To the class, bivalve mollusks include bilateral symmetric invertebrate water animals like mollusks. Fossil remains of these mollusks are found in the formations of the early Paleozoic period. During the evolution, animals of this class have reached a heyla in the chalk period. Many families are gradually completely extinct, and about 10 modern families of this class live in water bodies and so-inone, that is, about 400 million years have existed. Currently, about 130 families of this class are known, which unite about 10 thousand modern species. Representatives of bivalve mollusks are oysters, mussels, toothless, pearls, thidakne, sea scallops, etc. Bivalve mollusks are widespread in the waters of the world ocean and in fresh water reservoirs. These are bottom organisms feed on plankton or vegetable detritus by filtering. This method of nutrition does not require special mobility, so the structure of these animals is relatively simplified compared to representatives of other classes like mollusks. Bivalves lead a sedentary lifestyle, driving into the ground, just sitting on the bottom or attaching to any substrate. There are mollusks, which drill rocks or wood. Dimensions of bivalve shells are different: from 1 mm to 1.5 m in diameter. The largest marine molluste is the representative of the above-described class - the thidakna giant, the weight of which reaches 300 kg.
The structure of bivalve mollusks.
All bivalve mollusks have a similar structure. In contrast to the bruhogs, in bivalve mollusks, the body consists of a fond with the sides of the body and legs, no heads. Their lime sink has two sash (hence the name of the class), and not twisted with a spiral, like in the bucculent. The outer layer of the sink of horny, inside it is often a layer of pearl. The sink sash is connected along the spinal edge of the mollusk and closed when cutting the muscles in the body of the mollusk, which are attached to the inner sides of opposite sash. Most types of sink sinks from the inside on the dorsal side have protrusions and recesses (the so-called "castle"), which contributes to a more dense closure of the sash. The sink grows throughout the life of the mollusk, and concentric rings of annual growth are noticeable on its surface, resembling annual rings of trees. Many types of bivalves are well developed a pearl layer, so most marine species and rare freshwater are capable of forming pearls.
The body of the animal is entirely concluded in the sink. The foot from most individuals has a clinoid form, in those class representatives who lead an absolutely fixed lifestyle, reduced (mussels) or disappears at all (oysters). In a dangerous situation, the mollusk pulls the leg and slams the sink. The body of the mollusk is covered by mantle, the folds will grow and form siphons at the rear end of the body. Through the introductory siphon of oxygen-enriched water with nutrients enters the mantle cavity, and through the output - the mollusk is getting rid of undue residues of food and exchange products. Under the mantle from both sides of the body is the respiratory authorities - the gills - various buildings in different species, most of the bivalves are gill plates. The digestive system of bivalve mollusks is represented by the mouth, esophagus, stomach, liver, intestines, and the anal opening opens into the mantle cavity. The blood system of these mollusks is open, three-chamber heart, consisting of two atrial and ventricle, is located in the spinal part of the body. The nervous system is three pairs of ganglia, sense organs (equilibrium, tactile sensitivity, in some - eyes) underdeveloped. The excretory system is represented by two kidneys, which are opened in the mantle cavity.
The second in the number of species by a group of mollusks is bivalve. In nature, there are about 20 thousand their species. Each of them has its own special characteristics, structure. But they are all distinguished by small mobility.
Features of the structure
For most bivalves, a small mobility is characterized or a sedentary lifestyle. Being in the water, they bury in the bottom Il, lie at the bottom. They are attached to rocks or ship surfaces. Sea scallops can overcome long distances and are the only rolling representatives of the class.
Interesting fact! The biggest type of bivalve is a giant thidakna, the mass of which can reach up to 300 grams, and the size of the sink especially large instances is up to 1.2 meters. She lives for more than a hundred years.
Features of the external structure
Sink has symmetrical outlines, but sometimes asymmetry. From the stretched in the form of the painshop, the top (the dorsal) edge of the sash is moving. The opposite edge is called the bottom (abdominal).
The sink includes three layers:
- Outer kolhyolin;
- Internal lime;
- Nizhny pearl.
The sink is located the body of a double mollusk, formed by the torso and foot. It has the following features:
- The body of bivalve mollusks is characteristic of symmetry, it is oblong, flattened from the sides;
- Inside the body are the system of internal organs;
- The shape of the leg resembles a wedge, a number of representatives have no representative;
- The head is absent.
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Mantle and mantle cavity
The mantle is a pair of skin folds hanging from the dorsal part on the sides of the bug. At the bottom of the folds or remain free, or their splicing occurs. Between mantle and bivalve body is a mantle cavity.
In some species, the mantle forms siphons, which are a couple of long tubes. On the bottom, the inflow of water into the cavity is carried out, and water from the body is carried out on the top tube. Together with it there are particles of food and oxygen.
The mantle cavity includes a complex of organs consisting of a pair of Glabers, legs, two oral blades and an osfrady.
The mantle cavity leaves the holes of the most important systems of the body: selection, digestion and sex.
Digestion system
In bivalve mollusks, the structure of the digestion system has a feature that consists in the presence of a filtering machine. It is due to the type of power by filtering. Water delivered to the mantle cavity with the help of an introductory siphon enters the front side of the body. The flow of water provides a fixed epithelium, it covers the inner part of the mantle, gills and robust blades. According to receptors and ciliary chutes, food particles move into the mouth, located from the front side next to the muscle - the closer. Further, food is heading through the esophagus inside the entodermal stomach. There, with the help of a crystalline stem, the separation of digestive enzymes occurs.
Through the output siphon, the outwardness of excrement from the mantle cavity is made.
Respiratory system
Blow-related with such respiration organs like gills.
The primary gills are claddes with petals.
Summer types of breathing organs are filamentous or lamellar gills.
Partitioned - gill species breathe with a porous gill partition.
Circulatory system
In bivalve mollusks, the circulatory system is unlocked. It is represented by the heart consisting of a ventricle and a pair of atria. Arteries are opened in cavities that are between organs called lacunas. In the blood of several species there is hemoglobin.
Selective system
The system of the excretory system is a pair of kidneys having an elongated form. One of their side is open to the mantle cavity, and the second is in the window-shaped bag.
Area and distribution areas
Bivalve mollusks belong to the class of invertebrates, the habitat of which are freshwater and salty reservoirs throughout the planet. More than their number prefers to go into or attach to the bottom or attached to the subjects under water. Many bivalves are found in the oceans in littoral and sublittoral territories.
They live in tropical, temperate and arctic waters. There are varieties of bivalve mollusks that are not afraid of extreme conditions for life and development. Antarctic scallop lives in the sea under the ice at minus temperature. Some Pacific species accumulate next to hydrothermal sources at a depth of 3 kilometers.
The habitat is some freshwater varieties It is limited only to a small territory, and there are widespread varieties of bivalve mollusks.
Food and reproduction
Most bivalve - filtrators fed by plankton and existing particles of organic origin in water. They enroll the mantle cavity with water.
The most primitive varieties feed on the daddy, which they scrape from the bottom of the sea.
Mollusk with a suckling collects pieces of food from the bottom, sorting them. He discarded very large particles in size, and the remaining carries in the mouth.
There are predatory varieties of bivalves. These include plate-car mollusks. With the help of the most muscles, water enters into the introductory siphon, which sucks small crashes and worms. The extracted food falls on the mouth blades, with the help of which it enters the mouth. The device of the digestive system of plate-inhabitants allows them to digest large particles of food.
Food for other representatives of bivalve - ship worms - serves wood wooden items under water. With the help of the boring apparatus of the sink, they are bought in wood, as a result of a set of formed moves, its destruction occurs. Wood digested due to symbiotic bacteria.
Most of the bivalve mollusks are separately, but they meet among them and hermaphrodites. Special floor holes, located near the excretory holes, open sex grooves. Hermaphrodites are endowed with ovaries, and seeds, but most often a pair of hermaphroditic glasses.
Through the siphon of the male spermatozoa fall into the siphon of females, and from there in her mantle cavity. It is in her fertilization.
With the onset of spring from eggs, a larva sailboat is formed, which has some shells and legs. Shell itself originally looks like a unpaired killer plate. After some time, it goes into two parts. So the bivalve sink is formed. From the top of the larva, a sail is formed with numerous cilias, which are vehicle organs. The larva goes to the next phase of its development - a sailboat. Then he settles and his gradual transformation in an adult bivalve.
Similar reproduction option contributes to the wider resettlement of bivalve.
Representatives
Among the representatives of the class of bivalve mollusks allocate three detachments:
- Equal (Taxodonta);
- Disturbed (Anisomiaria);
- Platechybada (Eulamellibranchia).
This is the most primitive type of bivalve. Numerous teeth form the castle. The foot has a flat sole. Equals include arches, nuts and other representatives of the class.
Disturbed
In this detachment, numerous forms were combined, belonging earlier to the group of nitheraberous mollusks. On their gills are leaflets that have the kind of long threads. Representatives of the detachment detachment possess either one back muscle - the closer, or there is also a muscle in front, but a much smaller size. An example of this species are mussels, oysters, sea scallops and marine pearls.
Platechyabea
Representatives of this detachment are the overwhelming number of bivalve mollusks. The teeth of their castle look outwardly resemble the plates in the form of an arc. There are two muscles -thearter, the gills are complicated lattice plates, and the siphons are formed by the edges of the mantle.
Examples of this species are all freshwater mollusks of the family of Perchovers, pearls, chairs and Dreissen. This also includes ship worms, stone-skins and other specialized species.
Use by man
Predated many varieties of bivalves are used by a person for various purposes.
- Their meat belongs to delicacies, it has excellent taste characteristics, healthy and is easily absorbed by the body. It contains vitamins A, B, C, D. bivalve meat or rich in iron, iodine, copper, zinc and other minerals.
- In many countries, over the past few decades, mollusks have begun to relocate new territories, providing assistance in acclimatization. Artificial breeding in the seas, freshwater and artificially created water bodies is practiced. Also for breeding bivalve used shames and small bays, which are specifically prepared for this and are protected from predatory inhabitants.
- Bivalves found the use in the form of popular souvenirs and decorations. Pearls found use in jewelry production, and from the pearl sinks produced inexpensive jewelry and buttons.
- Often the sinks are used as a decorative element, sprinkling them into the plates, which serve to decorate the walls, paths in the gardens and photo frames.
Interesting fact! In the Indians, the seashell particles performed the role of a monetary unit. Obakov also manufactured objects of dishes, fishing hooks.
- Mollusks inhabiting contaminated reservoirs are able to accumulate pollutants of organic origin and heavy metals. Therefore, they are used as a biological indicator of environmental pollution.
- Often bivalves are used by man to purify water.
- Some varieties of mollusks allocate the Bissus used in the textile industry. It is one of the components for the production of Vison - an expensive type of fabric.
- Bivalves are considered to be the most valuable and popular food for fish, therefore reservoirs in which mollusks are found as places for fattening fishing fish.
- The food additive made of crushed seashells is used for feeding poultry.
- In agriculture, a number of countries of their shell are applied in the production of lime.
Attention! There are poisonous types of bivalves, provoking paralysis, long-term neurological disorders and even death. Raw and incorrectly cooked mollusks can cause infections in the human body. Therefore, it is better to cook this delicacy, abiding caution.
Conclusion
Summarizing information, it can be concluded that an important function in the ecosystem is assigned to bivalve mollusks. Filtering water, they purify it and reduce the content of harmful microorganisms in it.
Mollusc meat is a delicacious product. But not only a person eats bivalve. They feed fish and other water inhabitants.
The sinks of the shell embrace the body of the mollusk from the sides, but the forms lying on the ground, for example, at the sea scallop (pecne), or an earneering one with one sash, for example, in oysters, also covering the body from the sides, they occupy the lower and top position and are designated as abdominal and dorsal. Such forms have only one closure muscle that occupies the middle position.
The sinks of the shell are bonded by the majority of bivalves not only by the league and the closure muscles, like to remove, but also a lock.
The castle consists of a single sash of protrusions, or teeth, and the corresponding recesses on the other, in which the teeth are included.
Castles are arranged varied.
The most common two types of them are a taxonal, consisting of numerous small identical teeth, and a heterodonal, consisting of a few different in shape of teeth.
The reduction of the shell is observed in bivalve very rarely. An example is a ship worm (Teredo Navalis) - a very kind of marine mollusk, drilling wood and bringing it great harm. The body of this mollusk is strongly extended, and the sink covers only a small portion of the front end (Fig. 274). It serves as not to protect, but helps the mollusk drill a tree.
:
1 - reduced sink, 2- Rudimentary foot, 3 - Siphons, 4 - Mantle cavity, 5 - Zhabra, 6 - intestines, 7 - Heart, 8 - Gonads
Bivalve seas of the USSR :
1 - Misty (Mvtiius Edulis) - Northern Mori, 2- Portlandia Arctica - North Sea, 3 - Fazeolin (ModioLa Phaseolina) - Black Sea, 4 - White Shell (Spisula Sachalinensis) - Japanese Sea (A - Outside, B - with internal Parties; 1 - Heterodontal Castle), 5 - Cardium Edule - Cherry Sea, Baltic, Caspian and Aral Sea, Murmansk Coast, C - Leda Pernula - Northern Seas (A - Outside, B-from the Inner side; 1 - Castle Taxodnaya), 7, - scallop (pecten islandica) - Northern Sea
The formation of the shell occurs as follows. Blood brings to the cells of the edge of the mantle (or to the mantle plot lying under the damaged segment of the sink) complex complex of protein-lipoids. These substances "put out" on the surface of the mantle and accumulate in a narrow space between mantle and sink, i.e., where the formation of pearls occurs. From these substances, the special cells of the mantle build the so-called matrices - organic structures on which crystallization is first going on with calcium phosphorous acid substituted by carbon dioxide. Calcium carbonate is deposited in its least soluble form - in the form of aragonite. It is important to note that the matrices, being under the control of the body, can adjust the crystallization process. Blood is applied to the place of formation of sink and calcium salt; The amount of incoming salts is also regulated by the mollusk organism. Thus, the whole organism is involved in the construction of the shell, and not just the mantle, as previously believed.
General characteristics. Bivalve mollusks with double sink (Fig. 184). The head is not isolated. The foot is usually a wedge-shaped form, often in one degree or another reduced. The gills have the form of plates located in the mantle cavity on two sides of the legs (another name of the class - plate-contained).
More than 15 thousand species of bivalve mollusks are described. Most of them live in the seas, a smaller part - in freshly lodies. It is usually low-lifting or fixed animals living on the bottom of the reservoirs. Feed passively, by retention of food particles entering water into the mantle cavity. Development Direct (part of freshwater forms) OL with transformation.
Building and vitality. The body of bivalve mollusks bilateral symmetric. It consists of a torso and legs, the head is reduced. The foot usually has a wedge-shaped form. In rolling species, it can be torn out of the sink and break into the soil of the reservoir, tightening the weight of the animal then. At attached or lying on the ground mollusks, the foot is usually reduced to one degree or another.
Fig. 184. Double mollusk toothless:
/ - leg; 2 - gill siphon; 3 - Cloacade siphon; / - Top of the sink; 5 ...... bunch of sash
The covers of bivalve mollusks are rich in various glands that distinguish the mucus and other substances. So in mollusks of Komnetochetsev (Pholas.) the glands of the cover of the legs are distinguished by the acid that destructs lime, which allows the animal to delve into the limestone rocks. Dreisssen and mussels are distinguished by a special substance - Bissus, solidified in water in the form of thin yarns, which are firmly attached to the substrate.
The mantle falls from the sides in the form of two folds. They limit the mantle cavity, in which yoga and gills are located. The mantle cavity opens the back of the intestine, kidney ducts and genital paths. The water enters it through the gill siphon, and removes through the cloacade siphon, formed by the rear edge of the mantle. The flow of water is created by the movement of the cilia of the epithelium covering the mantle, gills and siphons.
The sink is formed by two sash connected on top of an elastic bunch or lock - teeth located on the upper edge of the sash (Fig. 185, 186). Most species have both sinks of the shell are the same, but in a number of lying on the bottom of the mollusks, they differ in size and shape. The disclosure of the flaps occurs automatically under the action of the elastic properties of the connecting bundle. Racing of the sash is caused by a reduction in powerful muscles - blocks that bind both sash. The growth of the shell occurs along the outer edge of the sash due to the discharge from the gland of the underlying mantle. In winter, the growth of mollusks is almost stopped, which is formed on the sash, the annual bands are formed, according to the number of which you can determine the age of the animal.
The nervous system is formed by three pairs of nerve nodes, one of which is located above the throat, the second - in the foot and the third - in the back of the body. The nodes are connected by nervous charges - the commissions. Nodes depart the nerves to different organs (Fig. 187).
Fig. 185. Perlove sink (from the inside):
/ ..... Cardinal. Chuck Aamka; 2 Popon. Chub and the form of the plate; I- muscle prints
Fig. 186. The cut of the shell and the mantle of toothless:
/ - Konchiolipopa layer Rakoshshu;
2
porcelause her layer; 3
mother of pearl elephant; 4
outdoor epithelium mantle; 5
- ■ Medium connecting layer of the mantle; in-inner epithelium mantle
Fig. 187. Internal structure of toothless:
/ - mantle; 2
-- leg; 3
- gill siphon; 4 --
cloacal siphon; 5
»Tamka Gelp muscles; in- robot blades; 7 - oral hole; N.stomach; I
liver, 10
intestine; // -
anus; 12
- a heart; 13
- Okolosraid bag; / / kidney; 15
Kebers organ; 16
-House gangliya; 17
- foot gangliya;
1Nvisceral gangli; 19
- Zhabra; 20
- Mantle cavity
The organs of feelings in bivalve mollusks are weakly developed. In the covers of different parts of the body, the feeling cells are scattered - the organs of the skin feeling. There are special chemical feelings in the gills - Offradia. In the foot are the equilibrium authorities - Stavtocists. A number of species have numerous eyes on the edge of the mantle.
Digestive system. The mouth is located at the front end of the body over the base of the leg. On the sides of it are two blades, covered with fiscal epithelium, whose cilia is caught up with food particles to the oral hole. A short esophagus spends food in a small stomach in which the liver fluid opens. The guts descending from the stomach forms a row of loops, rising to the spinal side of the body, passes back through the shallower bag and the ventricle of the heart and opens into the clock siphon.
Respiratory authorities are gills lying in the mantle cavity on both sides of the legs.
The blood system consists of a heart and blood vessels. The heart has one ventricle and several atrium, the number of which corresponds to the number of gills, since each of them represents the extension of vessels coming from the respiratory organs. The heart lies on the dorsal side of the body.
The separation bodies are represented by two kidneys lying under the Head. Each of them begins in a near-carrying bag with a funnel lined with fiscal epithelium. Ureterals open into the mantle cavity. The epithelium of the necrordic bag contains special pericardial glands (KEBSROV organ), also carrying excretory functions.
Reproductive organs. Most bivalve mollusks separatogs. Parry seeds and ovaries. The ducts usually open into the mantle cavity. Outdoor egg fertilization. In freshwater forms, it takes place in the mantle cavity of the female, where the spermatozoa penetrate along with water through the gill siphon.
The value of bivalve mollusks for the human farm is large. Many of them are eaten. These are oysters, scallops, mussels, etc.
Rig. 188. Glofhdia toothless:
/ Epshrki Rakopil; 2 her tynnu; ," i.muscle closer of Pnorok; 7 pinching Organon Chongsgn
Oysters ( Ostrca. ) - fixed marine clams with asymmetric sink.
Large convex sash, they come into contact with the substrate (Fig. 189, /). In small places, oysters often form huge clusters - oyster banks, where they are caught. In many countries, oysters are highly appreciated as a delicacy. Their meat on calorieness is not inferior to meat fish and rich in vitamins. In many countries they are bred in special farms.
Scallops ( Pecien. ) have beautiful rounded sinks with radial ribs (Fig. 190). They do not grow to the substrate, but lie at the bottom. With the danger of scallop quickly slams the sink, thrown away from it from it the water throws the animal that makes a smooth jump. Scallops in large quantities are mined in the seas of the Far East of the USSR. Used for cooking canned food.
Fig. 189. Commercial edible mollusks: / - oysters; // - Midiy
Fig. 190. GOLESHOK
Fig. 191. Pearl:
/ - appearance; // - Cut pearls (visible concentric layers of the pussy of the foreign body)
Fig. 192. Woodrel and his moves in a piece of wood
Mussels ( Mytitus. ) they have a dark sink of two sash, one side of which is almost straight, and the other is convex (see Fig. 189, II). Mollusk is attached to the substrate with a bunch of bissus threads. In the Black Sea there are huge reserves of mussels, which are widely exploited by seaside livestock farms.
For feeding of farm animals, our freshwater bivalve mollusks are also suitable - toothlessand perchitsy (Anodonta.,
Unio.).
They are widespread on rivers and lakes. The sinks of their extended, large, large, greenish-brown color, consist of two identical flaps. Mollusk moves, immersing a wedge-shaped foot in the sand. To the toothless sink thin-walled and sash it is connected by an elastic bundle without a lock. At the peak of the sink is more elongated and thickened, the sash is connected by a clove lock. Protlocking shells serve as raw materials for pearl production.
A number of types of bivalve mollusks give pearls. It is formed as a result of the deposition of the pearl layers around any foreign taurus (grades), which has fallen between the mantle and the sink animal. The largest and most valuable pearls give pearl (Ptern.), dwelling in tropical and subtropical seas (Fig. 191). In Japan, pearls are breed in special cells immersed in the sea. Young pearls in the mantle are introduced foreign trade, becoming a germ growth center. Pearls are also mined from river pearl (Mar.gAR.i.tana.), which meets in the rivers and lakes of the northern regions of the USSR.
Harmful representative of bivalve mollusks is dreissen (Dreissena.), living in fresh and salted water. They often accumulate on gateways and protective lattices of hydraulic structures, violating normal operation. Ship sharpened (Teredo.) it has a long black-shaped body and a small shell in the front end (Fig. 192). It serves as a drill apparatus. Deeply embedded in the wood of underwater structures and the bottoms of the wooden ships, they apply strong damage.
From the site www.sessystem.ru.
Species and illustrations taken from computer determinant of freshwater invertebrates Russia (Bogolyubov A.S., Kravchenko M.V., Moscow, "Ecosystem", 2018).
Bivalve Class - Bivalvia
Brief characteristics of the class. Bivalve, or plate-intensive (Bivalvia) - the class of marine and freshwater sedental clams that unlike From the bruhlogs of mollusks (Gastropoda) do not have a separate head and self-associated organs (mouth, throat). They have sink of 2 sash - right and left (in contrast to the top and bottom in the shoulder), which cover the body from the sides and movably articulated on the dorsal side of the ellastic ligament - ligamy, and from the inside are connected 2 or 1 muscular-closer.
Body bivalve body split from sides and carries 2 wide mantle blades, closely adjacent from the inside to the sinks of the shell. Leg more often than a cylinder form and can be torn out of the sink; often it has an iron highlighting bissusserving to attach to substrate. Paired gills located on the sides of the body and are modified in the gill plates (clad), performing not only the respiratory function, but also the role of filters to push the food particles from the water, therefore, the type of bivalves is predominantly filtrates. The central nervous system consists of 3 pairs of ganglia. There are sensitive tentian-shaped appendages, and some forms are complexly developed eyes. A heart Usually permeated with the rear intestine and has 2 atrium. Usually separations, less likely hermaphrodites.
To bivalve relate Such famous marine mollusks like oysters, mussels, sea scallops, and from our freshwater - toothless, pegs, hacketers, skids, pearls and Dreissen. Most bivalve mollusks are buried in the bottom Il, escaped from predators, some lie on the seabed or fill the rocks and other surfaces. Few species, such as sea scallops, are capable of short acting swimming.
Shape and size shells Bivalves vary greatly: among them are found both very large mollusks, such as a giant thidakne (Tridacna Gigas), which can reach 1.4 m in length and weigh up to 200 kg and very small. The most small representative of the type of mollusks is just a bivalve mollusk Condylonucula Maya, adults of which are only 0.5 mm in length
Body shape Bivalves are also very different. For example, the hearts have an almost spherical body and can jump, bending and flexing the leg. At the same time, marine cuttings (ENSIS) in connection with a specialization to the coming lifestyle have an oblong shell and a strong leg, intended for injection into the ground. The ship worms of the Teredinidae family have a strongly elongated black-shaped body up to 2 meters long with a reduced sink, located at its front end and modified into a drilling body, thanks to which the mollusk "scans" branched moves in the wood. Most of the same types of bivalve body is oblong, more or less flattened from the sides, bilateral-symmetrical. The head is reduced, and mollusk, in fact, consists of a torso and legs.
The history of the study of bivalves. For the first time name Bivalvia. Used Karl Linney in the 10th edition of his work "Nature System" in 1758 for the designation of mollusks whose sink consists of two sash. In addition, this class is known under other names, for example:
- PELECYPODA (which means "fighter"),
- LamelliBranchia (plate-intensive) and
- Acephala ("Headless", because bivalve, unlike all other mollusks, have lost their head).
In Russian science, the end of the XVIII, the beginning of the XIX centuries bivalve (live and fossils) were called - centen.
The structure of bivalve shells. Sashes of the shell in bivalve mollusks most often symmetrical (for example, at heart, toothless). However, some species may be observed. asymmetry sash. So, the oyster is a sash, on which the animal lies, convex, and the other flap is flat and plays the role of a lid, covering the first. A similar phenomenon takes place in the case of the scallop lying on the bottom.
From the outer surface of the shell, outstanding and several elevated part - tipor Makushka (See Figure). This is the most old Part of the sink, as the sink grows on the edge from the opposite side.
- The edge of the shell on which the top is called, is called topor spinalor Keystone sinks, and the opposite to him - nizhina, or abdominal.
- The wider end of the shell is in front of him, and narrower, somewhat elongated and removed from the top - rear end (edge).
- If you arrange the sink of the sink up and the front end from yourself, then the sash located on the left of the plane of the sash closure is called leva, and the sash located on the right, - right.
At the rear end Shells is an elastic plate or ligamentWith which both sink sash are connected to each other. The ligator consists of two keratin proteins - tenzylium and resilience. In various groups, bivalve ligators may be inner and outdoor. Its function, in addition to the fastening of the sash, is to participate in the disclosure of the shell.
The upper surface of each shell sash concentrically executed; Some of the arcs are more dramatically issued among others, stretch along the entire length of the sink and have a somewhat wrinkled form; this is annual arcscorresponding to winter slaughter suspensions for which the age of seashells can be determined with some approximation. At the rear end of the shell also open holes siphons.
In the front end Sinks are located leg and bissowel gland (if there).
Wall of the shell Consists of three layers: exterior kigholic ( periostrakum ), carrying strip, internal lime ( ostracum ) and the lower pearl ( hypiosteracum ).
The mineral component of the sink (that is, the composite and hysporakum is part of the urinary and hysporakum) can be represented exclusively by calcite, like oysters, or calcite and aragonite. Sometimes an aragonite also forms a pearl layer, as in the case of a PTterioida squad. The rest of the mollusks of the aragonite and calcite layers alternate.
Outer layer (periostrakum) consists of solid organic (Konchiolin) and secrets the edge of the mantle. On the tops of the sash, the koliolin layer is often erased. This top layer has patronizing color, usually brown or olive. Sometimes, under the influence of any mechanical effect, the outer horny layer of the sink is destroyed, opening the prismatic layer under it, which is easily dissolved in the rich carbon dioxide of soft waters, and the sinks of mollusks in such waters are reversible.
Inner layer (Ostrachum) is called porcelain. The microscopic study of the transverse grinding of the porcelain layer showed that it consists of limestone prisms, closely lying on each other in the direction perpendicular to the surface of the sink.
Pearl layer (hypiosteracum) consists of a variety of thin also lime layers lying on each other and the connected kihiolin. Such structure of the mother of pearl causes light interference, i.e. The layers refracted and reflect the light falling on them, as a result of which the inner surface of the sink and distinguishes various colors or urizes. The pearl sampling varies depending on which side and at what angle when viewed by the shell, the light falls on it. The pearl layer thickens with the age of mollusk and the growth of its shell.
On the inner surface of the shell (see Figure) Vidnicted Her thickened top edge or lock plate. It is called so because it carries growth or teethwhich come in the deepening of the opposite record, forming the so-called " lock". Teeth, located in front of a ligator, short and massive, are behind it - extended and thin. The lock prevents the displacement of the sash relative to each other. The lock may be equally (tactful) and a different-riveted (heteroodont).
Inside the surface of the peaks of the peaks
The most fully castle is expressed in the perch; In the pearl, some teeth are reduced, and the toothless they are missing at all, which is why their name occurs.
The front and the rear ends of the inner surface of the sink are stains formed by the irregularities of the pearl layer in the attachment places muscles-clocks mollusk; The power of reducing these pretty powerful muscles of the sink sash is tightly compressed alone on the other, and if the shekchuch is pulled out of the water, the body keeps the moisture in it for some time. The sashs are revealed without the involvement of any muscles, but only under the influence of a ligance playing the role of the spring. The action of the opening mechanism can be seen on the dead mollusks - from drying out the sash ligate of the shell themselves are open.
Rising shell It occurs due to the gradual laying by the edge of the mantle of the konchiolin layer, as well as the accumulation of minerals in the sink. The sink is noticeable concentric linesindicating its uneven increase in changing environmental conditions (growth lines). However, the determination of the age of the shell in the number of such concentric lines is not quite accurate. More precisely, the age of mollusk can be said in terms of the amount of layers on the cross-cut sink.
If there is an extraneous body (not mandatory only pearl mantle) between the sash (not mandatory only pearls), it has an annoying effect on the animal, and the skin of the mantle begin to highlight the solidifying pearl substance, which is gradually enveloping foreign The body with concentric layers (see Figure). In the end, forms pearl, consequently, in the mollusc, the formation of painful origin.
Pearl sink with pearl (left) and pearl formation scheme (right)
In the form and structure of the shell You can distinguish three main systematic groups Our freshwater bivalve mollusks. In addition to the pronounced difference in the structure of the teeth, and the locking apparatus, the pekloves are distinguished by an elongated hardening sink with an alpine approximate to the front end; The sink of toothless is widely oval, thin-walled, the top of it is weakly outstanding, the keel of the upper edge in some species is high. Pearl sink is large, elongated, thick-walled, with almost direct or even a few concave bottom edges; The top edge is almost parallel.
Mantle and mantian cavity. In bivalve mollusks, the mildlets have the form of two folds of the skin drying from the back on the sides to the abdominal side. From the bottom of its folds can be free (like toothpick) or grow together, leaving only the holes for the leg and siphons. On the edge of the mantle, small tentacles and eyes can sometimes develop. The outer layer of the mantle is distinguished by the sink, and the internal lined with a flicker epithelium, whose cilia bias provides a water current in the mantle cavity. In addition to the shell, the mantle also forms a ligator, bissing gland and a castle.
In the digging forms of the mantle forms siphons - Two long tubes, at the bottom of which (input siphon), water enters the mantle cavity, and along the top (output siphon) - it turns out. Oxygen and food particles are delivered to the water in the mantle cavity.
Like all other mollusks, the bivalve mantle forms the mantle cavity in which it includes mantle complex Organs: leg, two gills, two oral blades and offrases. The mantle cavity also opens the holes of the digestive, sexual and excretory systems.
In most bivalve mollusks on the inside of the shell, a line that is noted parallel to the edge of the sink and often connecting tracks from two adductors muscles (closers) is noticeable. It is called pallialic (Mantina) LinenIt is the lines of attachment of the mantle to the sash of the shell. Attachment is carried out by a narrow number of small retractor muscles. With the help of these muscles, the mollusk may in case of danger hide the protruding edge of the mantle inside the sink. Siphons can also be drawn (in normal condition treated from the sink). For this, serves a special brother-shaped deepening in the mantle cavity. A pallial sinus, or a mantle sine, or a mantle bay, or a siphonic bay, is corresponding to the sink of the sink, or the siphonic bay - the curvature of the pally lines.
Leg. The leg (muscular unpaired abdominal wall) in bivalve mollusks wedge-shaped, serves to break into the ground and crawling. At the most primitive forms (the squad of Protobranchia), the leg, like in the bucculent, with a flat crawling sole. Some bivalve, attaching to the substrate, have a special bissowic glageMixing bissus threads, with which the mollusk "grows" to the surface of the bottom (mussels). Many bivalve, leading motionless lifestyles, the foot is reduced completely (oysters).
Musculature. The main muscles in the body of bivalve mollusks are the front and rear muscles closers (adductors)Although the front closer in some species can be reduced or completely lost. Reducing, these strong muscles cover the sash, and when they relax them, the sash opens. In addition, the ligatment is involved in the mechanism of revealing sash. When the sink is closed, it is, as a spring, is in a stretched state. With the relaxation of the closers, it returns to its original position, opening the sash.
In bivalve mollusks lying on one sash (for example, oysters and mussels), the front adductor is lost, and the rear occupies the central position. The mollusks of the Limidae family, floating at the expense of flaking sash, also have the only central closer. Closeters consist of two types of muscle fibers: cross-striped intended for fast movements, and smooth, supporting long-term muscle stress.
As mentioned above, the mantle is attached to the sink due to small muscles forming arcoid mark On the sash of the shell - a pallial line. Paired protective (flexible) and retractable (extensible) muscles provide two-way movements. In bivalve, devoid of legs, these muscles are not. Other paired muscles control the siphons and bissing gland.
Digestive system. In connection with the passive way of nutrition by filtration The digestive system of bivalves has some features. Water entering through the introductory siphon is sent to the front end of the body, washing the gills and 2 pairs of long triangular robust blades. There are sensitive cells on the gills and robust blades ( body taste) And small grooves, for which food particles are transported in the mouth, located near the front closer.
Food is made to a short esophagus and then into a bag-shaped entodermal stomach. Since the bivalves are reduced to the head, then the throat, Radules and salivary glands are missing. A few digestive glands are opened in the stomach, often through a pair of diverticulus, such as, for example, a two-blade liver. The liver not only highlights digestive enzymes: its cells also phagocytic food particles. Thus, bivalves are available intracellular digestion.
In addition, there are in the stomach crystal stalkconsisting of mucoprotein and enzymes (amylases, glycogenase, etc.). The stalk is located in a special blind bag-shaped gross and go into the lumen of the stomach. The cilias are located there forcing the stalk to rotate, separating enzymes and stirring the contents of the stomach. Due to the constant movement of food particles in the stomach possible sorting In its rear end: small particles are sent to the digestive glands and there are absorbed by phagocytosis, and the larger are sent to the intestine. The middle intestine is departed from the stomach, which further makes several bends and heads along the dorsal side of the body to the rear end, goes into the rear, which opens with anal hole in the mantle cavity above the rear closer. The excrement with the flow of water is thrown through the output siphon outside. The render usually passes through the ventricle of the heart (a specific feature of bivalve).
Food and digestion in bivalve synchronized With daily and tidal rhythms.
The features of the digestive tract described above are characteristic of filtering bivalve. In predatory Bivalve stalk can be strongly reduced, but in some cases there is a muscular stomach, lined with chitin, in which food is peers even before the start of digestion. In other cases, the digestive tract of predatory bivalve is similar to those in bivalve-filter.
Nervous system. As with most other mollusks, a nervous system in bivalve scattered-node-type. It is easier for them than the bruhogs. Because of the reduction of the head, cerebral ganglia was merged with pleural; Thus formed pair cerebroploral dual nodes, located on both sides of the esophagus and connecting over the throat with a thin cerebral commissioner. The formation of cerebroploral nodes by the capture of ganglia is proved by the fact that primitive protobranchia pleural nodes are still isolated from cerebral. They innervate the mantle cavity and sense organs (except forces). In the foot there are pedal ganglia, innervating legs related to connecting with cerepropleral nodes. Under the rear muscular-closer there is a third pair of nodes - commercial, controlling internal organs, gills and offrases. They are connected by even longer connotatives with cerepropleral nodes. The third pair of nodes is especially well developed in floating bivalves. Bivaluble with long siphons may have special siphon ganglia controlling siphons.
Sense organs. The organs of feelings in bivalve mollusks are weakly developed. In the leg is available stavtocians - Equilibrium authorities innervating cerebral ganglias. In the mantle cavity at the base of the gills are located offradia. - chemical feelings; Perhaps the offrases of bivalves are non-homologous for fradicated buchelogs. Individual receptor cells scattered on the gills, oral blades, on the edge of the mantle and on the siphons. The tactile function is also performed tentacles, developing on the edge of the mantle. In the predatory bivalves from the detachment of Anomalodesmata, the siphons are surrounded by tentacles sensitive to vibrations; With their help, the mollusks detect prey.
Many bivalves lained eyes, however, representatives of Arcoidea, Limopsoidea, Mytiloidea, Anomioidea, Ostreoidea and Limoidea have simple eyeslocated along the edge of the mantle. They consist of a fossa lined with photosensitive cells, and a light crystal leaf. Sea scallops have inverted eyes of a rather complex structure consisting of a lens, two-layer retina and a concave reflective surface. Also known cases of eye formation on siphons at hearts. All bivalves have whole-sensitive cellsThanks to which the mollusk determines when it completely covers the shadow.
Respiratory system. Most bivalves breathe Glabrai. Each of the two gills consists of a growing up to the body of the Zhaba axis and two rows of gill threads from it. The totality of the threads of each row forms a gill sheets, or semi-fiber. The exception is a small group of bivalve mollusks - septibrankhium, representatives of which are deprived of the gills, but their mantle cavity is separated by a horizontal partition, penetrated by rows of holes. Food is also peculiar - they are predators. Bending partition, they suck together with the water of small animals, such as crustaceans.
Probrichnaybnoe (protobranchia)The most primitive bivalves have a pair of typical claddiev with gill petals.
W. nitzaberny (Filibranchia) There are filamentous gills. The filamentous gills are characterized by the fact that their gill petals lengthened into the threads forming down first, and then the rising knee. Neighboring threads are fastened with each other with hard cilia, forming plates; Part of the representatives of gill threads are free. Fit-shaped gills are peculiar to mussels, oysters, scallops.
W. euLamellibranchia detachment There are lamellar gills. This is a further modification of the filamentous gills: they have partitions between adjacent threads, as well as ascending and downward sites of one thread. So the gill plates are formed. Each gown consists of two semi-fiber: 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 this Changedide. Such gills are at the arm and toothless.
Label cavity (left) and direction of respiratory currents (right) bivalve mollusk
W. septibranchia (Septibranchia) The gills are reduced and converted into a gill partition with pores. The partition shields the upper part of the mantle cavity, forming a breathing cavity. Its walls are permeated with blood vessels, where gas exchange occurs.
Finally, at species devoid of gills (like, for example, representatives subclass Anomalodesmata.), Gas exchange goes through the wall of the mantle cavity.
Bivalves, living in the tidal and tidal zone, are capable of several hours survive without water, tightly closing the sash. Some freshwater shapes, being pulled out on the air, slightly open the sash, so that they have gas exchange with atmospheric air.
Circulatory system. Blood system of bivalve, like all other mollusks, unlocked, that is, the blood circulates not only by vessels, but also by lacunas (gaps between the organs). The heart is located on the dorsal side and consists of 1 ventricle and 2 atrial. As mentioned above, the back intestine passes through the ventricle. This fact is explained by the fact that the heart is laid in embryogenesis as a steam room on the sides of the intestine, and then these primitives are connected above and under the intestine (the pair origin of the heap in bivalves is confirmed by the presence of two hearts from representatives of the ARCA genus). Primitive forms of the Protobranchia detachment, the merger occurs only above the intestine.
Powerful front and rear aorti, branching B. arteries; Of these, blood ( hemolyimfa) Pouring into lacuna and gives oxygen to the tissues. Front artery goes forward over the intestine and supplies the inside, leg and front of the mantle, and rear Directed back under the intestine and soon disintegrates on the rear mantle artery. Some bivalve mollusks have only one aorta. Next, already becoming venous blood is going to large longitudinal lacuna Heat and sent to bringing gill vessels. Then oxygen-saturated arterial blood on the existing vessels returns from the gills in the heart. Blood is also poured into the wearing gill vessels, which, bypassing the gills, passes through the kidneys, where it is released from the exchange products.
Blood bivalves is usually devoid of any respiratory pigment, although representatives of the Arcidae and Limidae families have hemoglobindissolved directly in blood plasma. In the predatory bivalve mollusk Poromya in the blood is red amosombitescontaining hemoglobin.
Selective system. The separation system of bivalves, as in most other mollusks, is represented by pair nephonds (kidney). Bivalves kidney with ferrous walls called boyanus organs. The kidneys are long V-shaped tubes, which one end open into the pericardine hearts, to others - into the mantle cavity, from where the exchange products are carried out with a current of water.
In addition to the kidneys, the excretory function also performs the pericardium wall, modified in paired pericardial glands. Sometimes they are isolated from the rest of the pericardium in the form of two bags - keberov organs. Products for the selection of these glasses fall into pericardium, and from there through the kidneys are transported outward.
Sex system. Bivalve mollusks separationsHowever, there are also cases of hermaphroditism (so, at the type of Arca NOAE, a protaryan hermaphroditism has been established, in which the individuals first function as males, then as females). Some species, for example, thecaliacon Camerata, expressed sharply sexual dimorphism.
Gonads and ducts (seeds and eggs) pair; The gonads lie in the front of the body, close to the intestine, entering the base of the leg, and have the form of two prag-shaped formations. However, some species have sex ducts, and the gamets come out of the gonad through the tissue breaks into the mantle cavity. Primitive Protobranchia, as well as a number of other bivalves (pecent, ostrea, etc.), gonads open into the kidneys.
In some species, for example, representatives of the genus Lasaea, male genital cells outward through the siphon, and then with a current of water are tightened into the mantle cavity of females, where fertilization occurs. The offspring of such species is developing in the Maltian cavity of the mother and leaves it at the stage of the larvae - Veliger or the young individual. Most species outdoor fertilization. In this case, females and males throw sperm and eggs in the thickness of water. This process can go continuously or run the external environment factors, such as the duration of the day, water temperature and the presence of sperm in water. Some bivalves produce grounds gradually, others - large parts or all at the same time. Mass emissions Games sometimes occurs when all bivalves in the district synchronously release sex cells.
Life cycle. In bivalve, as in all mollusks, spiral crushing. It goes about the same way as the bruhogs.
Most bivalves develop with metamorphosis. Usually from fertilized eggs plankton larva - Verger (sailboat). The formation of Veliger is preceded by the Trophorm Stage, passing in the egg. Trophorm education occurs quite quickly and takes several hours or days. On the dorsal side of three phophores, the sink is laid in the form of a solid plate, which only later drives over the median line, becoming a bivalve, and the place beggar Save in the form of ligation. The upper part of the tripod with a rivic church becomes sail Veliger - covered with long cilia disk serving for swimming. The bivalve sink covers the whole body of the Veliger, when swimming the sail is set from the sink. The organization of Veiger is very close to the organization of an adult mollusk: he has a gentleman, gown, ganglia, stomach, liver and other organs, but protonphride remains allocating. In the future, the Gaver will settle on the bottom, fastened by the bissus thread, loses the sail and turns into an adult mollusk.
In some freshwater mollusks (for example, imlocks and toothless) there is a special larva - glocheidiumhaving a thin-walled bivalve sink with rounded sash and hooks on the abdominal edge. Most of the Glokhidod organs are still underdeveloped: there are no gills, a rudimentary foot. In these mollusks, fertilization occurs in the mantle cavity of the females, and Gloches develop in its gills. Each ripening glokidiy represents a small double mollusk, the sink of the shell of which is widely disclosed and quickly slaughte due to the reduction of a highly developed circuit muscle. The lower edges of the sink are equipped with sharp teeth, and a long adhesive thread of Bissus is moving from tiny sizes of the legs.
Double mollusk larva - Glocheidium and female Gorchaka with eggs
Economic meaning. For a long time, many bivalve mollusks are used by a person, they served and serve production subject. Their sinks are constantly found in the so-called "kitchen piles" of a prehistoric person who lived off the coast of the seas, rivers, lakes. In the excavations of the Paleolithic person's parking lots in the Crimea invariably find a large number of oysters shells, mussels, scallops and other mollusks, which are also industrial.
Bivalve mollusks are mined due to their delicious, very useful and easily digestible by the human body meat (as, for example, oysters, mussels, sea scallops, Petushkov-Tapese and regionwear, Mactra, sandy shell, heart leaders, arches, sea cuttings and synoval, freshwater peklocks, lampsiline, toothless, korbicul, etc.).
By calories they can even exceed the meat of many fish, both sea and freshwater. The nutritional value The mollusks meat is also due to the large content of vitamins A, B, C, D, etc., high content of such rare in the usual human food of minerals, like iodine, iron, zinc, copper, etc.. The latter is known to be in the composition A number of enzymes, hormones, play an extremely large role in the oxidative, carbohydrate and protein metabolism, in the regulation of hormonal activity.
In recent decades, due to the fact that the natural natural reserves of the most valuable edible mollusks (even in the seas) are exhausted, and the demand for them continues to increase, in many countries there are reset in new areas, acclimatize, as well as divide artificially both in marine and in fresh waters, on "farms" - specially prepared fellows and protected from predators of small bays, artificial reservoirs. Successfully bred and cultivate not only marine mollusks (oysters, mussels, nautical cocks, tapes), but also freshwater (Lammoxiline).
In 2010, 14.2 million tons of mollusks were grown in aquacultural facilities, which is 23.6% of the entire mass of mollusks used in food. In 1950, when the UN Food and Agriculture Organization began publishing similar statistics, the total mass of consumed bivalve mollusks was estimated at 1,007,419 tons. In 2000, this value was already 10 293 607, and in 2010 - 14,616,172. In particular, the consumption of mussels in 2010 amounted to 1 901 314 (decade earlier - 1,568,417) tons, oysters - 4,592,529 (3 858 911 ) Tons, scallops - 2 567 981 (1 713 453) tons. In China, the consumption of bivalve mollusks from 1970 by 1997 increased 400 times! Some countries regulate the import of bivalve and other seafood, mainly to minimize the risk of poisoning to toxins, which accumulate in these organisms.
Currently, the extraction of bivalve mollusks is inferior to them. artificial breeding in mariculture. Thus, mussels and oysters are grown on special farms. Especially great success such farms reached in the USA, Japan, France, Spain, Italy. In Russia, such farms are located on the shores of the Black, White, Barents and the Japanese seas. In addition, in Japan, the mariculture of sea pearls is developed (PinCTADA).
I. continues prey bivalve mollusks in natural reservoirs, where they are now produced on large courts specially designed fishing instruments; Molluscs are widely used. The greatest amount (about 90%) of bivalve mollusks is mined in the northern hemisphere - in quiet and in the Atlantic Oceans. Fisher freshwater Bivalve mollusks give only a few percent of the total world production.
The fishery of bivalve mollusks in countries such as Japan, USA, Korea, China, Indonesia, Filipino Islands and other islands of the Pacific Ocean is especially important. Thus, in Japan, about 90 species of bivalve mollusks are mined, of which about two dozen species have a large commercial importance, and 10 species are divorced artificially. In European countries, fishing and breeding bivalve mollusks are most developed in France and Italy.
In Russia, commercial significance They have mainly a large seaside scallop - pecnet (Patinopecen) YESSOENSIS, as well as various mussels, a white sewer (Spisula Sachalinensis), a sandy sewer - Mua (Arenomya) Arenaria, Petushki (Tapes, Venerupis) and some others.
Mollusks enter the market not only in fresh and dried, but especially in ice cream; Also developed the preparation of various canned mollusks.
In addition to food use, a person uses some types of bivalves (many freshwater pegs and pearls, marine pearls - Pinktada, Pteryi, etc.) as a source material for decorations (nacre and pearl), as well as souvenirs. Pearls are used in jewelry, and a pearl with bivalve sinks - in the manufacture of buttons or cheap jewelry, as well as for inlays. Among the natural pearls, the pearls of bivalve mollusks of Pinctada Margaritifera and Pinctada Mertensi, living in the tropical and subtropical part of the Pacific Ocean, possesses the greatest value. Industrial pearl production on pearl farms is based on the controlled implementation of solid particles in oysters. The material for the framed particles is often served by the stacked sinks of other mollusks. The use of this material on an industrial scale has delivered some freshwater types of bivalve in the southeastern United States to the edge of extinction.
