The structure and diversity of bivalves. Freshwater bivalve mollusc perlovitsa: description, habitat, reproduction

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The class of bivalve mollusks, as is known, has four different names, each of which to some extent reflects the main features of their structure. Name "Bivalve"(Bivalvia) was first proposed by Linnaeus (1758) and is the most correct since it applies to all members of this class. Headless(Acephala), they were named by Link (1807), which reflected the fact that their head section of the body was reduced as they developed two shell valves in the process of evolution and these valves closed around the body of the mollusk. Third name - "lamellar"(Lamellibranchia), proposed by Blainville in 1814, can be fully applicable to only one detachment of this class, since the rest of the detachments have gills of a different structure; thus this title is inapplicable, as is the fourth - "axe-footed"(Pelecypoda, Goldfuss, 1880), since the structure of the leg in bivalve molluscs is very diverse. Thus, the most correct and comprehensive is the first, Linnean name, which should be retained in relation to this class.

Bivalves are widespread in the World Ocean and its marginal seas, in rivers and lakes, and even in ponds.

The total number of species of bivalves is about fifteen thousand, and most of them are associated in their habitat with salty sea waters, and only about one-fifth of the total number of their species inhabit fresh waters. On land, bivalves are not found.

In marine waters, they are extremely widespread, occurring in all climatic zones, from the warm waters of tropical seas to the Arctic and Antarctic and to the cold depths of the oceanic abyssal. They inhabit almost all the depths of the World Ocean - from the tidal zone (littoral) and coastal shallow waters up to the great depths of the depressions of the World Ocean, where they were: found at a depth of almost 10.8 km.

At present, the number of species of deep-sea bivalve mollusks living in the abyssal of the World Ocean (i.e., at a depth of more than 2000 m), judging by still incomplete data, is about 400 species, but this number should also be considered greatly underestimated.

There is a great variety of sizes, structures and colors of shells of bivalve mollusks. So, a giant among mollusks in general, an inhabitant of tropical seas, tridacna can reach 200 kg of weight, and the length of its powerful shell is 1.4 m. Along with this, the size of a number of ordinary deep-sea mollusks does not exceed 2-3 mm.

The shells and edges of the mantles of many tropical shallow-water mollusks, invisible among thickets of algae, white, pinkish, purple and yellowish corals, bright stars and other invertebrates, shine with variegated patterns and bright colors. A variety of outgrowths, spikes, scales and ribs adorn the shells of these mollusks, which helps them to strengthen themselves in these thickets and resist the action of sea waves and currents.

The shells of mollusks living on sandy or silty soils of the temperate or arctic region have a more modest color.

Deep-sea forms usually have a pale colored shell, often very thin and translucent.

Most of the freshwater forms are modestly colored in greenish and brownish tones.

The great diversity in the structure of the body and shell of bivalve molluscs is closely related to their way of life, their habitat, the depth and quality of the soil on which they live, or attach themselves, or burrow into. This affects primarily the structure of their shells; on the presence of ribs on it with one or another armament of its so-called lock, with the help of which the wings are fastened; on the presence or absence of siphons - special outgrowths of the mantle (two soft lobes that surround the body of the mollusk and secrete its shell); on the shape and size of the leg and the presence in it of a special gland that secretes the threads of the so-called b and c c c a with which they can be attached to the ground, as well as many other things. In forms that burrow more or less deeply into soft soil, special outgrowths of the mantle develop behind - siphons, through which water is sucked in and removed, which is necessary for breathing and feeding the mollusk immersed in the soil. These are the various makoms, tellins, yoldii and others. Mollusks living on the surface of the soil, crawling or slightly burrowing into it, have only rudimentary siphons or are completely devoid of them (for example, cockles, venusi, astartes and etc.). Mollusks that live in coastal shallow areas on harder sandy soils with an admixture of stones have stronger, thicker shells (for example, arches, scallops, sea scallops - pectenes and chlamys), and various inhabitants of soft silty soils have thinner shells ( batiarki, sea scallops propeamussiums and etc.).

Many forms living in coastal shallow waters attach byssus threads to stones, rocks, to each other, often forming whole clusters, intergrowths (many mussels), or even grow with their valves to stones, or grow together with each other ( oysters).

A very strong shell with sharp teeth on the ribs is possessed by many stone-boring clams; some of them secrete a special sour secretion that dissolves the lime of coastal rocks and stones in which they grind minks for themselves. Soft worm-like body of woodworm teredo(Teredo) is covered only in front by a small complex shell, which serves it for drilling, and not for protecting the body; spending their whole lives in tunnels gnawed through wood, these mollusks do not need to protect their weak, long body with a shell. Finally, a huge variety of various tropical mollusks, permanent inhabitants of coral reefs, is closely related to their life in a shallow, extremely diverse substrate zone.

Strong calcareous shells of bivalves, as well as other mollusks, are well preserved in sediments (clays and sands) for entire geological epochs. The remains of their settlements are extremely valuable for geologists and paleontologists. These remains can perfectly characterize not only the hydrological and climatic conditions under which these deposits were formed (i.e., under which the species of mollusks found here lived), but also the age of the given sequence of deposits. Thus, accumulations of fossil shells of a cold-water mollusk now living in the Arctic seas portland arctic(Portlandia arctica) in the deposits of the north of Europe perfectly indicate that these areas were formerly occupied by the cold, slightly freshened waters of the shallow so-called Yoldiev Sea. This sea with a cold-water fauna, where the Arctic Portlandia played a leading role, was associated with a period of cooling in the post-glacial period (about 8-10 thousand years ago). Conversely, the deposits of the warm Littorina Sea, which formed later (3-5 thousand years ago), are characterized by the presence of remains of completely different, warm-water mollusks, such as Icelandic cyprina(Cyprina islandica), edible cockles(Cerastoderma edu1e), cirphei comb(Zirfaea crispata) and others. These species now live only in the North Atlantic, in the warmest regions of the Barents and partly the White Seas, while in the era of the Littorina Sea they moved further north.

Representatives of the class of bivalve mollusks first appear in deposits in the Paleozoic, that is, in the oldest deposits of our planet, namely in the Upper Cambrian layers, the formation of which dates back about 450-500 million years. The first bivalve mollusks found here belonged to four genera, of which such as Ctenodonta and Paleoneilo had a comb lock and outwardly resembled modern walnut(Nuculidae) and mallecium(Malletiidae) from comb-toothed order(Tachodonta). The greatest species diversity of bivalve molluscs reached in the Cretaceous, ie, 100-130 million years before our time.

Thus, the class of bivalves is one of the most ancient groups of benthic invertebrates.

Since ancient times, many bivalve molluscs have been used by humans, they have served and are still being 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, which are hunted to this day, are invariably found. Bivalve mollusks are harvested for their tasty, very healthy and easily digestible meat (such as oysters, mussels, scallops, tapes and venerupis cockerels, mackerels, sand shells, cockles, arches, sea cuttings and synovacules, freshwater perlovitz, lampsilia, toothless, corbicula and etc.).

In terms of calories, they can even surpass the meat of many fish, both marine and freshwater. The nutritional value of mollusk meat is also due to the high content of vitamins A, B, C, D, etc., and the high content of such rare minerals in ordinary human food as iodine, iron, zinc, copper, etc. The latter, as you know, are included in in the composition of a number of enzymes, hormones, play an extremely important role in oxidative, carbohydrate and protein metabolism, in the regulation of hormonal activity. The meat and shells of molluscs are widely used for the manufacture of fodder flour for fattening poultry, as well as for the manufacture of fertilizer fats.

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

Currently, significantly more than half of the harvested bivalve mollusks are obtained as a result of their artificial breeding. The capture of mollusks in their natural habitats in water bodies and their artificial breeding have now become a profitable and important part of the food industry in a number of countries.

Bivalves are now often caught on large vessels with specially designed fishing gear; scuba diving is widely used. 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.

The extraction of bivalve mollusks has increased dramatically in recent decades. If before the start of the World War their annual production was about 5 million cu, then already in 1962 it increased to about 17 million cu and began to account for about 50% of the world production of all marine invertebrates, or 4% of the total world production (426 million v) all products of the sea (fish, whales, invertebrates, algae).

The largest number (about 90%) of bivalve mollusks is mined in the northern hemisphere - in the Pacific and Atlantic oceans. Fishing for freshwater bivalve mollusks 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, the fishing and breeding of bivalve mollusks is most developed in France, Italy, etc.

In the USSR, commercial importance is mainly large seaside scallop(Pecten (Patinopecten) yessoensis), as well as various mussels, white shell(Spisula sachalinensis), sand shell(Mua (Arenomya) arenaria), cocks(Tapes, Venerupis) and some others.

Bivalve mollusks have long been mined for the sake of their shells, which provide not only excellent raw materials for mother-of-pearl products (many freshwater pearls and pearl mussels, sea pearls - pinctadas, pteria, etc.) ”but also the most valuable pearls. At the beginning of this century, industrial methods were found to more quickly artificially obtain pearls (the finds of which are a rather rare accident under natural conditions), indistinguishable from naturally formed pearls. Farms for keeping sea pearls and growing pearls in them are especially successful in Japan. So, already in 1936, 140 thousand sea pearl shells were grown here and 26.5 thousand pearls were obtained.

In some countries, especially in the tropical Pacific, bivalve shells are widely used for lime production.

Almost all bivalve mollusks, with the exception of large forms with a strong, thick shell, serve as a favorite food for bottom fish - benthophages (i.e., feeding on bottom animals), including many commercial fish, both marine and freshwater: flounders, some cod (hadock), sturgeon, many cyprinids (bream, carp), catfish, gobies, etc. Due to the predominance of small mollusks in their food, some fish are called "mollusk-eaters", such as the Caspian vobla. Areas where, along with other benthic animals (polychaete worms, brittle stars, etc.), mass development of small bivalve mollusks is observed, serve as feeding grounds for various demersal commercial fish.

Mollusks are readily eaten by many large decapod crayfish (lobsters, hermit crabs, crabs), starfish are the original enemies of bivalve mollusks. Commercial oyster banks are periodically cleaned of starfish with the help of special mops, which are dragged along the bottom by small vessels.

An important role is played by bivalves in the diet of commercial Kamchatka "crabs" (Paralithodes kamtschatica).

What are the main structural features of bivalves? To make it easier to understand their structure, imagine a bound book placed spine up. Both halves of the binding will correspond to the right and left shell valves, covering the body of the mollusk from the sides. The spine of the book will be analogous to an elastic external ligament (ligament) connecting both valves on the dorsal side of the shell and at the same time stretching them. The first and last pages of the book correspond to the two lobes of the mantle, covering the body on the right and left sides, and the next two leaves of the book in front and behind will be analogous to two pairs of gills on each side of the body. And finally, between both pairs of gills, the body itself and the leg are located inside - usually a rather large muscular ax-shaped or wedge-shaped organ directed forward; in attached or inactive forms, the leg can turn into a small outgrowth, and, conversely, in actively moving species (for example, cockles), the leg becomes strong, slightly articulated, adapted for moving in soft sandy soil.

The arrangement of the body parts of a bivalve mollusk will become clearer if we consider an open mollusk, such as toothless, common in our freshwater reservoirs with a muddy bottom and slowly flowing or stagnant water. The most common is common toothless(Anodonta cygnea) is a rather large mollusk from detachment of true laminabranchs(Eulamellibranchia). When examining a mollusk, it is important to determine the anterior and posterior ends of the shell. The front end of the toothless is easily recognizable by the more rounded shape of the shell and by the forward-facing leg; at the posterior, somewhat narrower end, short outgrowths of the mantle-siphons are visible between the valves. Along the upper dorsal edge, behind the tops, there is a rather large external ligament, or ligament, - an elastic elastic cord, with the "reduction" of which the valves open. It consists of a fibrous horny substance close to chitin - conchiolin: it is formed from the outer cover of the shell itself (periostraca). The "work" of the ligament is determined by the interaction of differently located conchiolin fibers, of which it is composed. When the closing muscles, contracting, tighten the shell valves, the fibers in the lower part of the ligament are compressed, and in the upper part they are stretched, and when the muscles are relaxed, vice versa; therefore, in dead molluscs, the shell valves are always half open. In bivalve molluscs, the ligament may be external or internal, or both.

The anodont has no lock teeth and the dorsal edge is smooth, hence its name, toothless (Anodonta). In most bivalve mollusks, in order to more firmly connect the valves to each other under the crown, from the inside, on the dorsal or key edge of the shell, there are various (in shape, number and location) outgrowths, the so-called teeth, each of which enters into a corresponding recess on the opposite sash. All this, taken together, forms a shell lock. The device of the lock, the nature, number and location of its teeth is an important systematic feature in bivalve mollusks and is characteristic of the family, genus and species. The ligament is also part of the locking apparatus of bivalves, since it serves to connect the valves to each other.

The shell surface of most bivalve molluscs, including the toothless one, is covered with a differently colored outer layer, or periostraca. It is easily scraped off with a knife, and then a white porcelain-like, or prismatic, calcareous layer (ostracum) is exposed under it. Concentric lines are clearly visible on it - traces of growth of the shell, running parallel to its edges. The inner surface of the shell of many molluscs, including the toothless, is lined with a mother-of-pearl layer (hypostracum).

Periostrak, consisting of conchiolin, is resistant to external influences (both mechanical and chemical) and thus serves as a good protection for the inner calcareous layer of the shell. The resistance of periostraca to the action of carbonic acid dissolved in sea water is especially noticeable. It can accumulate near the bottom, in the very bottom layers, and in the soil where mollusks live (due to the decomposition of organic matter, partly due to the respiration of aquatic organisms), and increase with increasing depth and pressure. So, in the Kara Sea, very soft dead shells of astarte, joldium or portlandium are often found with a dissolved calcareous part of the shell, from which only one intact soft horny layer, the periostracum, remained.

Both other layers of the shell consist of calcareous prismlets or plates connected by a small amount of conchiolin. In the middle (porcelain) layer, they are located perpendicular to the surface of the shell, and in the inner (mother-of-pearl) layer, they are parallel to it; thanks to this arrangement, the interference of light is obtained, which gives the shine and iridescent play of mother-of-pearl. The thinner the plates of this layer, the more beautiful and brighter this shine. The most beautiful mother-of-pearl occurs in those mollusks in which the thickness of the mother-of-pearl plates in the layer is 0.4-0.6 microns.

The shell of a mollusk is formed as a result of the secretory work of its mantle: along its edge there are a large number of glandular cells that produce various layers of the shell. Thus, the cells of a special mantle groove running along the entire edge of the mantle form the outer conchiolin layer, the epithelial cells of the so-called marginal fold form the prismatic layer of the shell, and the outer surface of the mantle forms the mother-of-pearl layer.

The shell of bivalve molluscs, consisting of more than 90% of CaCO3, contains it in the form of calcite or aragonite, which are in various proportions. In tropical mollusks, the shell contains more aragonite, and also quite a lot of strontium. Crystallographic study of the composition of shells of fossil mollusks now makes it possible to judge the temperature of the seas in which these mollusks lived.

Calcium, which is deposited in the shell by the mantle, enters it not only through the blood, where it enters from food through the intestines, but, as recent experiments with radioactive calcium have shown, mantle cells themselves can extract calcium from water.

The growth of the shell occurs both through a general thickening of the valves as a result of the layering of more and more new calcareous plates on the inner surface of the valves, and through the growth of the entire shell along its free edge. When unfavorable conditions occur (in winter, when nutrition deteriorates, etc.), shell growth slows down or even stops, which is clearly visible on the surface of the shell in many mollusks, where characteristic thickening of lines is formed at this time, having the form of concentric stripes, running parallel to the ventral margin of the shell. By winter rings - seasonal growth stops - it is sometimes possible to determine the approximate age of a mollusk. However, in some species such rings are indistinguishable; in tropical forms, where there are no seasonal phenomena, such rings usually do not form at all. Our freshwater barley and toothless have such seasonal winter breaks in growth, so the annual rings are usually well expressed.

In order to open the shell valves of the toothless, it is necessary first of all to cut the two rather strong closing muscles that are inside it in front and behind, which tighten both valves in the transverse direction and close the shell. In a living toothless, it is easier to break its thin shell than to open it without cutting these muscles.

When the muscles are cut, the valves themselves open freely, stretched by the ligament, and one can see two soft translucent pinkish or yellowish lobes - a mantle covering the body from the sides. The edges of the mantle are slightly thickened. In this place, it is attached to the shell, on the inner surface of the valves of which the so-called mantle line is formed. The mantle of the toothless fuses behind to form two short siphons pubescent with short sensitive outgrowths.

Mollusks that burrow into the ground form long contractile siphons; the places of attachment of the muscles that draw them in form an imprint on the inner surface of the shell, the so-called sinus. The deeper the sinus, the longer the siphons of the mollusks, the deeper they can dig into the ground.

On the ventral side, a rather large wedge-shaped leg protrudes forward from under the edge of the mantle, pointing forward with its sharp end. The toothless leg is very mobile (like many other mollusks), and its action is easy to observe in the aquarium. As soon as the anodont has calmed down, its shell opens slightly, the pinkish-yellow edges of the mantle are shown, and the tip of the leg protrudes outward. If everything around is calm, the leg protrudes even further (for large anodonts by 4-5 cm), sinks into the sand, and the mollusk begins to move forward or dig into the ground with its front end, slightly rising on its leg. On the path it has traveled, a trace remains in the form of a shallow groove.

The great mobility of the toothless leg is mainly due to the contraction of the various groups of smooth muscles present in it. There are paired anterior and posterior muscles: retractors that pull the leg obliquely upward, protractors that push the leg forward, and a group of smaller muscles-lifters (elevators) of the leg upward. All these muscles are attached to the inner surface of the shell valves, where the imprints of their attachment sites are quite clearly visible (near the retractors along the hinge edge of the shell). In addition, there are a number of smaller muscles in the leg that are not attached to the valves and are located in the leg in layers and transversely.

If you turn the mantle lobe up, then the mantle cavity of the toothless will open, where its main organs are located: oral lobes, brownish gill sheets (two on each side of the body), a leg, the base of which is located between the right and left gills. In front, in the depression between the leg and the anterior muscle, the mouth opening is placed, surrounded by two pairs of small triangular contractible perioral lobes. Each gill of an anodont consists of two sheets of semigills, which in turn are composed of two plates - ascending and descending.

Each gill plate consists of rows of individual threads (filaments), and each thread, respectively, forms an ascending and descending knee. Anodonts have vascular connections (bridges) between adjacent filaments and between the knees formed by them, which is characteristic of the entire order of true lamellar gills. Each semi-gill is thus a trellis, complexly perforated two-layer plate.

In representatives of other orders of bivalve mollusks, the gills have a different arrangement (which will be discussed below).

The mantle cavity and the gills located in it are constantly washed by the current of water, which is created mainly by the flickering of the smallest cilia of the epithelium covering the surface of the mantle, gills, oral lobes and body walls. Water enters the mantle cavity of the toothless through the lower (respiratory) siphon, first entering its large, lower part - the respiratory chamber, then it is filtered through the cracks in the gills and goes into the upper part of the mantle cavity - into the respiratory chamber, from where it finally exits through the upper (output, or anal) siphon. Water is sucked in through the inlet siphon as a result of the difference in hydrostatic pressure between the subgill and supragill spaces of the mantle cavity and between it and the water surrounding the mollusk; this difference is caused not only by the work of the ciliary epithelium, but also by the contraction of the gill filaments and the muscles of the mantle and siphons. When the flow of water slows down, entering the large "inhalation chamber" of the mantle, coarse and large particles fall out of it and settle on the surface of the mantle, and then are removed from the mollusk. It is easy to verify the presence of intense currents of water entering the mantle cavity, if you put the toothless in a shallow vessel with water so that the water only slightly covers the shell. After letting it calm down, it is necessary to pour into the water near its rear end some kind of powder that remains suspended in the water (for example, ink, carmine, dry grated algae). Then you can see how the powder grains go through the lower (inlet) siphon into the shell and after a while with a strong jet of water are thrown out through the upper (output) siphon. From time to time, toothless, often without any external irritation, slams the shell valves with force and throws out jets of water, renewing all the water contained in the mantle cavity at once. Soon the shell flaps open again and the normal slow circulation of water resumes.

To verify the intensity of the work of the ciliary epithelium, you can cut out a piece of the toothless mantle and put it with the inner surface down on the bottom of the vessel. Due to the work of the cilia continuing for some time, this piece will move slightly and even crawl slightly along the inclined plane.

The suction of water and its circulation inside the mantle cavity provide the toothless not only with the oxygen necessary for its breathing, but also with food. Like all bivalve mollusks, the toothless one is devoid of a head and a number of organs associated with it - a separate pharynx, salivary glands, hard formations for chewing food (such as chitinous plates - a grater found in gastropods). Therefore, toothless can not eat large organisms. She and most bivalves (Eulamellibranchia and Filibranchia) are active filter feeders. Such mollusks feed on detritus suspended in the water column (the smallest remains of dead plants and animals) and microplankton (unicellular algae, bacteria and very small animals). With the help of a complex ciliary mechanism of gills and near-mouth lobes, mollusks filter them out of the water, separating inedible mineral suspension and large food particles for them.

The gill filaments of molluscs have rows of cilia of different sizes located in certain places, which can filter and sort food particles, envelop them with mucus, and then direct them to food grooves located along the ventral edge of the semi-gills (at the transition points of descending gill knees to ascending) or at their base. Rows of rather large lateral, most intensively working cilia on the gill filaments filter water through narrow gaps between the gill filaments and ensure its passage from the "inhalation" to the "exhalation" chamber of the mantle cavity. Particularly large lateral-frontal cilia, located on the sides of the gill filaments, strain food particles from the water or catch them in abundantly secreted mucus and push them to the outer side of the gill filaments. Here are the frontal cilia, which collect food particles and direct them down to the food groove. Food particles that collect in the food grooves are also enveloped in mucus, form lumps here, compact and, thanks to the work of the cilia of the groove, are directed to the oral lobes. The mouth lobes of mollusks are a very efficient sorting apparatus, freeing food from inedible particles. They are armed with many sensitive elements - chemo and mechanoreceptors. They have rows of transverse grooves, armed with especially long cilia; the smallest particles suitable for nutrition are guided along a series of such grooves to the oral groove (located at the base of both lobes), along which they are further directed to the oral opening, where they are swallowed. Along other grooves (with cilia working in the opposite direction to that of the previous ones), larger particles and slimy lumps, unsuitable for nutrition, roll down and fall onto the mantle. The strong cilia of the mantle margins drive these particles back to the base of the introductory siphon; as they move there, these particles stick together, compact and are thrown out in the form of so-called pseudofeces.

In bivalve mollusks from the Protobranchia group (walnut, yoldium, portlandium, etc.), which have the most simply arranged petal-shaped gills - ktenid and i, the oral tentacles are very large, contractile and equipped with a long grooved outgrowth. With the help of it, they collect small food particles from the soil surface - detritus, which are then transferred by cilia along the groove to the plates of the oral tentacles, where they are sorted; gills-ctenidia serve mainly to create currents of water. The work of the filtering and sorting apparatus of bivalves is quite perfect. So, mussels can filter out particles ranging in size from 40 to 1.5-2 microns (best of all - 7-8 microns), completely removing them from the water. They detain unicellular algae and flagellates; heavier particles of mineral suspensions, even 4-5 microns in size, are not retained by mussels. From a mixture of algae and purple bacteria, oysters extract only algae; they usually trap flagellates, algae and organic particles larger than 2-3 microns and let all particles 1 micron or smaller pass through.

Bivalves filter very large volumes of water. So, an oyster can filter about 10 liters of water in an hour; mussel - up to 2-5 liters (at a higher temperature there is more water, at a lower temperature - less); edible cockle at a water temperature of 17-19.5 ° C - from 0.2 to 2.5 liters, an average of 0.5 liters of water per hour; small scallops filter at a rate of 1 liter per hour per 1 g of their weight, while old ones filter only 0.7 liters.

The digestive system of the toothless, like that of all bivalves, consists of a short esophagus, a more or less rounded stomach, middle and hindgut; the ducts of a paired digestive gland, the liver, open into the stomach, and the end of the so-called crystalline stalk protrudes on the ventral side. The intestine (midgut) departing from the stomach at the base of the leg, forms 1-2 turns in the mass of the gonads, then passes to the dorsal side and, penetrating the lower wall of the pericardial sac, passes through the ventricle of the heart, goes beyond the pericardium through its dorsal part, passes above the posterior closing muscle and ends with an anus opening into the cloacal chamber of the mantle cavity with its excretory siphon. The part of the intestine that runs from the pericardium to the anus is commonly called the rectum or hindgut. The intestinal tract of bivalve mollusks does not have muscle fibers, and the movement of food in it occurs due to the work of the ciliary epithelium lining it. The removal of undigested residues is facilitated by the muscular ligament surrounding the anus.

Once through the short esophagus into the stomach, food particles are sorted into small and large due to the activity of the ciliary current and the gastric groove. Large food particles enter the intestines, while smaller ones are carried along the folds of the stomach and are collected at the protruding end of the crystalline stalk. Its protruding end rotates all the time, which contributes to the mixing of food particles and their sorting. The crystalline stalk is formed in a special sac-like organ and is a vitreous rod of a gelatinous substance consisting of a globulin-type protein with enzymes (amylase, etc.) adsorbed in it, capable of digesting carbohydrates (starch, glycogen). Once in the slightly acidic environment of the intestine, it begins to dissolve and release the enzymes adsorbed in it - the only ones secreted by bivalve mollusks into the intestinal tract for extracellular digestion of food. Small food particles, processed by enzymes of the crystalline stalk, come from the stomach to the outgrowths of the liver. It consists of a very large number of elongated blind tubules - a diverticulum and is not a digestive gland in the usual sense; it does not produce or release any digestive enzymes into the intestinal tract, and is an organ for intracellular (rather than extracellular) digestion and absorption. Intracellular digestion in bivalves is carried out mainly by phagocytic wandering cells - amoebocytes. They are found in abundance not only in the diverticula of the liver, but also in the stomach and midgut. Amebocytes have various enzymes and are able to digest not only carbohydrates, but also proteins and fats, etc. Wandering cells can pass through the epithelium of the intestinal tract into its lumen and return back to the tissues. Liver cells also swallow and digest food particles; they can also wander through the lumen of the diverticulum and return back to the liver walls. Wandering cells play the main role in the digestion of food in bivalves.

With the death of amoebocytes and liver cells, their digestive enzymes can enter the lumen of the intestinal tract. Therefore, traces of various enzymes (proteases, lipases) are found in extracts from the liver and stomach of bivalves.

Not all organisms that enter the intestinal tract are digested by bivalves. Often, especially with a large amount of food, living diatoms (unicellular algae with a silicon skeleton), small copepods, etc. are found in the fecal masses of mollusks. the concentration of planktonic algae they feed on.

From what has been said, it can be seen that digestion in bivalve mollusks is very peculiar. They can only digest carbohydrates extracellularly, and the protein and fat components of their food are digested by phagocytic wandering amoebocytes and their "liver" cells. Thus, bivalves are a very specialized group of animals adapted to feeding on detritus, unicellular algae, and bacteria.

The circulatory system of toothless mollusks, like all bivalve mollusks, is open, and blood - hemolymph - circulates not only through the blood vessels - arteries and veins, but also in the spaces between organs, and in the connective tissue through a whole system of lacunae and sinuses that do not have their own walls. Arterial blood flows mainly through the vessels, and the venous system has a predominantly lacunar character. Blood is driven through the entire system by the contraction of the heart as well as the musculature of the body. The heart of bivalves (anodonts) consists of a ventricle and two atria and lies in the pericardial cavity, or pericardial sac, located on the dorsal side of the body. The pericardium is an elongated thin-walled sac filled with hemolymph, and in bivalve mollusks it is part of their secondary body cavity, which is greatly reduced in volume. The ventricle has powerful muscular walls and looks like a pear-shaped bag, with its wide end facing backwards. The atria are very thin-walled, translucent and most often look like elongated triangles, the tops of which open into the ventricle; at the entrance to the latter, they are equipped with small semilunar folds - valves that allow blood to pass only from the atrium to the ventricle.

In toothless, like in most bivalves, the ventricle is pierced by the posterior intestine passing through it, but its cavity is completely closed and separated from the intestine by its wall. From the ventricle, blood diverges throughout the body: to the posterior end - through the posterior aorta, which is divided into two arteries that feed the vessels of the posterior part of the mantle and the posterior closing muscle; to the anterior end - through the anterior aorta and arteries extending from it to the leg, to the viscera and to the front of the mantle. From the arterial vessels, the blood pours into the gaps not filled with tissues, and through the system of lacunae, the blood that has become venous is collected through the sinuses and veins into a large longitudinal venous sinus, which lies between the excretory organs. Passing from here through the venous system of the kidneys, it merges into the afferent paired branchial arteries, passing at the base of each gill. Of these, venous blood flows through the afferent gill vessels of the descending gill plates along the gill filaments and their vascular lintels. Arterial blood oxidized in the gills, saturated with oxygen, flows through the efferent vessels of the ascending gill plates into the paired (on each side of the mollusk) gill veins, from where it enters the atria. The atria also receive that part of the blood that, bypassing the gills and kidneys, was oxidized in the vessels of the mantle folds and entered the external gill veins through the mantle veins. In bivalve mollusks, the mantle, with its highly branched blood vessels, plays a very important role in respiration and oxygenation of the blood.

The fact that in most bivalve mollusks the ventricle of the heart is permeated with the rectum finds its explanation in the peculiarities of their embryonic development and in the evolution of this entire group. A number of lower representatives of the bivalves have not only two atria, but also two separate ventricles lying on the sides of the intestine (near the arches); in others, the unpaired ventricle lies above the intestine (nutlets, anomia, limas), in others, it lies downward from the intestine (oysters, pearl oysters, etc.) - All this indicates that the location of the intestine and heart in relation to each other has undergone great changes in the evolution of bivalves, and the fact that they originally had two separate ventricles, which then merged together. The heart rate in bivalves, which are generally sedentary organisms, is low, usually no more than 15-30 times per minute, while in such mobile and active mollusks as cephalopods, the heart contracts 40-80 times per minute. All parts of the heart of bivalves can contract autonomously.

In bivalves, as in invertebrates with an open circulatory system in general, blood pressure is very low and highly variable.

The blood-hemolymph of bivalves plays a huge role in their life and metabolism. It performs a number of functions: it provides internal organs and tissues with oxygen and nutrients, carries away their metabolic products (carbon dioxide, nitrogen metabolism products, etc.), creates and maintains the constancy of their internal environment (ionic composition, osmotic pressure). Finally, it plays a very important role in creating the hydraulic mechanism of pressure, the necessary turgor (tension), as well as in the movement of mollusks. The study of blood circulation in the body of bivalve mollusks explained the phenomenon of swelling of the legs, which is observed when the animal moves and burrows. It occurs due to its filling with blood, which gives the leg the necessary elasticity, creates the necessary turgor. When the leg is stretched and the leg muscles relax, blood flows through the artery to the leg, and when it contracts, it goes back to the body. So, in a sea stalk, which can burrow very quickly, the leg first sinks into the ground and the blood quickly flows into it, expanding the end of the leg in the form of a disk; the latter serves as an anchor when the leg muscles, contracting, pull the mollusk down. When the mollusk rises from the ground to the surface, the leg muscles relax, and the end of the leg expands again, filling with blood; holding on to such an "anchor", the leg is extended, as part of the blood enters the upper part of the leg and pushes the mollusk up. The inflow, injection of the amount of blood necessary for swelling of the leg and its outflow are regulated by the so-called keberian organ, which plays the role of a valve.

In contrast to animals with a closed circulatory system, bivalve mollusks, like all invertebrates with an open circulatory system, have a fairly significant amount of blood - hemolymph. In mollusks (except cephalopods), it is 40-60% (volume percent) of their body weight without a shell. At freshwater pearl mussel(Margaritifera) and mussels(Mytilus californianus) per 100 g of body weight, the volume of blood is about 50 ml.

In the blood of bivalve mollusks there are many formed elements, mainly various forms of amoebocytes (leukocytes). Their number varies in different species from 6,000 to 40,000 per 1 mm3. Bivalves also have erythrocytes; sometimes they can even be more than some forms of leukocytes. Hemoglobin is found in quite a few species (arches, sea cuttings, tellins, pectunculus, astartes, etc.).

Important for gas exchange (for supplying organs and tissues with oxygen), the ability of blood to be saturated with oxygen in bivalve mollusks is very small and amounts to 1-5% of their blood volume. So, 100 ml of toothless blood can absorb only 0.7 ml of oxygen, while in mussels - 0.3 ml. Toothless consumes 0.002 ml 02 per 1 g of its weight in one hour (at 10°C); oysters, respectively - 0.006 ml 02 (at 20°C); mussels-0.055 ml 02. More mobile species usually consume somewhat more, such as, for example, scallops Pecten grandis, consuming 0.07 ml 02 per 1 g of their weight in 1 hour (or 70 cm3 02 per 1 kg of weight). Small forms also often consume more oxygen than large ones. For example, at the optimal water temperature (18°C), the horny charr consumes 0.05 mg 02 per 1 g of weight per hour, but when the water temperature drops to 0.5°C, oxygen consumption almost stops. In oxygen consumption, i.e., in the intensity of metabolism, many bivalves show seasonal fluctuations; Thus, mussels in the summer, the most active time of their life, consume approximately twice as much oxygen as in the winter, cold season.

Many bivalves can live for quite a long time with very little or no oxygen in the water. So, sand shell(Mua arenaria) can live in anoxic conditions at 14°C for up to 8 days, and at 0° even for several weeks; virgin oyster also endures such conditions for a week or more. Metabolism during such periods of anaerobiosis is sharply reduced, but mollusks can at the same time receive the oxygen necessary for their life through intramolecular respiration - glycolytic breakdown of their reserve substances (carbohydrates and fats) by the type of fermentation. This ability to temporary (facultative) anaerobiosis (anoxibiosis) is especially characteristic and necessary for species living in the littoral zone (such as, for example, for sand shells, mussels, Baltic poppy, edible cockle). At low tide, they close their shells, a small amount of oxygen in their mantle cavity disappears rather quickly, and they begin to live due to the processes of anoxybiosis. At high tide, they open their shells, continuously filter the water and breathe in the oxygen dissolved in the water; at first, they sharply (several times) increase the intensity of filtration and oxygen consumption, and then after a while it returns to normal, characteristic of their life in water.

The organs of excretion in bivalves are the kidneys, and also, but to a lesser extent, the so-called keber organ; the latter is a glandular thickening of the anterior part and anterior-lateral walls of the pericardial sac.

The kidneys, or Boyanus organs, open with their inner ends into the pericardium, and with their outer ends into the mantle cavity. Toothless kidneys look like two dark green curved tubular sacs; one end has glandular walls and represents the actual excretory part of the kidney, the other has the form of a bubble, where metabolic products accumulate to be removed from the body.

In bivalves, there is no such concentration of parts of the central nervous system (nerve nodules, or ganglia) as in gastropods. The toothless, for example, has one pair of ganglia above the mouth, slightly behind it, another pair deep in the leg, and a third behind the posterior locking muscle. Between the first and second pair of ganglia, as well as between the first and third, a pair of nerve trunks passes, and each pair of nodules is interconnected by transverse bridges (commissures).

The sense organs in bivalves are rather poorly developed compared to other classes of molluscs. However, these organs are quite diverse in their structure and are scattered in different parts of the body: along the outer edges of the mantle, at the ends of the siphons, on the first gill filaments, near the mouth opening on the near-mouth tentacles, on the edges of the posterior closing muscle, in the exhalation chamber, near the back intestines, etc. These sense organs are both rather complex formations - eyes, or photoreceptors, balance organs - statocysts, or statoreceptors, and simpler ones - osphradia, various sensitive outgrowths, and sometimes simply clusters of pigmented sensitive cells.

Photoreceptors in bivalve mollusks can be arranged very differently: from simple epithelial pigment (optical organelles) to rather complex eyes with a lens and retina. Such eyes can be very numerous, especially in free-living forms, such as the mantle eyes in combs, in which there are sometimes up to 100 of them on both edges of the mantle.

Differently arranged eyes and ocelli can also be located in bivalves on the first gill filaments (gill eyes in arches, anomies), on short outgrowths around siphon openings (in some cockles, etc.).

Many bivalves have so-called optical organelles, spherical or elongated, concentrating light on a special intracellular innervated formation (retinella). Such photoreceptors are scattered at the ends of siphons and in other parts of the body of molluscs.

The organs of balance in bivalves are a vesicular protrusion of the epithelium, well innervated, lined from the inside with ciliated epithelium, closed (statocyst) or open (statocrypt). They contain hard mineral grains (statolith) or small grains of sand (statoconia) inside. Usually, for example, in toothless, statocysts are located near the foot ganglion or on the dorsal side of the mollusk. Balance organs are well developed in free-living forms, such as scallops.

Osphradia are usually very small paired pigmented, well innervated groups of sensitive cells. They can be located in various places - on the leg, in the region of the gills, hindgut, etc. Their role is still not clear enough: whether they are chemoreceptors or organs of touch.

Anodonts, like most bivalves, have separate sexes, but in the conditions of reservoirs with stagnant water, individual hermaphroditic individuals or even their entire colonies can be found. At the same time, in order to avoid self-fertilization, male reproductive products are first produced - spermatozoa, and then female - eggs. Paired, strongly dissected sex glands (gonads) in bivalves (including anodonts) lie in the dorsal part of the leg, where they are surrounded by an intestinal loop and outgrowths of the liver; the excretory ducts of the gonads open into the mantle cavity next to the orifices of the excretory system. Only in some of the most primitive bivalves, the gonadal ducts open with a common opening with an excretory pore. In some freshwater bivalves, sexual dimorphism is so pronounced that males and females of the same species are sometimes referred to as different species.

The development of juveniles in bivalve mollusks is varied. Almost all marine forms that live in shallow waters lay their eggs directly into the water, where fertilization occurs, or it occurs in the mantle cavity of the mother. Eggs float freely in water, rarely stick together or attach to shells, algae. The exceptions are viviparous (more precisely, larval-bearing) forms, like some oysters, arches, etc.

Fertilized eggs of bivalve mollusks, having passed the stage of crushing of the spiral type, form a trochophore-like larva, similar to the larva of polychaete worms (polychaetes). However, in the process of embryonic development of bivalves, there is almost no segmentation process, which is so characteristic of the development of the larval stages of annelids. The larvae of bivalve molluscs have an rudiment of a leg and a primary shell (prodissoconch), which is initially laid in the form of a single plate located on the dorsal side of the larva. After a series of changes in the trochophore, in which the sail (velum) arises - the ciliary parietal disc, the bivalve shell and the rudiments of other organs, turn into a veliger. The presence of a free-swimming larva (veliger) is a very important life stage for mollusks, as it provides them with the possibility of wide dispersal, since adult bivalves usually lead a sedentary or even attached lifestyle. At the same time, this larval stage of life, like that of other invertebrates, is the most sensitive to unfavorable external conditions, and only the high fecundity of bivalve mollusks ensures the preservation of the species and its distribution.

In a number of marine cold-water and, apparently, in many deep-sea oceanic species of bivalve mollusks, development can occur with a very shortened stage of the swimming larva or without it at all. In the latter case, a few large eggs are formed, supplied with a large amount of nutrients. This allows them to develop regardless of the presence of food in the surrounding water, which is especially important for deep-sea forms, where the amount of food for juveniles at the bottom is very limited.

Based on the structure of the shell, castle, gills, on the location and number of muscle-contacts, ligaments, etc., detachments are distinguished: Comb-toothed, Ligamentous-toothed, True lamellar-branched and Septoid-branched.

Animal life: in 6 volumes. - M.: Enlightenment. Edited by professors N.A. Gladkov, A.V. Mikheev. 1970 .

Nervous system. There are three pairs of nerve nodes - head, foot and trunk. The ganglia of each pair are interconnected by short commissures; in addition, the head nodes are connected to the trunk and foot nodes.

sense organs. The sense organs are poorly developed and are mainly represented by separate sensitive cells scattered in the epithelium of the oral lobes, papillae of the inlet siphon and mantle. There is also a pair of small statocysts lying close to the foot ganglia and an osphradia.

Digestive system. The mouth opening is located in front of the animal. On both sides of it lies a pair of oral lobes. The mouth opens directly into a short esophagus, which passes into a rather voluminous stomach.

1 - head ganglia, 2 - trunk ganglia, 3 - foot ganglia

Surrounding the stomach is the digestive gland, or liver. A peculiar formation is placed on the ventral side of the stomach, the so-called crystalline stalk - a vitreous-transparent gelatinous stick, it sorts the particles entering the stomach and releases substances that stick them together.

A long intestine begins from the stomach, forming several loops and opening with an anus near the excretory siphon.

Respiratory system. The respiratory organs, or gills, are located on the sides of the leg and are represented on each side by its two half-gills - internal and external. Each semi-gill is composed of a series of curved gill filaments, closely interconnected by connective tissue crossbars and forming two latticed gill plates - descending and ascending, passing one into the other below and interconnected by crossbars.

Thus, each half-gill is a two-layer lattice formation.

Circulatory system. The circulatory system is not closed.

A heart is placed on the dorsal side, consisting of a ventricle lying in the midline and perforated by the intestine, and two atria lying on its sides. From the ventricle, two main vessels depart back and forth - the aorta, further disintegrating into arteries and capillaries. Capillaries pour blood into lacunae located among the parenchyma. Blood saturated with carbon dioxide is collected in the veins, passes through the gills and excretory organs; then enters the atria, and from them into the ventricle.

excretory system. The organs of excretion are a pair of kidneys, lying one on each side of the pericardium and called the boyanus organs. The kidneys are highly modified coelomoducts (see above) and open on one side into the pericardium, and on the other into the mantle cavity. In addition, the excretory organ is the Keberian organ, which is a glandular thickening of the anterior wall of the pericardium.

Lab #4

Topic. The external structure of the shells of freshwater and marine mollusks (optional - point 2 or 3).
Target. Establish similarities and differences in the structure of mollusk shells.
Equipment: tweezers, mollusk shells: scallop, mussel, barley, toothless, horn coil, large pond snail, etc.

Working process

Consider scallop shells and mussels. Find out their similarities and differences. Explain the presence of protrusions and depressions on the dorsal side of the shells. Pay attention to the shape and color of the outer and inner mother-of-pearl shells.
Examine the shells of barley (or toothless), determine the front and back. Note the similarities and differences in the external structure. Determine the age of the molluscs by the growth rings located on the shell. Scrape off part of the stratum corneum to the calcareous layer with a scalpel. Consider the inner mother-of-pearl layer.
Examine the shells of a large pond snail and a horn coil. Note the similarities and differences in the external structure of the shells. Count the number of turns in the whorl of each shell.
Draw one shell from each pair. Indicate in the figure the main parts of the external and internal structure of the shells. Write the names of these parts.
Write the main distinguishing features of the shell of each mollusk. Explain which of them can be used to determine the habitat, age and lifestyle of a mollusk.

Bivalves are widely distributed in the seas. They are water purifiers. Their body is enclosed in a bivalve shell. There is no head. A person uses these mollusks for food, extracts pearls and mother-of-pearl from them.

Lesson learned exercises

Name the representatives of bivalves using Figure 76 (p. 107). What are the distinguishing features of their external structure?
What are the layers of a mollusk shell? What substances are they formed by?
What are the features of the internal structure and vital processes of bivalve molluscs? Explain with the example of toothless and mussels.
Describe the importance of bivalves in nature and human life.

The class of bivalve mollusks includes bilaterally symmetrical invertebrate aquatic animals such as molluscs. The fossil remains of these mollusks are found in the layers of the early Paleozoic period. In the course of evolution, animals of this class flourished in the Cretaceous period. Many families gradually became completely extinct, and about 10 modern families of this class still live in water bodies, that is, they have existed for about 400 million years. Currently, about 130 families of this class are known, uniting about 10 thousand modern species. Representatives of bivalve molluscs are oysters, mussels, toothless, pearl mussels, tridacna, scallops, etc. Bivalve molluscs are widely distributed in the waters of the World Ocean and in fresh water bodies. These are benthic organisms that feed on plankton or plant detritus through filtration. This way of feeding does not require special mobility, therefore the structure of these animals is relatively simplified compared to representatives of other classes such as mollusks. Bivalves lead a sedentary lifestyle, burrowing into the ground, simply sitting on the bottom or attaching to some kind of substrate. There are carpenter mollusks that drill through rocks or wood. The sizes of bivalve shells are different: from 1 mm to 1.5 m in diameter. The largest marine mollusk is a representative of the class described above - Giant Tridacna, whose weight reaches 300 kg.

Structure of bivalves.

All bivalve mollusks have a similar structure. Unlike gastropods, in bivalve molluscs the body consists of a body flattened from the sides and legs, there is no head. Their calcareous shell has two valves (hence the name of the class), and is not twisted in a spiral, as in gastropods. The outer layer of the shell is horny; inside it there is often a layer of mother-of-pearl. The shell valves are connected along the dorsal edge of the mollusk and close when the muscles in the body of the mollusk, which are attached to the inner sides of opposite valves, contract. In most species, the shell valves on the inside on the dorsal side have protrusions and recesses (the so-called “lock”), which contributes to a more dense closing of the valves. The shell grows throughout the life of the mollusk, and concentric rings of annual growth are visible on its surface, resembling the growth rings of trees. Many species of bivalves have a well-developed nacreous layer, so most marine species and rare freshwater species are able to form pearls.

The body of the animal is entirely enclosed in a shell. The leg in most individuals has a wedge-shaped shape; in those representatives of the class who lead an absolutely immobile lifestyle, it is reduced (mussels) or disappears completely (oysters). In a dangerous situation, the mollusk retracts its leg and slams the shell shut. The body of the mollusc is covered with a mantle, its folds coalesce and form siphons at the posterior end of the body. Through the inlet siphon, oxygen-enriched water with nutrients enters the mantle cavity, and through the outlet siphon, the mollusk gets rid of undigested food residues and metabolic products. Under the mantle on both sides of the body there are respiratory organs - gills - of various structures in different species, in most bivalves they are gill plates. The digestive system of bivalve molluscs is represented by the mouth opening, esophagus, stomach, liver, intestines, and the anus opens into the mantle cavity. The circulatory system of these mollusks is open, a three-chambered heart, consisting of two atria and a ventricle, is located in the dorsal part of the body. The nervous system consists of three pairs of ganglia, the sense organs (balance, tactile sensitivity, in some eyes) are underdeveloped. The excretory system is represented by two kidneys, the excretory ducts of which open into the mantle cavity.

The type of molluscs, numbering about 130,000 species, is second in number of species only to arthropods and represents the second largest type of animal world. Mollusks are predominantly aquatic; only a small number of species live on land.

Mollusks are of various practical importance. Among them there are useful ones, like pearl and barley, which are mined in order to obtain natural pearls and mother-of-pearl. Oysters and some other species are harvested and even bred for food use. Some species are pests of agricultural crops. From a medical point of view, mollusks are of interest as intermediate hosts of helminths.

General characteristics of the type

Animals belonging to the type of molluscs are characterized by:

three-layer, - i.e. formation of organs from ecto-, ento- and mesoderm
bilateral symmetry, often distorted due to displacement of organs
non-segmented body, usually covered by a shell, whole, bivalve, or consisting of several plates
skin fold - a mantle that fits the entire body
muscular outgrowth - a leg that serves to move
poorly defined coelomic cavity
the presence of the main systems: the apparatus of movement, digestive, respiratory, excretory, circulatory system, nervous and sexual

The body of mollusks has bilateral symmetry, in gastropods (they include, for example, a pond snail), it is asymmetrical. Only the most primitive mollusks retain signs of segmentation of the body and internal organs; in most species, it is not divided into segments. The body cavity is secondary, presented in the form of a pericardial sac and a cavity of the gonads. The space between the organs is filled with connective tissue (parenchyma).

The body of mollusks consists of three sections - the head, trunk and legs. In bivalves, the head is reduced. Leg - a muscular outgrowth of the abdominal wall of the body - serves for movement.

A large skin fold, the mantle, is developed at the base of the body. Between the mantle and the body there is a mantle cavity, in which there are gills, sensory organs, openings of the hindgut, excretory and reproductive systems open here. The mantle gives off a shell that protects the body from the outside. The shell can be solid, bivalve or consist of several plates. The composition of the shell includes calcium carbonate (CaCO 3) and organic matter conchiolin. In many mollusks, the shell is more or less reduced (for example, in some cephalopods, in naked slugs, etc.).

The circulatory system is not closed. The respiratory organs are represented by gills or a lung formed by part of the mantle (for example, in pond snails, grape and garden snails, naked slugs). The excretory organs - the kidneys - are connected by their inner ends to the pericardial sac.

The nervous system consists of several pairs of nerve nodes connected by longitudinal trunks.

The type of molluscs includes 7 classes. The most important of them:

gastropods (Gastropoda) - slowly crawling snails
bivalves (Bivalvia) - relatively sedentary molluscs
cephalopods (Cephalopoda) - mobile molluscs

Table 1. Characteristic features of bivalves and gastropods
sign	Class
sign	Bivalves	gastropods
Symmetry type	Bilateral	Asymmetric with reduction of some right organs
Head	Reduced together with related organs	Developed
Respiratory system	Gills	gills or lung
Sink	Bivalve	Spiral twisted or cap-shaped
reproductive system	Dioecious	Hermaphroditic or dioecious
Nutrition	passive	Active
Habitat	Marine or freshwater	Marine, freshwater or land

Class gastropods (Gastropoda)

This class includes molluscs that have a shell (snails). Its height ranges from 0.5 mm to 70 cm. Most often, the gastropod shell has the form of a cap or spiral, only in representatives of one family does a shell develop from 2 valves connected by an elastic ligament. The structure and shape of the shell are of great importance in the taxonomy of molluscs. [show] .

A placospiral shell is a highly twisted shell, the whorls of which are located in the same plane.
Turbospiral shell - shell whorls lie in different planes
Right-handed shell - the spiral of the shell is twisted clockwise
Left-handed shell - the spiral is twisted counterclockwise
Hidden spiral (involute) shell - the last whorl of the shell is very wide and completely covers all previous ones.
Open-spiral (evolute) shell - all whorls of the shell are visible

Sometimes the shell is equipped with a lid located on the dorsal side in the back of the leg (for example, in meadowsweet). When retracting the leg into the shell, the lid tightly covers the mouth.

In some species that have switched to a floating lifestyle (for example, pteropods and keeleds), the shell is absent. Shell reduction is also characteristic of some terrestrial gastropod molluscs living in the soil and forest litter (eg, slugs).

The body of gastropods consists of a well-separated head, legs and torso - an visceral sac; the latter is placed inside the sink. On the head are a mouth, two tentacles and at their base - two eyes.

Digestive system. At the front end of the head is the mouth. A powerful tongue is developed in it, covered with a hard chitinous grater, or radula. With its help, molluscs scrape algae from the ground or aquatic plants. In predatory species, a long proboscis develops in the front of the body, which can turn out through a hole on the lower surface of the head. In some gastropods (for example, cones), individual teeth of the radula may protrude from the mouth opening and have the form of stylets or hollow harpoons. With their help, the mollusk injects poison into the body of the victim. Some predatory species of gastropods feed on bivalve mollusks. They drill into their shells, releasing saliva containing sulfuric acid.

Through the esophagus, food enters the sac-like stomach, into which the ducts of the liver flow. Then the food enters the intestine, which bends in a loop and ends on the right side of the body with an anus.

The nerve ganglions are collected in the peripharyngeal nerve ring, from which the nerves extend to all organs. On the tentacles are tactile receptors and organs of chemical sense (taste and smell). There are balance organs and eyes.

In most gastropods, the body protrudes above the leg in the form of a large spirally twisted bag. Outside, it is covered with a mantle and closely adheres to the inner surface of the shell.

The respiratory organs of mollusks are represented by gills located in the anterior part of the body and directed with their apex forward (anterior gill mollusks) or located in the right rear part of the body and directed backward with their apex (posterior gills). In some gastropods (for example, nudibranchs), real gills have been reduced. As respiratory organs, they develop the so-called. skin adaptive gills. In addition, in land and secondary aquatic gastropod mollusks, part of the mantle forms a kind of lung, numerous blood vessels develop in its walls, and gas exchange occurs here. The pond snail, for example, breathes atmospheric oxygen, so it often rises to the surface of the water and opens a round breathing hole on the right at the base of the shell. Next to the lung is the heart, which consists of an atrium and a ventricle. The circulatory system is open, the blood is colorless. The excretory organs are represented by one kidney.

Among gastropods, there are both dioecious species and hermaphrodites, the gonad of which produces both spermine and eggs. Fertilization is always cross, development, as a rule, with metamorphosis. All land, freshwater, and some marine gastropods have direct development. The eggs are laid in long slimy filaments attached to moving objects.

belongs to the class of gastropods

Common pond snail, often found on aquatic plants in ponds, lakes and rivers. Its shell is solid, 4-7 cm long, spirally twisted, with 4-5 whorls, a sharp apex and a large opening - the mouth. The leg and head can protrude through the mouth.
Intermediate hosts of trematodes also belong to gastropods.
The intermediate host of the cat's fluke - bithynia (Bithynia leachi) - is widespread in freshwater reservoirs of our country. It lives in the coastal zone of rivers overgrown with vegetation, in lakes and ponds. The shell is dark brown, has 5 convex whorls. Shell height 6-12 mm.
The intermediate host of the liver fluke - the small pond snail (Limnea truncatula) - is widely distributed in Russia. The shell is small, no more than 10 mm in height, forms 6-7 whorls. It lives in ponds, swamps, ditches and puddles, where it often occurs in large numbers. In some areas, there are more than 1 million pond snails per hectare of swamps. When swamps dry up, pond snails burrow into the ground, experiencing a dry time in the ground.
Intermediate hosts of the lanceolate fluke are terrestrial mollusks Helicella and Zebrina (Helicella and Zebrina). Distributed in Ukraine, Moldova, Crimea and the Caucasus. Adapted to life in arid conditions; live in the open steppe on the stems of herbaceous plants. During the heat, helicella often accumulate on plants in clusters, escaping in this way from drying out. Helicella has a low-conical shell with 4-6 whorls; the shell is light, with dark spiral stripes and a wide rounded mouth. Zebrina has a highly conical shell with 8-11 whorls; the shell is light, with brown stripes running from the apex to the base; the mouth is irregularly oval.

Class bivalve (Bivalvia)

This class includes mollusks with a shell consisting of two symmetrical halves, or valves. These are sedentary, sometimes completely immobile animals that live at the bottom of the seas and freshwater reservoirs. They often burrow into the ground. The head is reduced. In freshwater reservoirs, toothless or barley are widespread. Of the marine forms, oysters are of the greatest importance. Very large species are found in tropical seas. The shell of a giant tridacna weighs up to 250 kg.

Pearl barley, or toothless lives on the silty and sandy bottom of rivers, lakes and ponds. This inactive animal feeds passively. Toothless food is detritus particles suspended in water (the smallest remains of plants and animals), bacteria, unicellular algae, flagellates, ciliates. The mollusk filters them out of the water passing through the mantle cavity.

The body of the toothless, up to 20 cm long, is covered on the outside with a bivalve shell. Distinguish between an expanded and rounded anterior end of the shell, and a narrowed, pointed posterior end. On the dorsal side, the flaps are connected by a strong elastic ligament, which keeps them in a semi-open state. The shell closes under the action of two closing muscles - anterior and posterior - each of which is attached to both valves.

Three layers are distinguished in the shell - horny, or conchiolin, which gives it a brownish-green color on the outside, a middle thick porcelain-like layer (consists of prisms of carbonic lime; located perpendicular to the surface - shells) and an inner mother-of-pearl layer (in it, between the thinnest calcareous leaves, there are thin layers of conchiolin). The mother-of-pearl layer is underlain on each of the two flaps by a yellowish-pink fold of the mantle. The epithelium of the mantle secretes a shell; in some species of freshwater and marine pearl mussels, it also forms pearls.

The body is located in the dorsal part of the shell, a muscular outgrowth departs from it - the leg. In the mantle cavity on both sides of the body there are a pair of lamellar gills.

In the posterior part, both shell valves and mantle folds do not fit snugly against one another; two openings remain between them - siphons. The lower, introductory, siphon serves to introduce water into the mantle cavity. A continuous directed flow of water is carried out due to the movement of numerous cilia that cover the surface of the body, mantle, gills and other organs of the mantle cavity. Water washes the gills and provides gas exchange, it also contains food particles. Through the upper, output, siphon, the used water, together with excrement, is brought out.

The mouth is at the front end of the body above the base of the leg. On the sides of the mouth are two pairs of triangular oral lobes. The cilia covering them with their movement adjust the food particles to the mouth. Due to the reduction of the head in barley and other bivalve mollusks, the pharynx and associated organs (salivary glands, jaws, etc.)

The digestive system of the barley consists of a short esophagus, a sac-like stomach, a liver, a long loop-shaped curved midgut and a short hindgut. A sac-like outgrowth opening opens into the stomach, inside of which there is a transparent crystalline stalk. With its help, the food is crushed, and the stalk itself gradually dissolves and releases the amylase, lipase and other enzymes contained in it, which provide the primary processing of food.

The circulatory system is not closed; colorless blood flows not only through the vessels, but also in the spaces between the organs. Gas exchange occurs in the gill filaments, from there the blood is sent to the efferent gill vessel and then to the corresponding (right or left) atrium, and from it to the unpaired ventricle, from which two arterial vessels begin - the anterior and posterior aorta. Thus, in bivalves, the heart consists of two atria and one ventricle. The heart is located in the pericardial sac on the dorsal side of the body.

The excretory organs, or kidneys, look like dark green tubular sacs, they start from the pericardial cavity and open into the mantle cavity.

The nervous system consists of three pairs of nerve nodes connected by nerve fibers. The sense organs are poorly developed due to the reduction of the head and a sedentary lifestyle.

Cephalopoda class

unites the most highly organized mollusks leading an active lifestyle. Cephalopods include the largest representatives of invertebrates - octopuses, squids, cuttlefish.

The body shape of cephalopods is very diverse and depends on their lifestyle. The inhabitants of the water column, which include most squids, have an elongated, torpedo-shaped body. For benthic species, among which octopuses predominate, a sac-like body is characteristic. In cuttlefish living in the bottom layer of water, the body is flattened in the dorsal direction. Narrow, spherical or jellyfish-like planktonic species of cephalopods are distinguished by their small size and gelatinous body.

Most modern cephalopods do not have an outer shell. It turns into an element of the internal skeleton. Only nautiluses retain an external, spirally twisted shell, divided into internal chambers. In cuttlefish, the shell, as a rule, looks like a large porous calcareous plate. The spirula retains a spiral shell hidden under the skin. In squids, only a thin horny plate remains from the shell, stretching along the dorsal side of the body. In octopuses, the shell is almost completely reduced and only small crystals of carbonic lime remain from it. Female argonauts (one of the species of octopuses) develop a special brood chamber, shaped very much like an outer shell. However, this is only an apparent resemblance, since it is secreted by the epithelium of the tentacles and is intended only to protect the developing eggs.

One of the distinguishing features of cephalopods is their internal cartilaginous skeleton. Cartilage, similar in structure to the cartilage of vertebrates, surrounds the head cluster of ganglia, forming a cartilaginous capsule. Processes depart from it, reinforcing the eye openings and organs of balance. In addition, supporting cartilage develops in cufflinks, the base of the tentacles, and fins.

The body of cephalopods consists of a head with compound eyes, a crown of tentacles or arms, a funnel, and a torso. Large complex eyes are located on the sides of the head and are not inferior in complexity to the eyes of vertebrates. The eyes have a lens, cornea and iris. Cephalopods have developed not only the ability to see in stronger or weaker light, but also accommodation. True, it is achieved not due to a change in the curvature of the lens, as in humans, but due to its approach or removal from the retina.

On the head around the mouth is a crown of very mobile tentacles, which are one part of a modified leg (hence the name). In the vast majority of species, powerful suckers are located on their inner surface. Squids use tentacles to catch prey, in male octopuses one of the tentacles is used to carry sexual products. During the breeding season, this tentacle is modified, and during the mating period it breaks off and, due to its ability to move, penetrates into the mantle cavity of the female.

The other part of the leg turns into a funnel, which plays an important role in movement. It grows to the ventral side of the body, opening at one end into the mantle cavity, and at the other into the external environment. The mantle cavity in cephalopods is located on the ventral side of the body. At the point of transition of the body to the head, it communicates with the external environment through the transverse abdominal opening. For its closure, in most cephalopods, paired semilunar pits are formed on the ventral side of the body. Opposite them, on the inside of the mantle, there are two hard tubercles reinforced with cartilage, the so-called. cufflinks. As a result of muscle contraction, the cufflinks enter the semilunar recesses, tightly fastening the mantle to the body. When the abdominal opening is open, water freely penetrates into the mantle cavity, washing the gills lying in it. After this, the mantle cavity closes and its muscles contract. Water is pushed out with force from the funnel lying between two cufflinks, and the mollusk, receiving a reverse push, moves forward with the rear end of the body. This type of movement is called reactive.

All cephalopods are predators and feed on various crustaceans and fish. They use tentacles to capture prey, and powerful horny jaws to kill. They are located in the muscular pharynx and resemble the beak of a parrot. A radula is also placed here - a chitinous ribbon with 7-11 rows of teeth. 1 or 2 pairs of salivary glands open into the pharynx. Their secret contains hydrolytic enzymes that break down polysaccharides and proteins. Often, the secretions of the second pair of salivary glands are poisonous. The venom also helps to immobilize and kill large prey.

The intestines are branched, with digestive glands. In many species, the duct of the ink gland opens directly in front of the anus into the lumen of the hindgut. It secretes a dark secret (ink) that can cloud a large amount of water. The ink serves as a smoke screen, disorients the enemy, and sometimes paralyzes his sense of smell. Cephalopods use it to escape predators.

The circulatory system is almost closed. Heart with 2 or 4 atria, kidneys also 2 or 4, their number is a multiple of the number of gills.

The nervous system has the highest organization with developed structures of touch, smell, sight and hearing. The ganglia of the nervous system form a common nerve mass - a multifunctional brain, which is located in a protective cartilage capsule. Two large nerves depart from the posterior part of the brain. Cephalopods have complex behavior, have a good memory and show the ability to learn. For the perfection of the brain, cephalopods are called "primates of the sea."

The unique skin photoreceptors of cephalopods react to the slightest changes in illumination. Some cephalopods are able to glow due to the bioluminescence of photophores.

All cephalopods are dioecious animals; some of them have pronounced sexual dimorphism. Males, as a rule, are smaller than females, armed with one or two modified arms - hectocotyls, with the help of which "packages" with seminal fluid - spermatophores - are transferred during the copulation period. Fertilization is external-internal and occurs not in the genital tract of the female, but in her mantle cavity. It consists in the capture of sperm by the gelatinous shell of the eggs. After fertilization, females attach clusters of eggs to bottom objects. Some species take care of the offspring and guard the developing eggs. The female guarding offspring can starve for more than 2 months. In octopuses, cuttlefish and nautiluses, each egg hatches a mini copy of the parents, only in squid development comes with metamorphosis. The young grow rapidly and often reach sexual maturity by the year.

The value of shellfish

Freshwater pearl shells with a mother-of-pearl layer thickness of about 2.5 mm are suitable for making mother-of-pearl buttons and other jewelry. Some bivalves (mussels, oysters, scallops), a grape snail from gastropod mollusks (in some European countries it is bred in snail farms), squids are especially valuable from cephalopods in terms of caloric content and protein composition (more than 600 thousand of them are harvested annually in the world). . T).

River zebra mussel is found in huge numbers in the reservoirs of the Volga, Dnieper, Don, in lakes, estuaries of the Black Sea, and desalinated areas of the Azov, Caspian and Aral Seas. It overgrows stones, piles and various hydraulic structures: watercourses, technical and drinking water supply pipes, protective gratings, etc., and its amount can reach 10 thousand copies per 1 m 2 and cover the substrate in several layers. This makes it difficult for the passage of water, so constant cleaning of zebra mussels from fouling is necessary; mechanical, chemical, electrical and biological control methods are used. Some bivalve molluscs make passages in the bottoms of ships, wooden parts of port facilities (shipworm).

Perlovitsa and some other bivalves play an important role in marine and freshwater biocenoses as natural water purifiers - biofilters. One large barley is able to filter 20-40 liters of water per day; mussels inhabiting 1 m 2 of the seabed can filter about 280 m 3 of water per day. At the same time, mollusks extract organic and inorganic substances from polluted water, some of which are used for their own nutrition, and some are concentrated in the form of lumps that are used to feed microorganisms.

Thus, mollusks are one of the most important parts of the self-purification system of the reservoir. Of particular importance in the system of biological self-purification of water bodies are mollusks, which have special mechanisms of resistance to pollution of water bodies with toxic substances and mineral salts, and are also adapted to living in water with a reduced amount of oxygen. The basis of the molecular mechanism of such adaptation is the carotenoids contained in the nerve cells of molluscs. Pearl barley and other filter-feeding mollusks need protection. They can be bred in special containers and used to clean artificial reservoirs of pollution, dispose of waste and obtain additional food.

Shellfish fishing is especially important in Japan, the USA, Korea, China, Indonesia, France, Italy, and England. In 1962, the extraction of mussels, oysters, scallops and other bivalve mollusks amounted to 1.7 million tons, by now the natural natural reserves of valuable edible mollusks have been depleted. In many countries, marine and freshwater mollusks are bred artificially. Since 1971, mussels have been bred on an experimental farm in the northwestern part of the Black Sea (productivity is 1000 centners of mussels per year), studies on mussel breeding are also being carried out in the basins of other seas washing the shores of our country. Shellfish meat is easily digestible, it contains a lot of vitamins, carotenoids, trace elements (iodine, iron, zinc, copper, cobalt); it is used for food by the population, as well as for fattening domestic animals. Filter-feeding mollusks can also be used in a biomonitoring system for monitoring the chemical composition of water in reservoirs.

Cephalopods, common in all seas, except for desalinated ones, despite the fact that they are predators, often serve as food for many fish and marine mammals (seals, sperm whales, etc.). Some cephalopods are edible and are an object of fishing. In China, Japan and Korea, the use of these animals as food goes back centuries; in the Mediterranean countries it also has a very long history. According to Aristotle and Plutarch, octopuses and cuttlefish were common foods in Ancient Greece. In addition, they were used in medicine, perfumery and in the manufacture of first-class paints. Currently, innate programs of complex behavior are being studied in cephalopods under laboratory conditions.