base of the second pair of antennae.
Nervous system. The nervous system of cancer consists of a well-developed brain connected by the peripharyngeal nerve ring to the ventral nerve cord (see Fig. 43). From the brain, nerves travel to the eyes and sensory antennae. From the peripharyngeal ring - to the oral apparatus, and from the nodes of the abdominal nerve chain to the rest of the limbs and internal organs bodies.
Sense organs. The sense organs are well developed. The organs of touch and chemical sense are located on the head antennae. At the base of the first pair of antennae are the organs of balance - statocysts.
The equilibrium organs of the crayfish are located at the base of the antennules and are open sac-like invaginations of the integument, communicating with environment. The bottom of the statocysts is lined with a thin cuticle with sensitive hairs. Sand grains entering the statocyst through its external opening act as statoliths. When the position of the crayfish body in space changes, the statoliths irritate the hairs, and corresponding nerve impulses enter the brain. During molting, the cuticular lining of the statocyst is also torn off, so during this period the crayfish loses coordination of movements.
Complex faceted eyes consist of numerous simple ocelli, each of which functions individually and perceives the image of only part of the surrounding space. As a result, the total visual perception is made up of individual fragments. This vision is called mosaic. The eyes of cancer are mobile, they sit on special outgrowths - eyestalks.
Reproduction and development. Crayfish are dioecious, with pronounced sexual dimorphism. In females, unlike males, the abdomen is wider than the thoracic segments. The first pair of abdominal limbs of the male is transformed into a copulatory organ; in females the legs are rudimentary. In the maxillary thorax there are unpaired gonads with paired reproductive ducts, opening genital openings at the bases of the third (in females) and fifth (in males) pairs of thoracic walking legs. In late autumn or winter, mating occurs, during which the males, using the first pair of abdominal legs, glue packets of sperm next to the genital openings of the females. After this, the females lay eggs, which are glued to the abdominal legs. In this case, the abdomen is pressed against the cephalothorax, forming a brood chamber. Fertilization and development of eggs occurs inside the chamber. In the spring, small crustaceans hatch from the eggs, which remain on the mother’s abdomen for some time. Then the crayfish leave the female and move on to independent life.
Crustaceans have a wide variety of shapes and sizes of male gametes. In many species, gametes are very large and completely immobile. For example, some small crustaceans, whose length is less than 1 mm, have the longest sperm of all animals - they are an order of magnitude longer than the crustacean itself and reach 6 mm! Remember that male gametes that do not have organelles of movement are called sperm. It’s the same in botany: motile gametes of spore plants are called sperm, and immobile gametes of seed plants are called sperm.
Rice. 44. Commercial crustaceans: A- Kamchatka crab; B- lobster; IN- lobster
The importance and diversity of crustaceans. Crustaceans are found in almost any body of water. Their numbers and biomass are very high, so crustaceans play an important role in aquatic ecosystems.
The plankton of fresh and marine waters is home to many small crustaceans that feed on unicellular algae. In turn, they serve as food for larger animals - from fish fry to whales. Thus, small crustaceans (cladocera and copepods, shrimp, etc.) represent an important link the food chain any aquatic community.
Among crustaceans there are many valuable commercial objects that humans eat: shrimp, lobsters, lobsters, Kamchatka and other crabs (Fig. 44). The crustacean fishery is widely developed and reaches 700 thousand tons per year in the world. Freshwater crayfish They are not only caught in the wild, but also successfully bred on specially created farms. In fish hatcheries, small crustaceans (for example, daphnia) are grown as food for fish.
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