Shells. Inner layer of a clam shell How many layers does a clam shell have?

Type or Class Gastropods or Gastropods, systems, biology, features, shell structure, body, organs, sole, representatives, similarities of gastropods and bivalves

Latin name Gastropoda

Class Gastropoda General characteristics, biology, features

The structure of the body, organs, shell, development, representatives of the habitat, significance are considered.

Most (about 105,000 species) of modern mollusks belong to class gastropods. Most of them inhabit the seas and oceans, some live in fresh water and on land. This is the only class of mollusks, some forms have passed to terrestrial existence. Gastropods or snails are animals with which our ideas about soft-bodied are primarily associated. These include the well-known grape snail, naked slugs, various freshwater snails (pond snails, meadow snails, coils), as well as many sea snails.

gastropods

External structure Body Leg Trunk

The body of a gastropod is distinctly divided into head, leg, and torso. The head has one or two pairs of tentacles and eyes, which are more often placed at the base of the tentacles, and in some species - at the top of the second pair of tentacles. In many gastropods, the near-mouth part of the head is extended into a proboscis.

The leg is an abdominal muscular part of the body, often with a wide sole, with which the molluscs crawl. Many gastropods can stick firmly to the substrate with the help of their legs. Some gastropods belonging to various orders lead a floating lifestyle, which is facilitated by a change in the shape of the leg. For example, in the marine keeled-legged mollusk Carinaria, the leg is turned into a swimming blade flattened from the sides. In pteropods, which also lead a pelagic way of life, wide lateral outgrowths of the legs serve for swimming.

The body of most of these mollusks is twisted into a curl. This significantly violates the bilateral symmetry. However, in a number of gastropods, the shell is not twisted into a spiral, but is a conical cap. In this case, the trunk does not sharply separate from the leg and maintains bilateral symmetry. In connection with the reduction of the shell in some gastropods (naked slugs, etc.), the body pouch is also reduced and the internal organs are placed in the upper part of the leg.

The mantle limits the mantle cavity with the organs located in it.

Shell of a gastropod mollusc

Sink at gastropods consists of three layers common to molluscs: conchiolin, prismatic and mother-of-pearl. The outer layer is chitinous, often colored.

The middle layer reaches its maximum development and can be multi-layered prismatic or porcelain-like. Composed of calcite or aragonite.

The mother-of-pearl layer is not always developed.

For gastropods, a typical shell is a long tube twisted into a conical spiral or into the so-called turbospiral. closed at the crown and opening outwards at the mouth. The last outer whorl has the largest dimensions. The line of contact between the turns is called a seam.

The shape of the shell is varied: cap-shaped, cochlear, flat-spiral and cone-spiral.

Examples of such a shell are the shells of the common pond snail, the sea mollusk Buccinum, and many others. As can be seen from the example of various freshwater snails, the degree of elongation of the shell cone can be very different, up to a shell twisted in one plane, for example, in freshwater coils.

In the shell, the mouth, top and curl are distinguished. Whorls of the curl, adjacent to each other, form a seam of the shell on the outer side. If you place the shell with the top up and the mouth towards you, then the mouth in most cases is located on the right. Such a shell is called right-handed or dexpotropic, it is characteristic of most gastropods. However, there are species with a left-handed shell - leiotropic, for example, in freshwater snails Physa and Aplexa. In some species of molluscs with a right-handed shell, mutant forms with a left-handed shell are known.

The inner walls of the whorls of the whorl, closely adjacent to each other, are soldered together, forming a column (or column), which can be clearly seen on a longitudinal cut of the shell.

Many gastropods have a special operculum on the dorsal side of the leg, behind the shell. When the animal's body is drawn into the shell, the operculum closes the mouth, such as that of a freshwater lawn.

In some mollusks, the shell is not spiral, but is a conical cap. Such, for example, is the shell of the sea saucer (Patella), common in the surf zone. This is a very inactive mollusk, firmly sticking to the stones with the sole of its foot. It is very difficult to tear off the saucer from the stone, since the disturbed animal draws the shell with strong muscles close to the stone on which it sits. Another sessile mollusk, Fissurella, has a cap-shell with a hole at the top. In many gastropods, the shell is reduced to a greater or lesser extent. The predatory pelagic mollusk carinaria has a thin and very small shell, in the form of a small cap. It cannot be protective. When swimming, it plays the role of a keel. In some pteropods, the shell is completely reduced. In carinaria and pteropods, the reduction of the shell occurred as a result of the transition to a floating lifestyle. In naked slugs, the shell is preserved only in the form of a rudiment - a small plate that is overgrown with a mantle, for example, in a garden slug (Limax). In others, this plate also breaks up into separate calcareous bodies, for example, in the garden slug (Arion). In both cases, only the mantle shield is visible on the back. In naked slugs, shell reduction is apparently associated with a nocturnal lifestyle. During the day, they hide under stones, leaves, and only crawl out at night in search of food.

Digestive system of molluscs

The mouth is located at the anterior end of the head, which can be elongated in the form of a muzzle or form a proboscis that can be drawn inward. The oral cavity passes into a muscular pharynx, at the very beginning of which the horny jaws are placed, and behind them the radula.

One or two pairs of salivary glands are connected with the pharynx. In some predatory gastropods, the salivary gland secretion contains free sulfuric acid (2-4%) or some organic acids. Such mollusks feed on other mollusks and echinoderms. Pressing the proboscis against the shell of a mollusk or against the shell of an echinoderm, they release an acid that dissolves calcium carbonate. A hole is formed in the shell through which they suck out food.

The pharynx is followed by the esophagus, usually expanding into a goiter, and then the stomach, into which the liver ducts open. The liver is laid down as a paired organ, however, due to the asymmetry of the body of gastropods in adults, the liver is usually preserved only on the left side, and is reduced on the right. The gastropod liver is a highly developed tubular gland that performs several functions. As a digestive gland, the liver secretes enzymes. In addition, a semi-liquid food slurry enters the liver tubules, and digestion (including intracellular) and absorption of food occur in them. The liver is also an organ that stores spare nutrients in the form of fat and glycogen.

The stomach is followed by the small intestine, which in different species forms one or more loops. The hindgut in some gastropods passes through the ventricle of the heart. The anus is usually located near the mouth, at the anterior end of the body.

Respiratory system of ctenidia

The respiratory organs of gastropods are most often ctenidia located in the mantle cavity. Ctenidium consists of an axial rod with two rows of gill sheets on both sides. At the base of such a bipinnate gill is osphradium. The right ctenidia are usually reduced due to the asymmetry of the structure, up to complete disappearance. More often, only one ctenidium is preserved. However, not all gastropods have ctenidia. In pulmonary mollusks (Pulmonata), the mantle cavity has turned into lungs - an organ adapted for breathing atmospheric air. In pulmonary mollusks, the edge of the mantle grows together with the body and the mantle cavity communicates with the external environment only through the respiratory opening. In the wall of the mantle cavity (lung) there are abundant ramifications of blood vessels.

In many marine gastropods, ctenidia are reduced. Instead, so-called adaptive skin gills develop, which are various, sometimes feathery skin protrusions on the back, on the sides of the body, or around the anus. In some forms, gills may be completely absent, and then skin respiration takes place over the entire surface of the body.

Mollusk circulatory system Heart, circular pulmonary sinus

Gastropods have an open circulatory system, characteristic of all molluscs.

The heart consists of a ventricle and one, rarely two atria, and is located in the pericardial cavity. Arterial blood flows in the heart of the mollusk. From the ventricle, during its contraction (systole), blood enters the aorta, which is divided into two trunks - the head aorta and the splanchnic aorta. From these vessels depart the arteries to the head, intestines, mantle, leg and other organs. From small arteries, blood enters the arterial sinuses between organs, and then is collected in the venous sinuses. From the large venous sinuses, most of the blood enters the afferent gill vessel and from the gill through the efferent branchial vein into the atrium. Part of the blood passes through the vascular system of the kidneys to the gills. It is necessary to emphasize this connection between the circulatory system and the kidneys, which extract dissimilation products from the blood.

In pulmonary molluscs, a circular pulmonary sinus runs along the edge of the mantle, into which blood enters from the body. Numerous afferent pulmonary vessels depart from this sinus, forming a dense vascular network in which blood is oxidized. Through the efferent pulmonary vessels, blood is collected into the pulmonary vein, which flows into the atrium.

excretory system kidneys

The kidneys of these mollusks are modified coelomoducts. They begin as funnels in the pericardial cavity (whole) and open with excretory openings into the mantle cavity. Only the most primitive gastropods have two kidneys; the rest retain only one left kidney. In the lungs, due to the transformation of the mantle cavity into a light excretory opening, it is placed near the respiratory opening and opens directly outward.

Nervous system and sensory organs nerve nodes, or ganglia

In most gastropods, the nervous system consists of five main pairs of nerve nodes, or ganglia, located in various parts of the body.

Ganglia of one pair can be interconnected by transverse bridges - commissures. Different ganglia of the same side of the body are connected by longitudinal trunks - connectives.

Gastropods have five pairs of ganglia of the nervous system. In the head, above the pharynx, is placed a pair of head, or cerebral, ganglia. They are connected to each other by a transverse commissure passing over the pharynx. From the cerebral ganglia, nerves depart to the head, eyes, tentacles and statocysts. Somewhat behind and to the side of the cerebral ganglia is a pair of pleural ganglia. These ganglia are connected by connectives to the cerebral and pedal ganglia. The pleural ganglia innervate the anterior half of the mantle. Significantly lower, in the leg, is a pair of pedal ganglia that innervates the muscles of the leg. They are interconnected by commissures and connected by connectives to the cerebral and pleural ganglia. Further, back and above, in the lower part of the visceral sac, lies a pair of parietal ganglia. Usually, these ganglia are connected by long connectives to the pleural ganglia and to the fifth pair of splanchnic or visceral ganglia. From the parietal ganglia, nerves depart to the ctenidia and osphradia. The visceral ganglia lie higher, in the visceral sac. They are close to each other, connected by short commissures or even merge. They innervate the internal organs: the intestines, kidneys, genitals, etc. In addition to these five pairs of ganglia, there is another pair of small, literal ganglia in the head, connected by connectives to the cerebral ganglia and innervating the pharynx, esophagus, and stomach.

The described structure of the nervous system of gastropods is a typical scattered-nodal nervous system of molluscs.

In many gastropods, the so-called chiastoneuria is observed, which consists in the fact that two pleuroparietal connectives connecting the pleural and parietal ganglia on each side cross each other, with the right pleuroparietal connective directed above the intestine to the left side, and the left one under the intestine to the right side of the body . As a result, the right parietal ganglion lies on the left and above the intestine (supraintestinal ganglion), and the left one lies on the right and under the intestine (subintestinal ganglion).

In many gastropods, all pairs of ganglia move to the head section without changing their location in relation to each other. Such a concentration of ganglia near the head is observed in pulmonary molluscs. Chiastoneuria in this case disappears.

Despite this peculiarity of the nervous system of gastropods, it is not difficult, however, to understand that the scattered-nodal system developed from the scalene nervous system of their ancestors, similar to that which we observe in modern chitopes. So, in some of these mollusks, the ganglia are poorly differentiated, and instead of pedal ganglia, there are pedal trunks connected by commissures and forming a ladder. If we disregard the presence of a criss-cross of pleuroparietal connectives in them and imagine them untwisted, then, in essence, we get a picture that is very reminiscent of the nervous system of chitons.

The emergence of cerebral ganglia can be easily imagined as the isolation of ganglion nodes in the supraesophageal part of the ring. Other ganglia - pleural, parietal and visceral - differentiated in the form of thickenings in different parts of the pleurovisceral trunks, which turned into connections between the ganglia. The pedal ganglia developed from the pedal trunks. Thus, the connection between the ladder system of chitons and the scattered-nodal system of gastropods is undoubted. The phenomenon of chiastoneuria finds an explanation in connection with the origin of the asymmetry characteristic of gastropods.

The organs of vision - the eyes - are located at the base of the tentacles or at their top. The eyes are very different in complexity of structure - from the visual fossa to goblet eyes with a lens and a vitreous body.

Touch in gastropods is carried out by tactile cells scattered throughout the skin and specialized tactile tentacles.

The organs of smell, apparently, are the second pair of head tentacles.

The organs of chemical sense are represented by osphradia. According to the external structure, osphradia resemble small bipinnate gills. Osphradia are located at the base of the gills, in the mantle cavity.

The organs of balance in all gastropods are statocysts. They are located on the sides of the body, near the pedal ganglia, and are innervated from the cerebral ganglia. The statocyst is most often a vesicle, in the walls of which sensitive cells carrying cilia or hairs are placed. Nerve endings approach sensitive cells. Inside the bubble with liquid there is one large or several small calcareous bodies - statoliths. Due to gravity, statoliths press on the hairs of sensitive cells, and their irritation is transmitted to the nerve endings and further along the nerve to the cerebral ganglion. If the normal position of the body of the mollusk in space is disturbed, the signals from the statocysts cause a response, leading to the restoration of its position.

Reproductive system Dioecious and hermaphrodites

Many primitive gastropods (anterobranchials) are dioecious, while posterior branchial and pulmonale are hermaphrodites. The sex gland - the gonad - is always single. In molluscs with the most simply arranged reproductive apparatus, the gonad does not have its own ducts and the reproductive products are excreted by the right kidney.

The sexual apparatus reaches the greatest complexity in hermaphrodite pulmonary mollusks, for example, in the grape snail. In these gastropods, the gonad, which simultaneously produces both eggs and sperm, is called hermaphroditic. One hermaphroditic duct departs from the gland, which forms an extension - the genital pocket, where fertilization occurs. Further, the common plug is divided into two channels, closely adjacent to each other: the wider one is the oviduct, the narrower one is the vas deferens. A protein gland opens into the initial section of the oviduct, secreting mucus that covers the eggs. Toward the anterior end of the body, the genital ducts separate, and the oviduct passes into the vagina, which opens into the genital cloaca.

A long canal of the seminal receptacle also opens into the vagina, into which sperm enters during copulation, and ducts of the finger glands, the secret of which forms the eggshell. Finally, a sac-like organ opens there - the “bag of love arrows”, in which calcareous needles are formed that irritate the partner during copulation.

The vas deferens passes into the ejaculatory canal, which passes inside the copulatory organ - the penis and opens into the genital cloaca. At the base of the penis, a very long flagellated gland, a scourge, opens into the vas deferens. Its secretions stick together a mass of spermatozoa into compact spermatophores. In some mollusks (grape snail, etc.), during copulation, mutual fertilization of two partners occurs. In other hermaphroditic mollusks, the same individuals at different times play the role of either male or female.

Development, spiral crushing of a fertilized egg

Gastropods are characterized by spiral crushing of a fertilized egg. In the most primitive gastropods, a trochophore emerges from the egg, very similar to the larva of annelids. A significant difference from the latter is the non-segmentation of the mesoderm rudiments. Soon the trochophore turns into a sailboat, or veliger. It is characterized by a leg bud on the ventral side and a shell gland on the dorsal side.

The visceral sac grows on the dorsal side and forms a protrusion covered with an embryonic shell in the form of a cap. Veliger is initially bilaterally symmetrical. The anus lies in the same plane as the mouth on the back of the body. At this stage, larval twisting, or torsion, occurs, which consists in the fact that the visceral sac and shell turn counterclockwise by 180 ° in a short time. This process is associated with increased growth of the left side of the base of the visceral sac, while the right side is almost non-growth. Torsion leads to the movement of the anus and the rudiments of organs associated with the mantle cavity (gills, heart, kidneys, etc.) forward to the head of the mollusk. In this case, the intestine forms a loop, and the above-described intersection of the nerve trunks (pleuroparietal connectives) occurs - chiastoneuria. The pleural ganglia lie below the site of twisting, while the parietal ganglia lie above.

The unequal growth of the right and left sides leads to the reduction or complete disappearance of the organs of the right side. This is how asymmetry typical of gastropods develops. Spiral twisting of the shell and visceral sac occurs later. In many freshwater and terrestrial gastropods, development is direct: a small mollusk similar to an adult emerges from the egg.

Asymmetry of the class of gastropods and its origin

Gastropods are the only group of animals in which a violation of bilateral symmetry is observed, expressed in the asymmetry of the shell and the asymmetric arrangement of organs. The asymmetry of the shell structure is expressed in its spiral shape, typical for gastropods. Since the trunk pouch follows the whorls of the shell, the shell is asymmetrical in shape.

In most gastropods, asymmetry also consists in the disappearance of pairing of many organs: gills, atria, kidneys. Asymmetry in different groups of mollusks is expressed differently. In the main features, each of these groups differs in the following features.

1. In mollusks belonging to the order of biatrial anterior gills (Diotocardia) (subclass anterior branchial - Prosobranchia), asymmetry affected the trunk curl and the internal organs lying in it (liver, part of the digestive tract, genitals), other organs are quite symmetrical. The mantle cavity is located in front and has symmetrically located organs of the mantle complex: a pair of ctenidia, a pair of osphradia, the anus occupies a median position, and two excretory openings lie on the sides of it. Double atrial - two kidneys. The heart is also symmetrical and consists of ventricles and two atria. Of the modern gastropods, the biatrial anterobranchials most fully preserved bilateral symmetry and more primitive features of organization. At the same time, chiastoneuria - the intersection of pleuroparietal connectives - is clearly expressed in them.

2. Single-atrial anterior gills (Monotocardia), which make up the second order of the subclass of the anterior branchial gastropods, also have a mantle cavity lying in front of the trunk sac. Unlike biatrial, they have a pronounced asymmetry of the organs of the mantle complex. The anal and genital openings are displaced to the right. All organs of the right side are reduced, only the organs of the left side are preserved. The monoatrial ones have one gill and, accordingly, one atrium (hence the name of the detachment), one osphradium, one kidney and one excretory opening. The gill is directed with its free end forward and lies in front of the heart. In monoatrial chiastoneuria is also clearly expressed. An example of such molluscs is freshwater meadow grass and bitinia and many marine mollusks.

3. The asymmetry is no less pronounced in the third group, which makes up a special subclass of the posterior gills (Opisthobranchia). They also retain one gill, one osphradium, one atrium, one kidney, but the mantle cavity is not placed in front, but on the side and on the right. The free end of the ctenidium is not directed forward, as in the anterior gills, but backwards. In posterior gills, various degrees of shell reduction are observed. They are characterized by the absence of chiastoneuria. This includes exclusively marine gastropods, such as pteropods and nudibranchs.

4. The fourth type of organization is characteristic of most freshwater and all terrestrial gastropods, which make up the subclass Pulmonata. By the degree of asymmetry and partly by the position of the mantle cavity, they approach the single-atrial anterior gills. But they have neither gills nor osphradium, and most of the mantle cavity is isolated and turned into an air respiratory organ - the lung. Chiastoneuria is absent.

Origin of asymmetry

Undoubtedly, the ancestors of modern gastropods were completely bilaterally symmetrical forms, in which the mantle cavity was located behind, the anus also occupied a posterior and central position.

The further evolution of the ancestors of the gastropods was associated with the development and increase in the size of the shell, into which the entire body of the animal could be drawn. If we assume that the primary shell had the shape of a cone not twisted into a spiral, then it is easy to understand that the elongation of this cone could lead to the appearance of a spirally twisted shell, as the most economical and convenient form. At the same time, there is reason to believe that this symmetrical spiral shell was originally twisted forward over the head, similar to the case of the cephalopod nautilus and the fossil gastropod Bellerophontidae. Apparently, the distant ancestors of the gastropods led a floating lifestyle.

The next stage in the evolution of gastropods is associated with the transition from a floating lifestyle to a crawling one. At the same time, the position of the spiral shell, twisted around the head and pressing on the anterior part of the body, should have been clearly unfavorable for the movement of mollusks. This position of the sink is much more convenient when it is twisted back. Mollusks tend to temporarily change the position of the visceral sac and shell by twisting its muscular base. This physiological twisting, or torsion, turned out to be beneficial for the mollusks, since in this case the shell no longer pressed on the head. Further, it can be assumed that in the evolution of gastropods, a turn of the shell occurred and became fixed along with the visceral sac and the mantle complex of organs by 180°. The most adapted were those forms in which the position of the shell and body in relation to the leg and head changed. That this actually took place in the evolution of gastropods is proved by the larval twisting of the shell by 180° in the pronotogranial mollusks described above.

The process of twisting in the narrowed place between the sac and the leg leads to: 1) a change in the position of the shell, now coiled backwards, 2) to the anterior position of the mantle organ complex, and 3) to chiastoneuria. There is no asymmetry yet, except for the very place of twisting and chiastoneuria. The further evolution of gastropods went in the direction of changing the shape of the shell. Apparently, the most advantageous is the compact shape of the turbospiral shell, rather than the shape of the shell twisted in one plane. Thus, the shell becomes asymmetric, and this entails the development of asymmetry in the visceral sac, following the whorls of the shell, and the internal organs located in it (reduction of one lobe of the liver). A conical spiral shell cannot remain in a position where its apex is directed to the right (with a right-handed shell) or to the left, since this requires additional muscular efforts so that the weight of the visceral sac and shell does not overturn the mollusk. Therefore, such a change in the position of the shell is inevitable, in which the position of the center of gravity would be most convenient during crawling. The shell should have acquired an inclination to the left, and its top was somewhat laid back, i.e., some reverse rotation of the shell should have occurred. This, in turn, led to the development of asymmetry in the organs of the mantle complex. Due to the narrowing of the right part of the mantle cavity, the right gill (primarily left), the right osphradium, the right atrium and the right kidney are reduced.

The lateral position of the mantle cavity in posterior gills is explained by a more or less significant reverse rotation of the shell and visceral sac. This process was apparently associated with a decrease in the value and size of the shell in these mollusks.
In connection with the elucidation of the question of the origin of the asymmetry of these mollusks, one can quite clearly imagine the phylogenetic relationships between the most important groups of this class. The most primitive and ancient should be considered the biatrial anterior branchials, from which the single atrial anterior branchials are derived primarily. Undoubtedly further, some groups of anterobranchials (probably hermaphroditic forms) gave rise to posteriorgill and pulmonary mollusks.

The most important representatives of the gastropod class and their practical significance

The class of gastropods is divided into subclasses and orders as follows. 1st subclass - Anterior gill (Prosobranchia) - includes the detachments: 1. Double atrial (Diotocardia); 2. Single atrial (Monotocardia); 2nd subclass - Pulmonary (Pulmonata); 3rd subclass - Posterior gills (Opisthobranchia).

From the order of the two-atrial anterior gills (Diotocardia), living in the seas, in the surf zone, various types of sea limpets (Patella), which belong to the so-called round gills, are common. They do not have ctenidia, they breathe with the help of adaptive gills, sitting on the edges of the mantle. The edible mollusk abalone (Haliotis), which is found in our Far Eastern seas, also belongs to the double-atrial ones. The abalone shell is perforated from above with holes. This mollusk is hunted for its mother-of-pearl, and is eaten in China, Japan, and the United States.

Among the second, most numerous order - single-atrial anterior gills (Monotocardia), in addition to a significant number of marine forms, there are also some freshwater ones. This order includes the lawns Viviparus viviparus, V. contectus, the tentacle bithynia (Bithynia tentaculata), which is often found in our reservoirs, and others. Their belonging to the anterior gills is easily detected due to the presence of an operculum and gill breathing. Viviparus means "viviparous". So the meadow is called because its eggs develop in an expanded oviduct and already small snails with a shell covered with hard bristles enter the water.

Of the marine monoatrial gastropods found in the seas of Russia, the Littorina rudis, common in the northern seas, should be noted. These are molluscs, sitting in masses on coastal stones and algae, on which they remain even at low tide.

In the northern seas and in the Sea of ​​Japan, large molluscs (shell height up to 10 cm) tubehorns (Buccinum) are common at great depths. The predatory mollusk rapana (Rapana bezoar) is found in the Far Eastern seas and harms commercial molluscs. Rapana was brought to the Black Sea not so long ago, where it multiplied strongly.

Of great interest are the keel-legged mollusks (Heteropoda) from the pronebranch subclass. These are predatory mollusks adapted to a pelagic way of life, with a strongly reduced shell. The leg, flattened laterally, is adapted for swimming. The transparency of the body compensates for the reduction of the shell. They are found mainly in warm seas.

Representatives of the subclass of pulmonary molluscs (Pulmonata) are terrestrial or freshwater forms. The pulmonary snails belong to the grape snail (Helix pomatia) and various naked slugs: the field slug (Agriolimax agrestis), the forest slug (Arion bourguignati), etc. Slugs differ from the grape snail and other terrestrial gastropods in the reduction of the shell. They are pests of garden and other cultivated and wild plants.

Land snails are distributed in various latitudes, up to the Far North. In the snail and in the slugs, the eyes are at the ends of the head tentacles. The same subclass includes freshwater pond snails (Lymnaea) and coils (Planorbis). They differ from terrestrial lung snails in that their eyes are located at the base of the second pair of tentacles.

Representatives of the posterior branchial subclass (Opisthobranchia) are exclusively marine inhabitants. In many of them, the shell is reduced. Of the posterior gills, an interesting order is the pteropods (Pteropoaa), which, like the keel-legged of the anterior gills, are adapted to a floating lifestyle. Their shell is either completely reduced, or small and has a conical shape. They swim with the help of pterygoid lateral outgrowths of the poga. Of the other posterior gills, the detachment of nudibranchs (Nudibranchia) should be noted, which are characterized by the absence of a shell and ctenidia and breathe with adaptive gills. This order includes the mollusk Dendronotus, which has branched skin outgrowths that play the role of gills.

In addition to the above negative value of some groups of gastropods (snails and slugs - pests of agriculture, freshwater and land snails - intermediate hosts of flukes, etc.), it is also necessary to note the positive value of gastropods. Many representatives of the class gastropods

Schematic diagram of the structure of the shell margin of Conchifera. 1 - outer layer of the periostracum; 2 - inner layer of the periostracum; 3 - ostracum; 4 - hypostracum; 5 - mantle epithelium; 6 - periostracum gland; 7 - the place of secretion of the inner part of the periostracum; 8 - place of secretion of ostracum; 9 - place of hypostracum secretion

The shells of all Conchifera (classes Gastropoda, Cephalopoda, Bivalvia, Scaphopoda, Monoplacophora) are built, in general, according to the same scheme.
Initially, the shell consists of three layers:

• Periostracum- the outer thin layer, consisting exclusively of protein - conchiolin. In fact, it is represented by two layers tightly adjacent to each other.
• Ostracum- the middle layer of the shell, consists of crystalline prisms of calcium carbonate (СaCO 3) in a conchiolin wrapper. Its structure can be very diverse.
• Hypostracum or mother-of-pearl layer - the inner layer of the shell, consists of CaCO 3 plates, also wrapped in conchiolin.

Often, especially in highly organized gastropods, the mother-of-pearl layer is absent; on the other hand, the ostracum in such cases can consist of many layers of different structure.

Calcium carbonate in the shell of molluscs can be in the form of three modifications:

• Aragonite - characteristic of the most ancient mollusks; the mother-of-pearl layer always consists only of aragonite.
• Calcite - apparently, this modification is a later acquisition of molluscs.
• Vaterite - used for reparation.

There are various combinations of aragonite and calcite in the shells of various molluscs.

The secretion of the shell is carried out by the mantle epithelium on its growing edge. At its base is the gland of the periostracum, which secretes the outer layer of the periostracum. Further along the course of the mantle epithelium, the remaining layers of the shell are sequentially secreted.
In the space between the mantle epithelium and the periostracum (extrapoly cavity), the process of biomineralization takes place. It is carried out due to the constant pumping of Ca 2+ and HCO 3 ions and pumping out hydrogen ions. This creates a favorable environment for the formation of calcium carbonate (CaCO 3). In addition, mucopolysaccharides and proteins are secreted into the extrapoly cavity to form the conchiolin wrapper of calcium carbonate crystals.

shell chitons

Shell morphology

Morphology of gastropod shells

shell shape

In most gastropods, the shell is twisted into a spiral, while the whorls of the spiral are most often in different planes. Such a spiral is called a helical spiral. The vast majority of shells are twisted to the right, they are called dexiotropic. However, there are also left-handed shells, which are called sinistral. If you look at the shell from the side of the mouth, then for right-handed ones it is located on the right side, for left-handed ones - on the left.

Most gastropods have a shell whose whorls do not crawl on top of each other, but only touch - such shells are called evolutionary. The same shells, in which each new turn completely covers the previous ones, belong to involute or convolute. Involute shells are characteristic of cypriae, trivia, and some other genera of gastropods. Convolute shells are distinguished by the fact that the last whorl hides all the previous ones, and they have a fusiform shape from the mouth side. In this part, they are more elongated; the siphonal and posterior canals are easily distinguishable against the background of a large outer lip and a smaller penultimate whorl. Such shells are characteristic of ovula and volva. Shells resembling a spiral, the turns of which are not closed together, but twisted many times in different directions, are called devolute, or untwisted.

The mouth of the shell can be round, oval, oblong, semicircular; narrow or wide. Internal whorls of the shell, growing together, form inner column or columnella. In a number of species, the internal canal of the columnella opens outward at the base of the shell in the form of an opening called the navel. This morphological feature occurs in molluscs of the genus Natica. The inner lip of the mouth can be wide, narrow, everted, and also contain teeth. Sometimes this lip can bear a thickened influx of enamel, called callus.

The outer lip of the mouth has a variety of morphological variants. For example, the last axial ridges, ribs and plates border the mouth of the murex shell, often resembling fish fins in their shape. All members of the family Strombidae have a special notch in the lower part of the outer lip of the mouth, which allows the mollusks to look around without protruding their organs of vision from the shell. Also, some representatives of this family have shells with a wide bent outer lip. Representatives of the genus Lambis have numerous curved outgrowths of the outer lip of the shell mouth.

In the lower part of the mouth of the shells of some gastropods, there is a trough-shaped or closed siphonal outgrowth, which in the latter case contains a siphonal canal that opens at the end of the outgrowth with a hole.

Cones and cyprees have peculiar shells. This led to the emergence of specific terms describing certain structural features of these shells. In Cyprian shells, it is customary to distinguish between the dorsal (upper), base (lower) surfaces, as well as the basal (lateral) edge and the median platform. In cones, a base (base) is distinguished, on which spots can occur, a body and a top, which can be smooth or have a circular row of teeth.

Sculpture

Sculpture of gastropod shells can be superficial (in which case it is called microsculpture) or real sculpture, formed by the deeper layers of the shell. Examples of microsculpture are scales, tubercles, or spiral grooves. Real sculpture is represented by keels, ribs, ribs, ridges and plates. Sometimes the latter can be high, low, pterygoid. The high wavy ridges and plates of some Murex are commonly referred to as varixes. In the case of a vertical arrangement, the sculptural formations are called axial, in the case of a transverse arrangement, they are called spiral. In some cases, one speaks of a diagonal sculpture.

Coloring

The general color of the shell can be monochromatic, spotted, striped or complex, patterned. In some species, the spots on the shell may be indistinct, vague, in others, they stand out in contrast against the general background of the shell, taking an oval, triangular or square shape, which may be a species feature. Stripes, depending on the location, are divided into axial, in the case of a vertical arrangement, spiral - in the case of a horizontal, diagonal and zigzag. The shells of some species of gastropod molluscs are remarkably complex in color. Each shell within the same species has its own unique, but common features, pattern. Some patterns have special definitions. Thus, a light spot on the dorsal surface of cypriae shells is often called a window, rounded spots with a contrasting inclusion are called eyes, and thin calligraphic lines that form a picturesque cascade of triangles of different sizes and adorn the shells of some types of cones are called a scaly pattern.

Morphology of bivalve shells

Bivalves molluscs- bilaterally symmetrical animals, the body of which is located in the shell, consisting of the left (upper) and right (lower) valves. A more or less convex rounded tubercle on the upper part of the dorsal surface of the valve is called top of the head. In equilateral shells, the crown occupies a median position, while in most non-equilateral shells, it is shifted forward or backward. A number of species, such as scallops, spondyluses, have flat triangular outgrowths called ears on the sides of the crown.

The shell valves are interconnected by an elastic ligament located on the dorsal surface behind the umbos. The shell lock, which is present in most mollusks of this class, is represented by teeth and notches on the lock platform. Each tooth of one valve corresponds to the notch of the other, providing a reliable articulation of the closed shell valves.

On the inner surface of the valves there are rounded imprints of the adductor muscles (closers). There may be two or one. A thin and wavy mantle line is noticeable between them, which runs along the edge of the valve. In species with well-developed siphons, this line, which bounds the mantle sinus, makes a bend in the posterior part of the shell.

A number of bivalve molluscs have shells with shells that vary in size, color, or even shape. These are, for example, oysters, some scallops and spondylus. Often a deeper and lighter lower leaf is complemented by a flat and brightly colored upper one.

shell shape

The shape of the valves varies greatly among different species. Most bivalves have an oval or triangular shell. There are also molluscs with rectangular, disc-shaped, wedge-shaped and trapezoidal valves.

Sculpture

The outer surface of the valves can be smooth or sculpted. A distinction is made between microsculpture and real sculpture. Microsculpture (setae, grooves, wrinkles) is formed on the surface of the periostracum, while real sculpture (ribs, keels, spines) is formed by deeper layers of the shell. In the case of intersection of concentric ribs equal in width and height with radial ribs, a net-like texture is formed. Small scales on the surface of the valves can be flat or convex. They cover a smooth surface or are located on the ribs. Large scales can be arranged in rows, giving the latter the appearance of steps, or wrapped in long tubes on the surface of the shells.

Coloring

The general color of shells of bivalve mollusks can be very diverse: mostly monophonic, spotted, with various lines and patterns. Thin or wide radial lines are called rays, concentric lines are called stripes. The lines can be wavy, zigzag, branch, or form intricate patterns such as diamonds, triangles, or crosses.

Morphology of cephalopod shells

The shell of cephalopods was originally a conical tube, straight or curved, in the living chamber of which a soft body is located, and the back part performs the function of a hydrostatic apparatus. The acquisition of planospiral coiling by the shell is an adaptive mechanism that enabled them to locate the center of gravity and buoyancy along one vertical line, or even at one point (in Nautilus, the difference between these centers is about 2 mm). Due to this, a minimum of effort is required to acquire any position in the water.

The mollusk is a unique and ancient representative of the animal world. Thanks to a special kind of protective formation - a shell - it cannot be confused with anyone else. Later in the article we will talk about what kind of shell gastropods and other representatives of invertebrates have.

Such different shellfish

They are found on the seashore, and on the river, and just in the forest. The variety of their shapes and sizes is truly amazing. Elongated, curved, like a spindle, flattened, consisting of two "petals", spherical with a sharp top and very small, built in the form of a spiral flattened from the sides ...

Some snails are so tiny that you can only see them with a good magnifying glass. But, for example, among the coral reefs of the oceans, there are huge bivalve mollusks with a shell size of more than a meter and a weight exceeding 300 kg.

The color of the shells is very diverse. It can be plain and modest, or it can be whimsical - decorated with specks, lines and stains.

A general concept of what function the shell performs in mollusks in the 7th grade gives us a course in zoology. But in order to thoroughly understand this topic, let us first dwell on some points.

Types of shellfish

Mollusks in the world, scientists have counted more than 100 thousand species. The most famous among them are bivalves and gastropods.

Among the typical, that is, the most famous, representatives of mollusks are called gastropods (about 40 thousand species). They live in almost any environment, can move around, and they have a shell.

The body of gastropods is divided into a head, a leg equipped with strong muscles, and a mantle. In the oral cavity is the so-called radula (grater), which serves to scrape food from various surfaces. As a rule, these are algae, fungi, plants, their rotting remains, carrion.

Slugs are the closest relatives of this type of snail, but they do not have a shell.

Gastropods live both in water and on land. You have seen many of them in forests, gardens and parks. For those who are fond of aquaristics, snails (most often Achatina) crawl on the glass and the bottom of the aquarium and bring undoubted benefits, clearing them of growing algae. The most common gastropods are the grape snail and the same Achatina. And whimsically curved, with a point at the end, shells of buccinums (or trumpeter clams) we bring with us from the sea as souvenirs.

There are also predators among snails. Their saliva contains sulfuric acid, with which they dissolve the shell of smaller molluscs and eat it.

The most sedentary, bivalve mollusks (such as oysters, mussels, scallops, freshwater toothless and barley) have a shell divided in two, and no legs or head. They do not need them - bivalves get food by opening the shell and filtering water through special internal siphons. These siphons also serve as gills - with their help, mollusks breathe.

Bivalves may simply lie on the sea or river floor, may live among corals, or may be partially buried in silt burrows. Some species, sticking to stones and various surfaces, literally stick around them. Only very few species of bivalves (scallops) have the ability to swim.

What is a sink

At its core, this is a shell protecting a living being or a future plant. It can be woody (like a nut), be the egg shell of a future bird, act as an insect or crab shell. In the latter case, fragments of the shell are fastened with soft tissues so that the animal can move around.

The basis for the shells can be wood, lime, bone tissue (like a turtle) and even quartz compounds.

Some shells are destroyed during the development of a living being living inside - this happens with a nut when it germinates, or with a chick hatching from an egg shell. Insects survive molting and change their shell for a new one several times during their life cycle.

But all molluscs, although their name comes from the Latin word Mollusca, which means "soft-bodied", carry their shell with them, building it up throughout their lives.

How a sink is built

The shell is made by the mollusk itself. The building material for the house is produced by its special glands. They filter out lime particles from the water and build them up along the edges of the shell, which is still soft and young, received at birth. The mollusk matures - its body grows, and its "suit" becomes thicker.

The thickenings along the edge of the shell in the form of scars are a kind of growth rings that trees have. Only in them they are hidden inside the trunk, and in mollusks (for example, in oysters) you can easily find them - these thickenings are located parallel to the entire edge of the shell shell.

What is the shell of a mollusk made of?

Shell shapes are very diverse. Snails (gastropods) most often have a shell in the form of a curled cone. The shell of bivalves, as the name implies, consists of two interlocking halves.

Slugs and cephalopods (these are predatory inhabitants of the sea kingdom - squid, octopus and cuttlefish) also belong to mollusks, but do not have a shell as such. They have a thickened skin that scientists believe is a hidden, evolved shell. The leg in cephalopods also evolved and turned into tentacles with suction cups.

The shell is usually formed by three layers:

1. In the outer there is no lime, it consists of a horny substance secreted by the mantle. This protein, called conchiolin, is the strongest among the layers of the shell. It serves as the strongest defense against the aggression of the surrounding world.
2. The next layer is calcium carbonate. It is an inorganic chemical compound widely distributed in nature. It occurs both in the form of independent minerals (calcite and aragonite) and rocks. This is chalk, limestone, marble. The lime base is part of the shell of bird eggs, it also makes up the middle layer of the shell of the mollusk.
3. The thinnest is the inner layer of the shell, which is called the "mother of pearls" or mother of pearl. It consists of a mixture of small particles of lime and the organic matter of conchiolin.

Mother-of-pearl, which could be appreciated from a jewelry point of view, however, you will find only in mollusks living in warm tropical seas. For example, in the waters of the Red Sea, the Pacific Ocean, in the Persian Gulf.

And in freshwater bodies of mollusks, "manufacturing" mother-of-pearl and pearls, there are practically no left.

What is the function of the shell in molluscs?

The shell is protection. A kind of natural spacesuit and the skeleton of a soft-bodied creature, which, if it were without it, would immediately become easy prey for many predators hunting nearby - from fish in reservoirs to birds and animals in the forests.

The function of the shell in mollusks is to be a refuge in which he can hide, as soon as there is any threat to his life.

So, bivalve molluscs, sensing danger, tightly close the valves. And the disturbed gastropod mollusk not only goes into the shell, but also slams the lid. All! Now it is not prey, but only a pebble, among others, hard and inedible. The predator will leave or swim away, having lost interest, and the mollusk, having waited out the danger, will again get out.

So, protection - this is the function of the shell in mollusks. It would seem that there are no other answers to this question. However, Japanese scientists managed to discover in some species of gastropods the so-called ability for active self-defense. It manifests itself, in particular, in such species as Karaftohelix selskii and Karaftohelix gainesi. They have large shell sizes and a wider entrance to the "house".

When attacked, for example, by a beetle, the snail can turn its "house" almost 180°, tilt it almost horizontally, defending itself and repelling the attacker. Of course, the muscles of snails with this ability are more developed than those of their relatives. Moreover, this ability did not appear in related species, as their feature, but was evolutionarily fixed in various habitats.

We hope you have received a fairly complete answer to the question of which the function is performed by the shell in molluscs.

Although the mollusk shell itself is a lifeless formation (a product of the excretion of living cells of the mantle), its structure very clearly reflects many biological features that characterize the life of these organisms.

On an empty shell, the valves always take a half-open position due to the tension of the elastic ligament connecting them. The ligament acts in the same way in a living shell: the valves open slightly without any effort on its part and remain in this position while the shell calmly stays in place with the help of its foot or slowly moves along the bottom.

But in order to close the shell tightly, the shells have to apply force by contracting their closing muscles - the anterior and posterior, attached with their ends to both shell valves (traces of their attachment are clearly visible in the form of matte rounded spots on the inner surface of the shell, at the anterior and posterior ends of each sashes).

On the valves of the shell, it is easy to find the most convex and at the same time the oldest part of it - the top, or top, and arcuate bands of annual growth that go one after another. The formation of these bands depends on the fact that the growth of the shell in cold, winter time is greatly slowed down, and with the onset of heat it intensifies (compare with the growth rings of wood). Our shells live up to 12-14 years.

Each shell shell consists of three layers:

1. The outer dark-colored organic layer, resembling a horny substance;
2. A china-like layer, actually composed of lime (mostly CaCO 3 ), and
3. The mother-of-pearl layer, which also consists of lime, which is deposited here in the thinnest layers. As a result of this structure, the mother-of-pearl layer casts with iridescent colors (just as the thinnest walls of soap bubbles or oil stains that have spilled a thin film on the surface of water are cast with all the colors of the rainbow).

Further, examining the shell valves, one can see, firstly, that the oldest parts of the shell are at the same time thicker-walled, and the youngest growth band, which forms the very edge of the shell, is also the thinnest.

Secondly, on larger, that is, older shells and on their tops, the dark, organic layer often turns out to be destroyed during the life of the mollusk, exposing a white porcelain-like layer here. All this depends on the fact that the organic layer is formed only by the outer edge of the mantle, that is, only on the youngest band of annual growth, and lime is released by the entire lobe of the mantle, which is why the calcareous shell becomes thicker and more durable every year.

Sometimes small tubercles are visible on the smooth surface of mother-of-pearl. This means that here, between the living cells of the mantle and the shell, some grain of sand clogged and the mantle enveloped it with a layer of mother-of-pearl.

In our ordinary shells, the mother-of-pearl layer is thin and such tubercles remain very small. But in those bivalve molluscs, in which mother-of-pearl forms a thick layer, such tubercles turn into very large beautiful pearls (hence the name "pearl"), or pearls that go to various jewelry.

The content of the article

SINK, hard integuments of the body of some animals, such as snails, bivalves or barnacles. Of greatest interest, especially in terms of practical use and collection, are calcareous mollusk shells. To protect their soft, vulnerable body from natural enemies, mollusks secrete a substance that consists mainly of calcium carbonate and hardens into a material close in density to marble. They acquired this ability in the early periods of the geological history of the Earth, already by the beginning of the Cambrian (570 million years ago). Rocks of this age contain many of their fossilized shells.

Shell types.

There are five main classes of molluscs: bivalves, gastropods, shellfish, spadefoot, and cephalopods. Representatives of each of them have their own characteristic type of shell.

Bivalves.

The shells of bivalves consist of two halves (flaps) connected to each other by an elastic ligament and held in a certain position by interlocking teeth. The castle line - the side on which the valves are connected - is considered the upper, or dorsal (dorsal), and the opposite - where they can diverge, - the lower, or ventral (abdominal). In some species, the valves are identical, while in others they differ slightly in size, shape, and color. Oysters, cockles, mussels and scallops - all these marine molluscs are part of the bivalve group.

Gastropods.

The shells of gastropods, unlike bivalves, are whole, i.e. not divided into sections. Representatives of this group, often called snails, can be found on land, in fresh water and in the sea. Usually their shells are twisted clockwise around the central axis (column) like a spiral staircase. If you hold such a shell, called right-handed, with the sharp end (apex) up, then its "inlet" hole - the mouth - will be on the right. If the mouth is on the left, the shell is called left-handed. At the mouth, the inner and outer lips are distinguished, and its lower edge usually bears an outgrowth (anterior canal), which may resemble either a long tube or a curved teapot spout. If there are two channels, the second, located in the upper part of the outer lip, is called the back.

Gastropods move with the help of a muscular outgrowth - the legs. When the animal senses danger, it retracts its leg into the shell; the mouth at the same time is closed by a cap - a small solid formation attached to the back of the leg. Caps in different species are not the same in structure, size and shape (according to the closed mouth) and may resemble a thin disc, button or marble plate.

Each turn of the shell is called a whorl, and the last and largest one is called a body whorl. They are clearly visible, for example, in trumpeters, flattened and almost merged externally, like in cones, or not visible at all from the outside, like in cypriae.

Armored.

The shells of these mollusks consist of eight overlapping dorsal plates. These animals are also called chitons, because from below, from under the shell, a leathery belt protrudes, resembling the edge of ancient Greek clothing - a chiton. Shellfish usually keep under rocks and in crevasses; they are difficult to tear off from the substrate, to which they firmly stick with the sole of a muscular foot.

Spadefoot.

The shells of these mollusks are slightly curved tubes resembling elephant tusks in shape. Their length ranges from 2.5 to 12.5 cm; some are white and matte like chalk, others shine like porcelain.

Cephalopods.

Cephalopods are perhaps the most interesting of the molluscs in terms of evolution. Judging by the fossil remains, they once had shells up to 4.6 m long. Most modern cephalopods have only small internal shell rudiments. Squids, cuttlefish, octopuses belonging to this class are now protected by their powerful tentacles, camouflage coloring and "ink" curtains released into the water. The only current cephalopods with an outer shell are members of the nautilus genus. Decoration of any collection - view Nautilus pompilius. Its spiral, iridescent mother-of-pearl shell consists of a series of chambers and forms a flawless logarithmic spiral; whorl width increases, maintaining a constant ratio to its length. Growing up, the body builds new chambers and moves to live in the last, largest of them.

Shell composition and growth.

As mollusks grow, they secrete a substance that increases the size and thickness of their shells. This secret, secreted by the skin fold surrounding the body, called the mantle, consists of calcium carbonate mixed with phosphate and magnesium carbonate. In bivalves, the mantle covers the body from the sides, while in gastropods it forms a fleshy lining of the mouth. The growth lines on bivalve shells run parallel to their outer margin, while in gastropods new whorls are added to the shells.

There are three layers in the shell of molluscs. Outer (periostracum) rough, consists of organic matter conchiolin; the middle, or porcelain-like (ostracum), is formed by small prisms of calcite or aragonite, and the inner (hypostracum) is formed by parallel plates of aragonite and is often mother-of-pearl. The pearly iridescent sheen is due to the translucent layers of calcium carbonate. The shapes of the shells and the color of their outer surface are extremely diverse. Some of them are no larger than the head of a pin; they are so small that the beauty of their form cannot be fully appreciated without a magnifying glass. Others, such as the giant tridacna ( Tridacna gigas) from the Indian and Pacific Oceans, reach a diameter of 60–120 cm and a mass of 135–180 kg. They gave rise to legends about divers who fell underwater into a trap from the closed shells of this mollusk.

Spreading.

The current ranges of some 50,000 species of marine mollusks depend on the temperature and salinity of the water, as well as the contours of the primitive oceans. Probably the richest source of shells in the world is a wide belt stretching from the warm waters of East Africa across the Indian Ocean to Australia and the islands of the South Pacific. Many of their best specimens (cyprees, cones, terebras, venerids) are mined here - off the African coast between Kenya and Mozambique, in the waters off Queensland (Australia) and tropical seas surrounding some islands of Indonesia, the Philippines and the Ryukyu archipelago.

The second largest is the West Indies region, stretching from Bermuda through the Antilles to Brazil. This area abounds in shells such as Triton's horn, strombus, cassis, and fasciolaria. There are several other places in the world where interesting specimens of shell mollusks are found. Since the temperature in the Mediterranean is approximately the same as in the Caribbean, many species of scallops, trumpeters, fasciolaria and brooms are found in both of these areas. Along the East Coast of the United States, you can collect beautiful naticides, cones, anomia and olives, left-handed beusicons, as well as strombuses and graceful angel wings bivalves. Two small islands located off the west coast of Florida, Sanibel and Captiva, are considered the best places for collecting shells in the United States. Off the western coast of the country there are many fairly common species, as well as rarer haliotis and sea cuttings.

Approximately 50,000 taxa of freshwater mollusks are known, primarily related to bivalves and gastropods. They live not only in rivers and lakes, but also in hot springs, in caves, at the base of waterfalls, and even in the freezing waters of the polar regions. Most terrestrial mollusks are pulmonary gastropods, i.e. snails with special breathing apparatus. Their shells are often as brightly colored as those of the most colorful marine species. These snails live among damp vegetation, mostly in trees; one of their most famous species is the grape snail ( Helix aspersa); in France it is considered a delicacy.

Usage.

The history of the use of shells goes back over 10,000 years. Red cassis from the South Pacific found in prehistoric Cro-Magnon caves in Europe. Their presence thousands of kilometers from their homeland suggests that they served as money, which means that trade between these remote areas inexplicably existed already in the early stages of human history. Primitive man undoubtedly used shells as decorations. Shells with sharp edges, such as some common bivalves, were used as a cutting tool.

Particularly interesting is the role of shells as a currency. In the past, such "money" was widespread in America, Asia, Africa and Australia. The most valued in this sense was the cypriea-coin ( Cypraea moneta), or kauri. Even today, on some islands in the Pacific and Indian Oceans, the shells of another type of kauri, C. annulus. Among the peoples of Central Africa, the possession of bundles of large kauri served as evidence of personal or tribal wealth, and in West Africa these shells were paid until the middle of the 19th century. In some areas of the African continent, for example, in the territory of present-day Angola, coins were distributed from the cut shells of the ground snail Achatina mint ( Achatina monetaria). On the islands north of New Guinea, shells were also often ground to a suitable size to be used as change money of various denominations. Until 1882, trade in the Solomon Islands was carried out with the help of such "coins" of a standard shape and a certain size.

Shell money laid the foundation for the economy of the North American Indians. Shells of spadefoot (for example, a sea tooth - Dentalium pretiosum) were used by them as coins long before the emergence of the Hudson's Bay Company. A thread of 25 such large shells was enough to buy a canoe. A remarkable achievement of the "coinage" of the natives of America was the so-called. wampum. It consisted of polished cylindrical pieces of trumpeter shells, common mercenaria ( Mercenaria mercenaria) and common littorina ( Littorina littorea) strung on leather straps. This money was usually made in coastal areas, where the highly prized purple mercenaria shells and giant white trumpeters were more readily available. From here, ready-made money was transported into the interior of the country.

Shells have also been used for other purposes for centuries. The collections found in Roman dwellings testify that they were collected already in ancient times. Medieval pilgrims wore the comb of St. James ( pecten jacobeus) on their hats as a sign that they had crossed the sea and reached the Holy Land. Large shells of cypriae, trumpeters and other mollusks were often depicted by Renaissance artists. A famous example is the huge scallop in a Botticelli painting. Birth of Venus.