Most bivalve molluscs are inhabited. Bivalve class - bivalvia. Sedimentary rock formation

BIVALVES BIVALVES

(Bivalvia), a class of shellfish. Known from the early Paleozoic (modern D. m. - from the lower Ordovician), they reached the greatest species diversity in the Cretaceous. The body (length from several mm to 1.5 m, weight up to 30 kg) is bilaterally symmetrical, consists of a laterally flattened torso and legs. There is no head (hence one of the names - Acephala). In most, the foot is wedge-shaped (hence another of the names - Pelecypoda), in primitive forms it has a crawling sole, in those leading a motionless lifestyle it is reduced (mussels) or disappears completely (oysters). Many D. m. have a byssal gland on the leg that secretes strong threads (byssus), with the help of which the mollusk is attached to the substrate. The body is covered with a mantle, freely hanging in two folds, which can grow together, at the posterior end of the body there is a pair of long or short siphons. The shell consists of two valves (from a few mm to 1.4 m long), covering the body from the sides; some of them are lined with mother-of-pearl from the inside. The edge of the valves carries protrusions (teeth) that form a castle, the structure of which is one of the systematic. signs. The valves are closed by 1-2 adductor muscles, their antagonist, the elastic ligament, keeps the valves half-open. In some D. m. (pearl oysters, mussels, toothless), foreign particles that fall between the mantle and the shell shell are enveloped in layers of mother-of-pearl and turn into pearls. The mouth is equipped with two pairs of lobes. Stomach with a blind sac-like outgrowth bearing crystalline inside. stalk (takes part in digestion and supplies the body of the mollusk with oxygen under anaerobic conditions). The liver, unlike other molluscs, is three-lobed. In some (Solemyidae) (eg, Solemii) the liver and digestion. the system is completely reduced. The gills of primitive forms are bipinnate, in some they turn into a muscular septum, and in the majority - into gill plates (hence another name for D. m. - Lamellibranchia). The nervous system consists of three pairs of ganglia. The sense organs are poorly developed; some (scallops) have inverted eyes along the edge of the mantle or siphon, at the base of the gill plates there are primitive osphradia, there are statocysts. The circulatory system is not closed. Most are dioecious, rarely hermaphrodites. At nek-ry forms sexual dimorphism is sharply expressed (eg, Thecaliacon camerata). Fertilization is usually external. Many species development with a floating larva (veliger, glochidia). Some have developed care for offspring - gestation of juveniles in the gills (Unionidae), in brood chambers (eg, Mineria minima). It is customary to distinguish 3 orders: even-toothed (Taxodonta), heteromuscular (Anisomyaria), lamella-branchia proper (Eulamellibranchia), according to another system - 3 superorders with 14 orders, uniting 130 modern. families. OK. 1000 genera, including tridacna, zebra sen, mii, pinna, folads, cockles, makoms, trigonias, marrows, toothless. OK. 20,000 species (in the USSR there are 50 genera with 200 species in fresh water bodies, and about 160 genera with 400 species in the seas and brackish waters). Widely distributed throughout the world. from the littoral to the abyssal, as well as in fresh waters. Bottom sedentary animals. At the depth 100-200 m in terms of biomass and population density are often b. including benthic fauna. Filter feeders, detritus and plankton feeders, rarely predators; in some (tridacna, solemia) symbiosis with zooxanthels and thiobacteria. I write many fish and other sea. animals. Some marine D. m. drill wood and stones, many participate in fouling, causing great harm to ships and hydrotechnical. structures. Object of fishing (annual catch 2.9-3.1 million tons in 1978-80) and aquaculture. See also fig. in table. 31 and 32.

.(Source: Biological encyclopedic Dictionary." Ch. ed. M. S. Gilyarov; Editorial: A. A. Babaev, G. G. Vinberg, G. A. Zavarzin and others - 2nd ed., corrected. - M.: Sov. Encyclopedia, 1986.)

bivalve mollusks

Mollusk class. Includes approx. 20 thousand species widely distributed in the World Ocean, as well as in fresh water bodies (for example, toothless). Bivalve mollusks are benthic sedentary animals that inhabit marine and fresh waters. Their bilaterally symmetrical, laterally compressed body consists of a trunk and a leg (there is no head) and is entirely enclosed in a calcareous shell. It has two symmetrical doors (hence the name) dl. from a few millimeters to 1.4 m. Outside, the shell is covered with a stratum corneum; inside, it is often lined with a layer of mother-of-pearl. The shell valves are connected by dorsal edges, closed with the help of two muscles passing in the body of the mollusk from one valve to another. When the muscles are relaxed, the valves move apart, when they are compressed, they close. Inside the shell is the actual body of the mollusk, covered with a mantle, which hangs freely from the sides in the form of two large folds. Under the mantle on each side are 2 gills, between which the leg is located. With the help of the foot, the molluscs slowly (20-30 km/h) crawl along the bottom. In case of danger, they pull in their leg and slam the shell shut. If a foreign particle (for example, a grain of sand) gets between the mantle and the shell shell, it is enveloped in mother-of-pearl and turns into a pearl. The main suppliers of pearls are marine pearl oysters living in shallow waters in tropical seas. They are hunted and bred for pearls. artificially bred scallops, as well as mussels and oysters, which form large clusters (so-called banks) and are used for food.

Bivalve mollusks feed by filtering small organisms and organic particles from the water passing through the mantle cavity. Water enters and exits through siphons (holes in the back of the mantle between the right and left folds).

.(Source: "Biology. Modern Illustrated Encyclopedia." Editor-in-Chief A.P. Gorkin; M.: Rosmen, 2006.)

See what "Bivalve mollusks" are in other dictionaries:

Bivalves Tridacna Scientific classification Kingdom: Animals Subsection ... Wikipedia

bivalves- 17. Bivalve mollusks filtering lamellar gill mollusks... Source: SanPiN 2.3.4.050 96. 2.3.4. Enterprises of the food and processing industry ( technological processes, raw material). Production and sale of fish products. ... ... Official terminology

- (Bivalvia) a class of bilaterally symmetrical aquatic invertebrates such as molluscs. The shell consists of 2 valves covering the body of the mollusk from the sides. On the dorsal side, the valves are interconnected by an elastic bridge with a ligament ...

Tridacna (Trid ... Wikipedia

SHELLS, representatives of more than 80,000 species of invertebrates of the Mollusca phylum. These include well-known snails, bivalves and squids, as well as many lesser-known species. Originally inhabitants of the sea, mollusks are now ... ... Scientific and technical encyclopedic dictionary

Soft-bodied (Mollusca), a type of invertebrate. Arise presumably in the Precambrian; from the Lower Cambrian already known several. classes M. Probably descended from low-segment worm-like ancestors (annelids) or directly from flat ones ... ... Biological encyclopedic dictionary

"Mollusk" redirects here; see also other meanings. Shellfish ... Wikipedia

- (Mollusca) (from lat. molluscus soft), soft-bodied, type of invertebrates. 7 classes: Gastropods, Monoplacophores, Shellfish, Chute-bellied mollusks, Bivalves, Spadefoot mollusks and ... Great Soviet Encyclopedia

Class of marine and freshwater shell mollusks. Shell (length from a few millimeters to 1.4 m) of 2 valves connected on the dorsal side. About 20 thousand species. Widely distributed in the oceans, as well as in fresh waters. They live on ... ... encyclopedic Dictionary

This type of animal has about 130 thousand species. Mollusks live in fresh and marine waters, on land. They are known from the first half of the Paleozoic; they arose from polychaete rings as a result of the following aromorphoses:

the appearance of the mantle, mantle cavity and shell;
fusion of all segments of the body;
concentration of the nervous system in the ganglion;
the formation of the heart, consisting of the ventricle and atria.

The following idioadaptations contributed to the biological progress of molluscs:

the appearance of a shell;
the emergence of an apparatus for grinding food - radula;
the emergence of two forms of respiration - gill and pulmonary;
high fertility.

Features of the structure of mollusks:

bilaterally symmetrical animals, many of them have a pronounced asymmetry as a result of displacement of organs;
the body is not divided into segments;
secondary cavity animals; in general - residual, represented by the pericardial sac;
the body consists of a head, trunk and legs - a muscular unpaired outgrowth of the abdominal wall, which serves to move the animal;
the body is surrounded by a skin fold-mantle. Between the mantle and the body is the mantle cavity. Gills and some sense organs are located in the cavity. The excretory, genital and anus openings also open here;
on the dorsal side of the body is a shell secreted by the mantle. Shells can be solid and bivalve and have a wide variety of shapes;
part digestive organs the liver enters, the ducts of which flow into the middle intestine;
respiratory organs - primary gills (ctenidia), secondary gills or lungs (under the mantle cavity);
excretory organs are the kidneys, communicating with the inner ends with the pericardial sac, and the outer ends opening into the mantle cavity;
nervous system diffuse-nodular type, four longitudinal nerve trunks depart from the peripharyngeal ring, carrying several pairs of ganglia;
the sense organs are represented by the eyes, the organs of smell, balance and chemical sense;
sexual reproduction; molluscs are mostly dioecious animals; there are hermaphrodites. A larva emerges from the eggs - a trochophore, resembling the larva of annelids. Sometimes a larva develops immediately - a veliger, which turns into adult form. There are forms with direct development, in which a small mollusk emerges from the egg.

Class Gastropoda

These animals live in sea and fresh waters, there are also terrestrial forms. Their sizes are different - from a few millimeters to 60 cm (sea hare).

Structural features of gastropods:

body differentiated into head, trunk, and sole;
the shell is entire, sometimes reduced;
the body is asymmetrical, which is associated with the reduction of the right organs of the mantle complex. Shells are twisted spirally or have the form of a cap;
the shell consists of a thin outer and porcelain-like layer - several systems of calcareous plates crossing each other at right angles, some have a mother-of-pearl layer;
sense organs are represented by tentacles, a pair of eyes, chemical sense organs, statocysts - balance organs;
the nervous system is well developed;
digestive system represented by the digestive tract and glands (salivary and liver). There is a radula in the throat. IN oral cavity distinguish "jaws" - horny or calcareous thickenings of the cuticle. The anus opens in front of the body;
respiratory system represented by ctenidia (gills), and in terrestrial forms - by lungs;
circulatory system open, formed by the heart and blood vessels. From the ventricle of the heart departs the aorta, which is divided into head and internal. The heart contains arterial blood. Gas exchange takes place in lacunae connective tissue. The blood becomes venous and returns to the gills, where gas exchange between the body and the external environment also takes place. Blood is colorless;
excretory system initially consists of a pair of kidneys, of which one is reduced;
reproduction. There are both dioecious and hermaphrodite. Most fertilization is internal. From the eggs in the lower gastropods, a larva develops - a trochophore, which turns into a veliger (sailboat). An adult mollusk develops from a sailboat. In higher development, direct development occurs inside the egg.

Representatives of gastropods: grape snail, small And big pond slugs, slug.

Class Bivalve

This class contains about 1500 species.

The most progressive animals. There are about 700 species. The body of cephalopods is differentiated into the head, torso, and tentacles, into which the leg has turned. Their shell is underdeveloped, preserved only on the dorsal side. There is a radula. The ducts of the ink sac open into the hindgut. Animals breathe with gills. The circulatory and excretory system is similar in structure to the excretory system of bivalves and gastropods. Cephalopods have a well-developed nervous system and vision. The eyes are capable of accommodation. These are dioecious animals with pronounced sexual dimorphism. Their fertilization is internal, development is direct. All representatives of the class are predators. Cephalopods: squid, octopus, cuttlefish.

Bivalves- This is a class of type Mollusks, including about 20 thousand species that live mainly in marine as well as fresh waters. Often they breed strongly in the coastal shallow waters of the seas. Another name for bivalves - laminabranchial. Representatives are barley, toothless, mussels, oysters, scallops, pearl mussels, zebra mussels, etc.

The body size of bivalve mollusks varies depending on the species from a few mm to more than a meter.

There are no freely moving forms among the laminabranchs. All of them are either inactive or immobile. Sedentary mollusks can burrow into the ground.

Bivalves are filter feeders. They feed on plankton and organic particles suspended in water. At the same time, the water is purified.

characteristic feature is the presence of a sufficiently large shell, consisting of two wings(which gave the name to the class), covering the body from the sides. Thus, bivalves have bilateral symmetry, with the body laterally flattened. The size, shape and color of the shell varies depending on the species. During the life of the mollusk, the shells grow with their edges. In unfavorable periods of the year, growth stops, so there may be stripes on the shell.

The shell valves are connected by a ribbon of elastic substance from the dorsal side of the bivalve mollusk. The closure of the valves is provided by special muscles-contactors (one or two), with the reduction of which the valves close, and when relaxed, they diverge. In a number of bivalves, the shell is lined from the inside with mother-of-pearl, which has strength and brilliance. If a grain of sand gets between the mantle and the shell shell, then it is enveloped in mother-of-pearl and gradually turns into a pearl.

Bivalve mollusks have a completely reduced head, many species do not even have legs. If there is a leg (for example, toothless), then it serves for movement. In this case, the leg protrudes, attaches to the ground, after which the shell with the body of the animal is pulled up to it.

Inside the shell, the mantle hangs down the sides of the body, forming folds. Between the mantle and the body are lamellar gills. In many, at the posterior end of the body, the mantle grows together, forming a couple of siphons tube-like.

Bivalve mollusks do not have a head; along with it, the pharynx with a grater, characteristic of gastropods, is lost. The mouth is in front at the base of the leg, surrounded by lobes. Next comes the esophagus, followed by the stomach, into which the ducts of the liver flow. The intestine winds at the base of the leg, then goes towards the back, passes near the heart. The anal opening is located at the posterior end of the body.

On the gills of the laminabranchs there are many oscillating cilia that create a current of water in the mantle cavity. Water with food particles is sucked in through the inlet siphon, and ejected through the outlet. From the water, the mollusk precipitates edible particles and sends them to the mouth with the help of near-oral lobes. Also, oxygen is absorbed from the water by the gills, and carbon dioxide is released into the water. Excrement is excreted from the anus into the mantle cavity, and metabolic products are excreted from the kidneys. All of them are excreted with a current of water through the outlet siphon of the mollusk.

The respiratory organs are lamellar gills located under the mantle to the right and left of the body of the mollusk.

The circulatory system has a structure characteristic of the entire type of Mollusk. She is unclosed. The heart of a bivalve consists of two or more atria and one ventricle.

Bivalves have two kidneys. Their structure is approximately the same as that of the whole type.

The nervous system of the laminabranchs usually consists of three pairs of ganglia (above the pharynx, in the leg, in the back of the body) with nerves extending from them. The ganglia are interconnected by nerve trunks (commissures).

The sense organs are poorly developed. The leg contains balance organs (statocysts). There are tactile cells in the oral lobes, leg, mantle and gills. Some bivalves have small tentacles on the edge of the mantle. There are chemical sense organs at the base of the gills. Sometimes they are in large quantities eyes on the edge of the mantle.

Most bivalve mollusks have separate sexes, fertilization is external. In this case, often the spermatozoa released through the siphon of the male enter the mantle cavity through the siphon of the female. This is where fertilization takes place.

Bivalvia - Lamellibranchiata

Bivalve molluscs General characteristics

bivalves constitute the second largest group among modern mollusks. There are up to 20,000 species in the modern fauna. The vast majority of bivalve mollusks live in the seas, while some have adapted to life in fresh waters, such as barley. toothless, freshwater pearl mussel, zebra mussel, sharovka, etc.

Among mollusks, bivalves are characterized by low mobility. Even the slow movement of snails will seem very fast compared to the movement of toothless and barley. So, the speed of toothless movement along the bottom does not exceed 20-30 cm per hour. Some of the bivalves lead a completely immobile lifestyle. Attached to the substrate in the larval stage, they remain attached for the rest of their lives (oysters and some other marine mollusks). All bivalves are benthic animals living at various depths, from the tidal zone to deep sea trenches (almost 10 km).

Most bivalves feed on phytoplankton or plant detritus brought into the mantle cavity by water flow. This passive way of feeding explains both the very low mobility of bivalves and the development of a bivalve shell in them, which completely hides the body of the mollusk.

External structure

As the name of the class itself shows, bivalves have a shell, which in most forms consists of two symmetrical valves connected on the dorsal side by a ligament. The shell, with very rare exceptions, covers the entire body of the mollusk, and only the edges of the mantle, part of the leg, and sometimes the oral lobes can protrude from it.

Body a mollusk that fits in a shell is usually laterally compressed and consists of a body occupying the dorsal part of the shell and a leg located on the ventral side. The head is reduced, so bivalves are also called headless. Two mantle folds hang down on the sides of the body on the right and left, lining the inside of the shell and limiting the vast mantle cavity containing the leg and organs of the mantle complex.

shell shape can be quite different. In many bivalves, both valves may not be symmetrical, one more, the other less convex (oysters, scallops, etc.). The shell usually consists of the three layers described above: conchiolin, prismatic, and mother-of-pearl. Many bivalves have a well-developed mother-of-pearl layer (pearl, sea pearl, freshwater pearl, etc.).

The thickness and strength of the shell of both bivalves and other mollusks is determined by the conditions of their life. The marine environment provides more favorable conditions for the development of the calcareous skeleton (sponges, coral polyps, molluscs) due to great content salts. However, even in freshwater mollusks, the shell is not of the same strength.

In toothless, it is thin and brittle, while in barley it is much thicker and stronger. This is due to the fact that toothless live in the muddy ground of quiet, stagnant or low-flowing water bodies, while the barley lives on the sandy bottom of the rivers. The shell of the freshwater pearl mussel is even stronger - it is a resident of the northern fast-flowing and rapids rivers. Of marine mollusks, the shell reaches the greatest strength in species that live in the surf zone and in the tidal zone.

The shell is secreted by the epithelium of the mantle, and it grows along the entire margin, except for the junction of the valves. In most bivalves, annual layers of shell growth are clearly visible. All three layers of the shell are distinguished by the edge of the mantle. However, the surface of the mantle adjacent to the shell retains the ability to release the substances from which the shell is built, especially the mother-of-pearl layer, which thickens with age.

Many mollusks form pearls, but the sea pearl mussels from the genera Pteria and Pinctada (Pteria, Pinctada), living in the Indian and Pacific Oceans, and freshwater pearl mussels (Margaritana margaritifera) are the subject of fishing. Ancient Russian pearls were mined from freshwater pearl oysters found in northern rivers and lakes.

The shell valves are connected on the dorsal side in two ways: with the help of a ligament and a lock.

The ligament is an elastic cord composed of conchiolin. The shell valves open due to the elasticity of the ligament, which stretches the valves, and they close when the closure muscles contract. Toothless, barley, and other molluscs usually have two muscle-contactors - anterior and posterior, but many molluscs have only one. These are strong muscles that attach with their ends to the shell valves. Muscle attachment points are clearly visible on the shell valves.

The lock is part of the shell sash. It is located anterior to the ligament on the dorsal side. On one valve there are calcareous teeth, which correspond to depressions on the other valve. The lock does not connect the leaves like a ligament, but only ensures their correct position and prevents the leaves from moving relative to each other.

Structure

Mantle and mantle cavity

The mantle usually covers the entire body of the mollusk. If its lateral folds correspond to the size of the shell and do not fuse with each other, then such a mantle is called free. In some areas of the mantle edge, various thickenings, folds, tubercles, or papillae can often appear, preventing its two lateral folds from closing tightly. Thus, in toothless and barley, the edges of the mantle are not closed in two places located one above the other at the posterior end of the body, forming two siphons. The lower siphon is called gill siphon, water enters it; the upper one is cloacal, water comes out through it. The edges of the gill siphon are covered with sensitive papillae. The folds of the mantle move apart where the leg is located. When the toothless leg protrudes from the shell, the mantle on both sides fits snugly against it. In some mollusks, the edges of the mantle can grow together along the entire length, except for the siphons and the hole for protruding the leg, and the siphons in many forms that burrow into the ground turn into long tubes.

In the mantle cavity of bivalves, there are a large number of different organs: leg, gills, osphradik, oral opening, oral lobes, excretory openings, anus, openings of the excretory ducts of the sex glands. In the mantle cavity, water constantly circulates, which washes the gills, which ensures breathing. Small plankton and particles of plant detritus are brought with water, which are necessary for nutrition. From the mantle cavity with water, excrement and excretion products of the kidneys are carried out.

The leg of a bivalve mollusk is an ventral, muscular part of the body, often wedge-shaped and capable of protruding from under the shell. With the help of the foot, the molluscs burrow into the ground or move very slowly. In fixed forms, the leg can be reduced (oysters). In many bivalves, for example, in the edible seashell mussel, a byssal gland is located in the leg, which secretes organic matter in the form of very strong byssus threads, with the help of which the animal is attached to the substrate - to stones, piles, etc. In mollusks that do not have byssal gland in adulthood, it can be developed in larvae.

Respiratory system

In the lamellar gills proper (barley, toothless, etc.), two long gill plates hang from the ceiling of the mantle cavity on both sides of the leg. Each plate is double, lattice, with a complex system of crossbars. The gills are covered with ciliated epithelium. The circulation of water in the mantle cavity is due to the beating of the cilia of the mantle epithelium, gills, and oral lobes. Water enters through the gill siphon, washes the gills, passes through the cribriform plates, then enters the supra-gill chamber through the opening behind the leg, and exits it through the cloacal siphon.

In some groups of bivalves, the gills are arranged differently, and a comparative study of the gill apparatus makes it possible to understand the transformation of typical ctenidia into lamellar gills. So, in a small group of marine bivalves - even-toothed (Taxodonta) - there are two very little modified ctenidia. The stem of each ctenidium is attached to the ceiling of the mantle cavity on one side, and two rows of gill filaments are located on it.

In a large group of mixed muscles (Anisomyaria) a further change in ctenidium is observed. Its gill filaments have elongated into thin filaments, so long that, having reached the bottom of the mantle cavity, they bend upward. The descending and ascending knees of this thread and adjacent threads are linked to each other with the help of special hard cilia. Due to this, the gill, consisting of two rows of threads, looks like two plates. A similar structure of gills is found in scallops (Pecten), oysters (Ostrea), etc.

The structure of the gills of the true lamellar gills (Eulamellibranchiata) described above is a further change in the filiform gills. It consists in the formation of bridges between the ascending and descending branches of each filament and between adjacent filaments, as well as in the fusion of the ends of the ascending branches of the outer leaf with the mantle and the ascending branches of the inner leaf with the leg, and behind the leg with the formed inner gill leaf of the opposite side.

Thus lamellar gills are descended from true ctenidia, with two lamellar gills on each side corresponding to one ctenidium, and each plate representing a half-gill.

In a small group of animal-eating bivalve mollusks that feed on plankton and small polychaetes, ctenidia are reduced. The respiratory function is performed by the dorsal part of the mantle cavity, separated by a septum pierced by pores (in Septibranchia).

Digestive system

In connection with the reduction of the head and the passive way of feeding, the anterior ectodermic section of the digestive tract disappears: the pharynx, salivary glands, jaws, and radula. The mouth is placed in the anterior part of the body between the anterior adductor muscle and the leg. Oral lobes are usually located on the sides of the mouth. Small food particles are filtered out by a system of various cilia that cover the gills, are enveloped in mucus and enter the mouth through the food gill grooves, which leads to the esophagus, which passes into the stomach. The ducts of the paired tubular liver and the bag of the crystalline stalk open into the stomach. From the stomach, the small intestine begins, forming several loops at the base of the leg and passing into the rectum. The latter “pierces” the ventricle of the heart (in almost all bivalves) and opens with an anus not far from the cloacal siphon. The entire digestive tract is lined with ciliated epithelium, the movement of the cilia of which carries out the movement of food particles.

A bag of crystalline stalk secretes a gelatinous substance of a protein nature containing enzymes that can only digest carbohydrates. This substance solidifies in the form of a stalk sticking out in the stomach. Gradually, its end dissolves and enzymes are released that digest food particles of plant nature.

The liver of bivalve mollusks does not produce enzymes at all; absorption and intracellular digestion of food particles occur in its blind branches. Intracellular digestion is carried out mainly by mobile phagocytes capable of digesting proteins and fats. Bivalves feed on phytoplankton, detritus, and bacteria.

Bivalves belong to the group of biofilters that pass tens of liters of water per day. They play an important role in the formation of bottom sediments (silts).

Circulatory system

The heart usually consists of a ventricle and two atria and is placed in the pericardial cavity - the pericardium. Two aortas depart from the heart - anterior and posterior. The anterior one splits into arteries that supply blood to the intestines, sex glands, leg, etc. The posterior one forms two mantle arteries that go to the mantle and to the organs of the back of the body. Small arteries break off, and blood enters the gaps between the organs - lacunae, and from there it is collected in the longitudinal venous sinus. From the sinus, the blood partially goes to the kidneys, where it is cleared of metabolic products. Then, through the afferent gill vessels, it enters the gills, is oxidized, and is sent through the efferent vessels to the atrium (part of the blood from the mantle vessels also enters there, bypassing the gills). In many, the hindgut passes through the ventricle of the heart. This is due to the fact that the ventricle of the heart is laid as a paired formation on the sides of the intestine. Some mollusks (Area) have two ventricles in the adult state, located above the intestine.

excretory system

There are two large kidneys, called bayanus organs. They lie under the pericardial cavity and are V-shaped. In the anterior part of the pericardial cavity, each kidney begins with a ciliated funnel. The outlet openings open into the mantle cavity. In addition to the kidneys, the excretory function is also performed by the pericardial glands, or the so-called Keberian organs, which are isolated sections of the wall of the pericardial cavity.

Nervous system and sense organs

In bivalves, the nervous system is somewhat simplified compared to the nervous system of gastropods, which is explained by passive feeding and low mobility. Most often, the fusion of two pairs of ganglia is observed, as a result of which there are only three pairs of them. The cerebral and pleural ganglia merge into the cerebropleural ganglion, which lies between the esophagus and the anterior concha muscle. A pair of closely spaced pedal ganglia is placed in the foot, connected by connectives to the cerebropleural ganglia. The parietal and visceral ganglia also merged into the visceroparietal ganglia. They lie under the posterior adductor muscle and are connected to the cerebropleural ganglia by very long connectives.

The sense organs are represented primarily by tactile cells, which are very rich in the edge of the mantle and the oral lobes. On the edge of the mantle, some mollusks have small tentacles. Usually there are statocysts located on the sides of the leg near the pedal ganglia. Osphradia are located on the ceiling of the mantle cavity, at the base of the gills.

Bivalves do not have brain eyes, however, in some species secondary eyes appear in various parts of the body: on the mantle, siphons, gill filaments, etc. So, in scallops (Pecten), numerous eyes (up to 100 pieces) are placed along the edge of the mantle complex structure, which is explained by the ability of scallops to move by flapping flaps. The secondary eyes are innervated not from the cerebral ganglion.

Reproductive system and reproduction

Most lamellar gills are dioecious, but there are also hermaphroditic forms. The sex glands are paired and lie in the parenchyma of the body, occupying the upper part of the leg. In most cases, the ducts of the sex glands open with special genital openings located next to the excretory ones. Hermaphrodite forms have separate ovaries and testes, or more often one pair of hermaphrodite glands.

The eggs of most bivalves are laid singly in the water, where fertilization takes place. In freshwater shells from the Unionidae family (toothless, barley, etc.), eggs are laid on the outer plates of the gills and hatch there until the larvae emerge.

Development

Embryonic development of bivalves resembles the development of polychaetes. In almost all marine bivalves, a trochophore larva emerges from the egg. In addition to the typical signs of trochophore - the presence of preoral and postoral corollas of cilia, parietal plate, sultan, protonephridia, etc. - the trochophore of bivalves also has the rudiments of a leg and a shell. The shell is initially laid in the form of an unpaired conchiolin plate. Later, it bends in half and forms a bivalve shell. The place of inflection of the conchiolin plate is preserved in the form of an elastic ligament. The upper part of the trochophore turns into a sail covered with cilia (an organ of movement), and the larva passes into the second stage - the veliger (sailfish). Its structure already resembles the structure of an adult mollusk.

In other freshwater bivalves, such as Sphaerium, the embryos develop in special brood chambers on the gills. Already fully formed tiny mollusks emerge from the mantle cavity.

Biology and practical significance

The largest number of bivalves are typical benthic animals, often burrowing into the sand, and some of them even very deep into the ground. So, Solen marginatus, found in the Black Sea, burrows into the sand to a depth of 3 m. Many bivalves lead a sedentary lifestyle. At the same time, some of the sessile mollusks, for example, mussels (Mytilus), are attached with the help of byssal threads, but can, discarding the byssus, move to a new place, while others - oysters (Ostrea) - adhere to the substrate for the whole life of one of the shell valves.

Many laminabranchs have long been eaten. These are mainly mussels (Mytilus), oysters (Ostrea), cockles (Cagdium), scallops (Pecten) and a number of others. Especially widespread is the use of oysters, which are not only caught in oyster banks - places of their mass settlement, but also artificially bred in special oyster plants, which are a system of devices for growing oysters. We have oyster banks in the Black Sea inhabited by Ostrea taurica.

Bivalves Classification

The class of bivalves is divided into four orders, of which the most important are the following: 1. Equal-toothed (Taxodonta); 2. Miscellaneous (Anisomyaria); 3. Actually lamellar gills (Eulamellibranchiata).

Detachment. Even-toothed (Taxodonta)

The most primitive bivalves. The castle consists of numerous battlements. Gills of the type of true ctenidia, bearing rounded leaflets on an axis adhering to the ceiling of the mantle cavity. Foot with a flat sole. This order includes widespread walnuts (family Nuculidae), northern forms (genus Portlandia), arches (family Arcidae), etc.

Detachment. Miscellaneous (Anisomyaria)

The order combines a large number of forms that previously constituted the filamentary group, since the gill leaves of their ctenidia are turned into long filaments. There is either only one posterior closing muscle, or, if there is also an anterior one, it is much smaller. This order includes mussels, sea scallops: Icelandic (Pecten islandicus), Black Sea (P. ponticus), etc. This order also includes oysters (Ostreidae family), sea pearls (Pteriidae family).

Detachment. Lamellar gills (Eulamellibranchiata)

The vast majority of bivalve mollusks belong to this order. They are characterized by the structure of the castle, the teeth of which have the form of arcuate plates. There are two closing muscles. The edges of the mantle form siphons. Gills in the form of complex lattice plates.
This order includes all freshwater bivalves belonging to the family of barley (Unionidae): barley, toothless; to the family of freshwater pearl mussels (Margaritanidae), to the family of ballfish (Sphaeriidae), as well as to the family of zebrafish (Dreissenidae). More specialized forms also belong to this order: stone grinders (Pholas), shipworms (Teredo) and many others.

Gallery

From the website www.ecosystem.ru

Species descriptions and illustrations are taken from computer identification of freshwater invertebrates in Russia(Bogolyubov A.S., Kravchenko M.V., Moscow, "Ecosystem", 2018) .

CLASS DOUBLE - BIVALVIA

Brief description of the class. Bivalves, or lamellar gills (Bivalvia) - a class of marine and freshwater sedentary mollusks, which in contrast from gastropods (Gastropoda) do not have a separate head and associated organs (mouth, pharynx). They have a shell of 2 wings- right and left (in contrast to the upper and lower in brachiopods), which cover the body from the sides and are movably articulated on the dorsal side by an ellastic ligament - ligament, and 2 or 1 are connected from the inside closing muscle.

Body of bivalves flattened laterally and carries 2 wide mantle vanes closely adjacent to the shell shells from the inside. Leg often keeled and may protrude from the shell; often it has a gland that secretes byssus used to attach to the substrate. Paired gills located on the sides of the body and modified into gill plates (ctenidia), which perform not only a respiratory function, but also the role of filters for straining food particles from the water, therefore, according to the type of food, bivalves are predominantly filter feeders. The central nervous system consists of 3 pairs of ganglia. There are sensitive tentacles appendages, and in some forms complexly developed eyes. Heart usually penetrated by the hindgut and has 2 atria. Usually separate sexes, rarely hermaphrodites.

To bivalve relate such well-known marine mollusks as oysters, mussels, scallops, and from our freshwater ones - toothless, barley, balls, peas, pearl mussel and zebra mussel. Most bivalve mollusks burrow into the bottom silt, thus escaping from predators, some lie on the seabed or cling to rocks and other surfaces. Few species, such as scallops, are capable of short active swimming.

Shell shape and size bivalves vary greatly: among them there are both very large mollusks, for example, the giant tridacna (Tridacna gigas), which can reach 1.4 m in length and weigh up to 200 kg, and very small ones. The smallest representative of the type of molluscs is just the bivalve mollusk Condylonucula maya, whose adults reach a length of only 0.5 mm

body shape in bivalves, it is also very different. For example, cockles have an almost spherical body and can jump by bending and unbending their legs. At the same time, marine cuttings (Ensis), due to their specialization in a burrowing lifestyle, have an oblong shell and a strong leg intended for digging into the ground. Shipworms of the family Teredinidae have a strongly elongated worm-like body up to 2 meters long with a reduced shell located at its front end and modified into a drilling organ, thanks to which the mollusk "gnaws" branched passages in wood. In most species of bivalves, the body is oblong, more or less laterally flattened, and bilaterally symmetrical. The head is reduced, and the mollusk, in fact, consists of a body and a leg.

The history of the study of bivalves. First time title Bivalvia used by Carl Linnaeus in the 10th edition of his work "The System of Nature" in 1758 to refer to molluscs, the shell of which consists of two valves. In addition, this class is known by other names, for example:

Pelecypoda (meaning "axe-footed"),
Lamellibranchia (lamellabranchs) and
Acephala ("headless", because bivalves, unlike all other molluscs, have lost their heads).

In Russian science at the end of the XVIII, early XIX centuries, bivalves (living and fossils) were called - cranioderms.

Shell structure of bivalves. Shell valves in bivalve mollusks are most often symmetrical(for example, in heartworms, toothless). However, some species may have asymmetry sashes. So, in an oyster, the valve on which the animal lies is convex, and the other valve is flat and plays the role of a lid, covering the first one. A similar phenomenon occurs in the case of a scallop lying at the bottom.

From the outer surface of the shell, a protruding and somewhat elevated part is clearly visible - top, or top of the head(see picture). This is the most old part of the shell, as the shell grows along the edge from the opposite side.

The edge of the shell on which the apex is located is called top, or dorsal, or key edge shells, and the opposite - lower, or abdominal.
The wider end of the shell is in front of him, and narrower, somewhat elongated and remote from the top - rear end (edge).
If you place the shell with the top of the shell up and the front end away from you, then the valve located to the left of the plane of closure of the valves is called left, and the sash, located on the right, - right.

At the back end sinks there is an elastic plate or ligament, with which both shells of the shell are connected to each other. The ligament consists of two keratin proteins, tensilium and resilium. In different groups of bivalves, the ligament can be internal and external. Its function, in addition to fastening the valves, is to participate in the opening of the shell.

Upper surface of each shell valve concentrically streaked; some of the arches protrude more sharply than others, stretch along the entire length of the shell and have a somewhat wrinkled appearance; This annual arcs, corresponding to the winter pauses in the growth of the shell, from which, with some approximation, the age of the shell can be determined. Holes also open at the back end of the shell. siphons.

At the front end shells are arranged leg And byssal gland(if there is).

sink wall consists of three layers: outer conchiolin ( periostracum ), bearing growth bands, internal calcareous ( ostracum ) and lower mother-of-pearl ( hypostracum ).

The mineral component of the shell (that is, which is part of the ostracum and hypostracum) can be represented exclusively by calcite, as in oysters, or by calcite and aragonite. Sometimes aragonite also forms a nacreous layer, as in the case of the order Pterioida. In other molluscs, the layers of aragonite and calcite alternate.

outer layer(periostracum) consists of solid organic matter (conchiolin) and is secreted by the edge of the mantle. At the tops of the valves, the conchiolin layer is often obliterated. This top layer has patronizing coloration, usually brown or olive. Sometimes, under the influence of some mechanical action, the outer horny layer of the shell is destroyed, revealing the underlying prismatic layer, which is easily dissolved in soft waters rich in free carbon dioxide, and mollusk shells in such waters are corroded.

Inner layer(ostracum) is called porcelain. A microscopic study of transverse sections of the porcelain layer showed that it consists of calcareous prisms closely lying on top of each other in a direction perpendicular to the shell surface.

Pearlescent layer(hypostracum) consists of many thin, also calcareous layers, lying on top of each other and connected by conchiolin. Such a mother-of-pearl structure causes light interference, i.e. the layers refract and reflect the light falling on them, as a result of which the inner surface of the shell is cast in various colors or iridescent. The mother-of-pearl tint changes depending on which side and at what angle, when viewing the shell, the light falls on it. The mother-of-pearl layer thickens with the age of the mollusk and the growth of its shell.

On the inside of the sink(see figure) its thickened upper edge is visible or lock plate. It is called so because it carries outgrowths or teeth, which enter the recesses of the opposite plate, forming the so-called " lock". The teeth in front of the ligament are short and massive, those behind it are elongated and thin. The lock prevents the valves from moving relative to each other. The lock can be equal-toothed (taxodont) and different-toothed (heterodont).

The inner surface of the pearl shell

The lock is most fully expressed in perlovits; in the pearl mussel, some teeth are reduced, and in the toothless, they are completely absent, which is why their name comes from.

At the anterior and posterior ends of the inner surface of the shell there are spots formed by irregularities of the mother-of-pearl layer at the attachment points. closure muscles shellfish; by the force of contraction of these rather powerful muscles, the shell valves are tightly compressed one with the other, and if the shell is pulled out of the water, its body retains the necessary supply of moisture for some time. The valves open without the participation of any muscles, but only under the influence of the ligament, which plays the role of a spring. The action of the opening mechanism can be seen on dead mollusks - from the drying of the ligament, the shell valves open slightly by themselves.

shell growth occurs due to the gradual laying of a conchiolin layer by the edge of the mantle, as well as the accumulation of minerals in the shell. visible on the sink concentric lines, indicating its uneven growth in changing conditions environment(growth lines). However, determining the age of the shell from the number of such concentric lines is not sufficiently accurate. More precisely, the age of the mollusk can be determined by the number of layers on the cross section of the shell.

If any foreign body, for example, a grain of sand, gets between the shell valve and the adjacent mantle of a bivalve mollusk (not necessarily only a pearl oyster), then it has an irritating effect on the animal, and the skin of the mantle begins to secrete a hardening mother-of-pearl substance, which gradually envelops the foreign body. body in concentric layers (see figure). Eventually, it forms pearl, which, therefore, in a mollusk is a formation of painful origin.

Shell of a pearl oyster with a pearl (left) and a diagram of the formation of a pearl (right)

The shape and structure of the shell it is possible to distinguish three main systematic groups our freshwater bivalves. In addition to a pronounced difference in the structure of the teeth and the locking apparatus, barley is distinguished by an elongated hard-walled shell with a prominent apex close to the front end; the shell of the toothless is broadly oval, thin-walled, its apex is slightly protruding, the keel of the upper margin is high in some species. The shell of the pearl mussel is large, elongated, thick-walled, with an almost straight or even somewhat concave lower edge; the top edge is nearly parallel to the bottom.

Mantle and mantle cavity. In bivalves, the mantle has the form of two folds of skin hanging from the back on the sides to the ventral side. From below, its folds can be free (like a toothless one) or grow together, leaving only holes for the leg and siphons. Small tentacles and eyes can sometimes develop along the edge of the mantle. The outer layer of the mantle secretes the shell, and the inner layer is lined with ciliated epithelium, the beating of the cilia of which ensures the flow of water in the mantle cavity. In addition to the shell, the mantle also forms a ligament, a byssal gland, and a castle.

In burrowing forms, the mantle forms siphons- two long tubes, through the lower one (inlet siphon) water enters the mantle cavity, and through the upper one (outlet siphon) it exits. With the flow of water, oxygen and food particles are delivered to the mantle cavity.

Like all other molluscs, in bivalves, the mantle forms a mantle cavity, which includes mantle complex organs: leg, two gills, two oral lobes and osphradia. Openings of the digestive, reproductive and excretory systems also open into the mantle cavity.

In most bivalves, a line is visible on the inside of the shell, running parallel to the edge of the shell and often connecting traces from two adductor muscles (snappers). They call her pallial (mantle) line, it represents the lines of attachment of the mantle to the shell valve. Attachment is carried out by a narrow row of small retractor muscles. With the help of these muscles, the mollusk can, in case of danger, hide the protruding edge of the mantle inside the shell. Siphons (in the normal state, protruding from the sink) can also be drawn in. For this, a special pocket-shaped recess in the mantle cavity is used. On the shell valve, this depression corresponds to the pallial sinus, or mantle sinus, or mantle bay, or siphon bay, an inward curvature of the pallial line.

Leg. The leg (muscular unpaired outgrowth of the abdominal wall) in bivalve mollusks is wedge-shaped, serves for burrowing into the ground and crawling. In the most primitive forms (the order Protobranchia), the foot, like that of the gastropods, has a flat crawling sole. Some bivalves that attach to the substrate have a special byssal gland, which secrete byssal threads, with the help of which the mollusk "grows" to the bottom surface (mussels). In many bivalves, leading a stationary lifestyle, the leg is completely reduced (oysters).

Musculature. The main muscles in the body of bivalves are the anterior and posterior adductor muscles (adductors), although the anterior link may be reduced or completely lost in some species. By contracting, these strong muscles close the valves, and when they relax, the valves open. In addition, a ligament is involved in the valve opening mechanism. When the shell is closed, it, like a spring, is in a taut state. When the contactors are relaxed, it returns to its original position, opening the sashes.

In bivalve mollusks lying on the same valve (for example, oysters and mussels), the anterior adductor is lost, and the posterior adductor occupies a central position. Mollusks of the family Limidae, which swim by flapping their doors, also have a single central contact. The closures consist of two types of muscle fibers: striated, designed for fast movements, and smooth, maintaining long-term muscle tension.

As mentioned above, the mantle is attached to the shell by small muscles that form arcuate trace on the shell valve - a pallial line. Paired protractor (flexor) and retractor (extensor) muscles provide movement of the bivalve leg. Bivalves, lacking a leg, do not have these muscles. Other paired muscles control the siphons and the byssal gland.

Digestive system. In connection with the passive way of feeding by filtration The digestive system of bivalves has some peculiarities. Water entering through the introductory siphon is directed to the anterior end of the body, washing the gills and 2 pairs of long triangular oral lobes. There are sensory cells on the gills and oral lobes ( organs of taste) and small grooves through which food particles are transported to the mouth located near the front contactor.

From the mouth, food enters the short esophagus and then into the sac-like endodermal stomach. Since the head is reduced in bivalves, the pharynx, radula and salivary glands are absent. Several digestive glands open into the stomach, often through a pair of diverticula, such as a bilobed liver. The liver not only secretes digestive enzymes, its cells also phagocytize food particles. Thus, bivalves have intracellular digestion.

In addition, the stomach has crystalline stalk, consisting of mucoproteins and enzymes (amylase, glycogenase, etc.). The stalk is located in a special blind saccular outgrowth and protrudes into the lumen of the stomach. The cilia located there cause the stalk to rotate, separating enzymes and mixing the contents of the stomach. Due to the constant movement of food particles in the stomach, it is possible to sorting at its posterior end: small particles are sent to the digestive glands and are absorbed there by phagocytosis, while larger particles are sent to the intestines. The middle intestine departs from the stomach, which then makes several bends and goes along the dorsal side of the body to the posterior end, passes into the hindgut, which opens with an anus into the mantle cavity above the posterior contactor. Excrement with a stream of water is thrown out through the outlet siphon. The hindgut usually passes through the ventricle of the heart (a specific feature of bivalves).

Nutrition and digestion in bivalves synchronized with diurnal and tidal rhythms.

The features of the digestive tract described above are characteristic of filter bivalves. Carnivores bivalve stalk can be greatly reduced, but in some cases there is a muscular stomach lined with chitin, in which food is ground even before digestion begins. In other cases, the digestive tract of predatory bivalves is similar to that of filter-feeding bivalves.

Nervous system. Like most other mollusks, the nervous system in bivalves is of a scattered-nodular type. They have a simpler structure than gastropods. Due to the reduction of the head, the cerebral ganglia merged with the pleural ganglia; this is how paired cerebropleural double nodes were formed, located on both sides of the esophagus and connected above the pharynx by a thin cerebral commissure. The formation of cerebropleural nodes by fusion of ganglia is proved by the fact that in primitive Protobranchia the pleural nodes are still isolated from the cerebral ones. They innervate the mantle cavity and sensory organs (except for the osphradia). The foot has pedal ganglia that innervate the foot and are connected by connectives to the cerebropleural ganglia. Under the posterior muscle, there is a third pair of nodes - visceroparietal, controlling internal organs, gills and osphradia. They are connected by even longer connectives to the cerebropleural nodes. The third pair of nodes is especially well developed in floating bivalves. Bivalves with long siphons may have special siphon ganglia that control siphons.

Sense organs. The sense organs of bivalves are poorly developed. The leg has statocysts- balance organs innervated by cerebral ganglia. In the mantle cavity at the base of the gills are osphradia- organs of chemical sense; perhaps the osphradia of the bivalves are non-homologous to the osphradia of the gastropods. Separate receptor cells scattered on the gills, oral lobes, along the edge of the mantle and on the siphons. The tactile function is also performed tentacles developing along the edge of the mantle. In predatory bivalves from the order Anomalodesmata, the siphons are surrounded by tentacles that are sensitive to vibrations; with their help, mollusks detect prey.

Many bivalves deprived of eyes, however, members of the Arcoidea, Limopsoidea, Mytiloidea, Anomioidea, Ostreoidea, and Limoidea groups have simple eyes located along the edge of the mantle. They consist of a pit lined with photosensitive cells and a light-refracting lens. Scallops have inverted eyes of a rather complex structure, consisting of a lens, a two-layer retina, and a concave reflective surface. There are also known cases of the formation of eyes on siphons in cockles. All bivalves have photosensitive cells, thanks to which the mollusk determines when the shadow will completely cover it.

Respiratory system. Most bivalves breathe with gills. Each of the two gills consists of a gill axis attached to the body and two rows of gill filaments extending from it. The combination of threads of each row forms gill sheets, or half-gills. An exception is a small group of bivalve mollusks - septibranchia, whose representatives are devoid of gills, but their mantle cavity is divided by a horizontal partition pierced by rows of holes. Their food is also peculiar - they are predators. By arching the septum, they suck in small animals, such as crustaceans, along with water.

Primary branchial (Protobranchia), which are the most primitive bivalves, have a pair of typical ctenidia with gill filaments.

At filamentous (Filibranchia) there are threadlike gills. Filamentous gills are characterized by the fact that their gill filaments have elongated into filaments, forming first a descending and then an ascending knee. Neighboring threads are fastened to each other with the help of hard cilia, forming plates; in some representatives, the gill filaments are free. Filamentous gills are characteristic of mussels, oysters, scallops.

At order Eulamellibranchia there are lamellar gills. This is a further modification of the filiform gills: partitions appear in them between adjacent threads, as well as ascending and descending sections of one thread. This is how gill plates are formed. Each gill consists of two semi-gills: the outer, adjacent to the mantle, and the inner, adjacent to the leg. Thus, Eulamellibranchia has 4 gills, but each of them corresponds to only one half of a real ctenidium. Barley and toothless have such gills.

Gill cavity (left) and direction of respiratory currents (right) of a bivalve

At septumbranchia (Septibranchia) the gills are reduced and transformed into a gill septum with pores. The septum encloses the upper part of the mantle cavity, forming a respiratory cavity. Its walls are permeated with blood vessels, where gas exchange takes place.

Finally, in species lacking gills (as, for example, in representatives subclass Anomalodesmata), gas exchange occurs through the wall of the mantle cavity.

Bivalves living in the intertidal zone are capable of several hours survive without water by closing the doors tightly. Some freshwater forms, when exposed to air, slightly open the valves, so that they exchange gases with atmospheric air.

Circulatory system. The circulatory system of bivalves, like all other mollusks, open, that is, blood circulates not only through the vessels, but also by gaps(gaps between organs). The heart is located on the dorsal side and consists of 1 ventricle and 2 atria. As mentioned above, the hindgut passes through the ventricle. This fact is explained by the fact that the heart is laid in embryogenesis as a pair on the sides of the intestine, and then these rudiments are connected above and below the intestine (the paired origin of the heart in bivalves is confirmed by the presence of two hearts in representatives of the genus Arca). In primitive forms of the order Protobranchia, fusion occurs only above the gut.

Powerful anterior and posterior aorta branching into arteries; of them blood hemolymph) pours into the gaps and gives oxygen to the tissues. anterior artery goes forward over the intestine and supplies blood to the insides, leg and front of the mantle, and rear directed backwards under the intestines and soon splits into the posterior mantle arteries. Some bivalves have only one aorta. Further, the blood that has already become venous is collected in a large longitudinal gap under the heart and is sent to the afferent gill vessels. The oxygenated arterial blood then returns through the efferent vessels from the gills to the heart. Blood is also poured into the efferent gill vessels, which, bypassing the gills, passes through the kidneys, where it is released from metabolic products.

The blood of bivalves is usually devoid of any respiratory pigment, although members of the families Arcidae and Limidae have hemoglobin dissolved directly in blood plasma. The predatory bivalve mollusc Poromya has red amoebocytes containing hemoglobin.

excretory system. The excretory system of bivalves, like most other mollusks, is represented by paired nephridia (kidneys). Bivalve kidneys with glandular walls are called boyanus organs. The kidneys are long V-shaped tubes that open at one end into the pericardium of the heart, and at the other end into the mantle cavity, from where metabolic products are carried away with a stream of water.

In addition to the kidneys, the pericardial wall also performs an excretory function, modified into paired pericardial glands. Sometimes they are isolated from the rest of the pericardium in the form of two sac-like formations - keber organs. The excretory products of these glands enter the pericardium, and from there they are transported out through the kidneys.

Sexual system. bivalves separate sexes, however, there are also cases of hermaphroditism (for example, in the species Arca noae, protandric hermaphroditism was established, in which individuals first function as males, then as females). In some species, such as Thecaliacon camerata, there is a pronounced sexual dimorphism.

Gonads and ducts (vas deferens and oviducts) are paired; the gonads lie in the anterior part of the body, close to the intestine, going into the base of the leg, and look like two lobed, vine-like formations. However, in some species, the genital ducts are absent, and the gametes exit the gonads through tissue ruptures into the mantle cavity. In primitive Protobranchia, as well as a number of other bivalves (Pecten, Ostrea, etc.), the gonads open into kidneys.

In some species, for example, representatives of the genus Lasaea, the male germ cells go out through the siphon, and then with the flow of water they are drawn into the mantle cavity of the females, where fertilization occurs. The offspring of such species develops in the mantle cavity of the mother and leaves it at the stage of a larva - a veliger or a young individual. In most species fertilization external. In this case, females and males release sperm and eggs. into the water column. This process can be continuous or triggered by environmental factors such as the length of the day, the temperature of the water, and the presence of sperm in the water. Some bivalves release gametes little by little, while others - in large parts or all at the same time. A massive release of gametes sometimes occurs when all the bivalves in the area simultaneously release germ cells.

Life cycle. In bivalves, like all mollusks, spiral crushing. It goes about the same as in gastropods.

Most bivalves develop with metamorphosis. Usually from fertilized eggs comes out planktonic larva - veliger (sailfish). The formation of the veliger is preceded by the trochophore stage, which takes place in the egg. The formation of trochophores occurs quite quickly and takes several hours or days. On the dorsal side of the trochophore, a shell is laid in the form of a whole plate, which only later bends along the median line, becoming bivalve, and the place inflection preserved in the form of a ligament. The upper part of the trochophore with the corolla of cilia becomes sail veligera - a disc covered with long cilia used for swimming. A bivalve shell covers the entire body of the veliger; when swimming, the sail is exposed from the shell. The organization of the veliger is very close to that of an adult mollusc: it has a leg rudiment, a mantle, ganglia, a stomach, a liver, and other organs, but protonephridia remain the organs of excretion. Subsequently, the veliger settles to the bottom, is fixed by a byssus thread, loses its sail and turns into an adult mollusk.

Some freshwater mollusks (for example, barley and toothless) have a special larva - glochidia, which has a thin-walled bivalve shell with rounded valves and hooks on the ventral margin. Most organs of the glochidia are still underdeveloped: there are no gills, the leg is rudimentary. In these mollusks, fertilization occurs in the mantle cavity of the female, and glochidia develop in her gills. Each mature glochidium is a small bivalve mollusk whose shell flaps open wide and close quickly due to the contraction of a strongly developed closing muscle. The lower edges of the shell are equipped with sharp teeth, and a long sticky thread of byssus extends from the tiny foot of the larva.

A larva of a bivalve mollusk - glochidia and a female bitterling with an ovipositor

Economic value. Since ancient times, many bivalve mollusks have been used by humans, they served and serve prey. Their shells are constantly found in the so-called "kitchen heaps" of prehistoric man, who lived near the shores of the seas, rivers, lakes. In the excavations of Paleolithic human sites in the Crimea, a large number of shells of oysters, mussels, scallops and other mollusks are invariably found, which are still hunted today.

bivalves are mined because of their tasty, very healthy and easily digestible by the human body meat(such as, for example, oysters, mussels, scallops, tapes and venerupis cockerels, mactres, sand shells, cockles, arches, sea cuttings and synovacules, freshwater barley, lampsilin, toothless, corbicula, etc.).

In terms of calories, they can even surpass the meat of many fish, both marine and freshwater. The nutritional value shellfish meat is also due to the high content of vitamins A, B, C, D, etc., the high content of such rare minerals in ordinary human food as iodine, iron, zinc, copper, etc. The latter, as is known, are part of a number of enzymes, hormones, play an extremely important role in oxidative, carbohydrate and protein metabolism, in the regulation of hormonal activity.

In recent decades, due to the fact that the natural resources of the most valuable edible mollusks (even in the seas) are depleted, and the demand for them continues to increase, in many countries they have become resettle to new areas, to acclimatize, and also breed artificially both in marine and fresh waters, on "farms" - specially prepared shallows and in small bays and artificial reservoirs protected from predators. Successfully bred and cultivated not only marine molluscs (oysters, mussels, cockatiels, tapes), but also freshwater (lampsilin).

In 2010, 14.2 million tons of shellfish were grown in aquaculture farms, which is 23.6% of the total mass of shellfish used for food. In 1950, when the Food and Agriculture Organization of the United Nations began publishing similar statistics, the total mass of bivalve molluscs consumed was estimated at 1,007,419 tons. In 2000, this value was already 10,293,607, and in 2010 - 14,616,172. ) tons, scallops - 2,567,981 (1,713,453) tons. In China, the consumption of bivalve mollusks from 1970 to 1997 increased 400 times! Some countries regulate the import of bivalves and other seafood, mainly to minimize the risk of poisoning by the toxins that build up in these organisms.

At present, the production of bivalves is inferior to their artificial breeding in mariculture. Thus, mussels and oysters are grown on special farms. Such farms have achieved especially great success in the USA, Japan, France, Spain, and Italy. In Russia, such farms are located on the shores of the Black, White, Barents and Japan Seas. In addition, mariculture of sea pearl mussel (Pinctada) is developed in Japan.

It also continues booty bivalve mollusks in natural waters, where they are now harvested on large vessels with specially designed fishing gear; scuba diving is widely used. The largest number (about 90%) of bivalve mollusks is mined in the northern hemisphere - in the Pacific and Atlantic oceans. fishing freshwater bivalve molluscs provides only a few percent of their total world production.

Of particular importance is the bivalve fishery in countries such as Japan, the United States, Korea, China, Indonesia, the Philippine Islands and other Pacific islands. Thus, about 90 species of bivalve mollusks are mined in Japan, of which about two dozen species are of great commercial importance, and 10 species are artificially bred. In European countries, fishing and breeding of bivalve mollusks are most developed in France and Italy.

In Russia, commercial value have mainly a large seaside scallop - Pecten (Patinopecten) yessoensis, as well as various mussels, white shell (Spisula sachalinensis), sand shell - Mua (Arenomya) arenaria, cockerels (Tapes, Venerupis) and some others.

Shellfish enter the market not only in fresh and dried form, but especially in ice cream; the preparation of various canned molluscs also developed greatly.

In addition to food use, a person uses some types of bivalves (many freshwater barley and pearl mussels, sea pearl mussels - pinctadas, pteria, etc.) as a source material for jewelry (nacre And pearl), and also as souvenirs. Pearls are used in jewelry, and mother-of-pearl from bivalve shells is used in the manufacture of buttons or cheap jewelry, as well as for inlays. Among natural pearls, pearls of bivalve mollusks Pinctada margaritifera and Pinctada mertensi, living in the tropical and subtropical Pacific Ocean, have the highest value. Commercial pearl farming is based on the controlled incorporation of solid particles into oysters. The ground shells of other mollusks are often used as material for the introduced particles. The use of this material on an industrial scale has brought some freshwater bivalves in the southeastern United States to the brink of extinction.