Brief description of the type of mollusk. Classes of mollusks and their characteristic features Animal mollusks

photo:Art

In living nature, mollusks are found almost everywhere - they are found in the thickness of the ocean waters, high in the mountains, in salt and fresh water, on the ground and underground. They do not live only in sandy deserts and snow cover.
The diversity of the habitat explains that mollusks differ from each other in body structure, its color and shape, methods and speed of movement and other features.

photo:jacinta lluch valero

But despite this, they have some common features: the mollusk consists of a head, torso and legs. Most species are “equipped” with an external shell, which consists of protein substances and calcium carbonate. The shell plays a protective function - the mollusk hides in it at the slightest danger. It can be single or double-leafed. The body of the mollusk, like the shell, in most individuals is a spiral.

photo: divemecressi

The internal organs of mollusks are located in the body; in some species they are shifted to the leg. There are mollusks that do not have a head - it has disappeared as unnecessary. The absence or presence of eyes, tentacles, the shape and size of the shell, the location of the internal organs - all this depends on the method of evolution, habitat, and nutrition of certain species of these invertebrate animals. It should be noted that mollusks are the first animals to have a liver.

photo: Martin LaBar

The first fossil mollusks date back to the Cambrian period. Their ancestors are considered to be annelids, which have external segmentation. At the beginning of their evolution, cephalopods predominated, later bivalves and gastropods appeared. Modern scientists pay special attention to the study of the evolution of mollusks, which can be traced through preserved shells. This makes it possible to understand the developmental features of not only invertebrates, but also the rest of the animal world, because mollusks are an important component of the diet of fish, birds and mammals.

photo:Art

Today, there are seven classes of mollusks. The most popular of them are gastropods, bivalves and cephalopods.

Gastropods- one of the many species that live literally everywhere. These are aquatic and land snails. Moreover, in aquatic gastropods, the respiratory organ is the gills, while in land animals they are transformed into lungs, which allows the snails to be independent of the reservoir and live a full life on land. Bivalve mollusks are difficult to confuse with other species - they have a characteristic shell consisting of two closed valves. The body of the mollusk is enclosed between the valves. At the slightest danger, the doors slam shut, but in the normal state they are slightly open. Bivalves live in aquatic environments, both in salt and fresh water at various depths.

photo:Art

Cephalopods are considered the most “intelligent” representatives of this animal species. Cephalopods have good memory and a very large brain. They are found only in water - these are squids, cuttlefish, octopuses. Such mollusks can actively move thanks to tentacles equipped with special suction cups.
Other classes of mollusks are less common in wildlife.

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Molluscs (soft-bodied)
There are a lot of mollusks on the planet. About 130,000 biological species. All these animals form a type of mollusc. Depending on the structure of the body, several classes of soft-bodied animals are distinguished.

Class of bivalve molluscs

These soft-bodied creatures live in water. They are protected like armor by the shell. The mollusk “builds” its shell itself, from calcium carbonate molecules. So the chemical composition of shells is similar to human bones. The sink consists of two halves (leaves). The doors are connected on one side. They can open and close with little force. Bivalves feed on small edibles, filtering them from the water. At the same time, they clean the water from turbidity.

Class of gastropods (snails)

Gastropods are protected by a single, coiled shell. The muscular “leg” allows gastropods to crawl from place to place. On land and under water, they move slowly and smoothly, looking for food and scraping it with their tongue. There are also predatory snails.

Cephalopods (Cephalopoda)

These soft-bodied animals do not need a protective shell - they themselves are predators (the cephalopod nautilus, however, has a shell). Squids, octopuses, and cuttlefish swim quickly in the water column. Cephalopods have an amazing “engine” - a kind of “water jet”. They take in water and then forcefully throw it out through a special hole - a funnel. A stream of water flies out in one direction, and the mollusk, like a living “rocket,” moves in the opposite direction.

On the head of these mollusks there are tentacles, “arms” (or “legs”). The tentacles are covered with suckers, and in squids they are also covered with hooks sharp like claws. With their tentacles, mollusks deftly grab fish and crabs. Their strong beak bites through the hardest shells. Cephalopods can instantly change their color and blind the enemy with a cloud of “ink liquid”.

Collection

You can collect a very interesting collection of shells. The collection sink must be empty. It is thoroughly washed in fresh water. It is advisable to fix the dried shell on cardboard and write the name of the mollusk, where it was found, and when. It would be nice to indicate the scientific name in Latin.

You can make a necklace out of shells. They are strung on a strong thread (nylon, for example).

The largest shell belongs to the giant tridacna. The ribbed doors are more than a meter across and weigh up to 250 kg. The mollusk itself, living in such a fortress, weighs up to 30 kg. Tridacna lives for more than a century.

The predators of the soft-bodied world are Murex brandaris. They attack other mollusks by drilling holes in their shells.

Charonia newt, a huge (more than 30 cm) predatory gastropod. Charonia destroy starfish, including the crown of thorns. This star eats coral polyps. But Charonia were caught by lovers of beautiful shells. Then the “crowns of thorns” multiplied and destroyed many beautiful coral reefs.

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Shellfish- bilaterally symmetrical or secondary asymmetrical three-layer animals. They live in marine and fresh water bodies, on land.

The body of most species of mollusks can be divided into three sections: head, trunk and leg. The head contains the mouth and sensory organs. The strongly thickened ventral side forms various types of legs. The leg, as an organ of locomotion, can have a different shape: in swimming forms it turns into wide blades or tentacles, in crawling forms it turns into a flat sole.

The body is surrounded by a fold of skin - the mantle. Between the mantle and the body, a mantle cavity is formed, into which the openings of the digestive, excretory and reproductive systems open. The mantle cavity also contains the respiratory organs and chemical sense organs (osphradia). All of the above is called the mantle complex of organs.

The muscles of mollusks are well developed and consist of muscle bundles. They are especially strongly developed in the animal’s leg.

The whole is reduced to the pericardial sac and the cavity in which the gonads are located. The space between the other organs is filled with parenchyma.

The digestive system is divided into three sections: anterior, middle and posterior. The anterior and posterior sections are of ectodermal origin, the middle section is of endodermal origin. In the pharynx of many species there is a specific organ for grinding food - the radula, or grater. The ducts of the salivary glands open into the pharynx, and the ducts of the liver open into the midgut.

The respiratory organs are represented by gills or lungs. Lungs are present not only in terrestrial species, but also in forms that have secondarily transferred to an aquatic lifestyle. Gills and lungs are modified sections of the mantle. In aquatic species, gas exchange can also occur through the skin.

The circulatory system is not closed: blood flows not only through the blood vessels, but also through the lacunae located in the space between the organs. Mollusks have a heart consisting of two or more chambers. The heart is located in the pericardium (pericardium).

The excretory organs are the kidneys, which are modified metanephridia. The kidney begins as a funnel in the pericardial sac and opens with an excretory opening into the mantle cavity.

The nervous system of most mollusks is represented by several pairs of nerve ganglia, which are located in different parts of the body. This type of nervous system is called scattered-nodular. In addition to reflex activity, the nervous system performs the functions of regulating growth and reproduction by releasing various neurohormones. Mollusks have organs of chemical sense (osphradia), balance, and numerous tactile receptors are scattered in the skin. Many species have eyes.

The predominant number of species of mollusks are dioecious animals, but bisexual species are also found. Development in all terrestrial species, in most freshwater and some marine inhabitants is direct. If development proceeds with metamorphosis, then either a trochophore-type larva or a veliger (sailfish) larva emerges from the egg.

The phylum Mollusks are divided into classes: Gastropoda (Gastropoda), Bivalves (Bivalvia), Cephalopoda (Cephalopoda), etc.

The question of the origin of mollusks is still discussed by zoologists. Currently, the most proven hypothesis is the origin of mollusks from primary coelomic trochophore animals, from the same group from which annelids originated. The relationship of mollusks and annelids is evidenced by the similarity of embryogenesis (spiral fragmentation, metamerism of the rudiments of some organs, teloblastic anlage of the mesoderm) and the presence in lower mollusks of a trochophore larva, similar to the trochophore of polychaetes. It is assumed that the primary mollusks were bilaterally symmetrical animals with a low body, covered with a slightly convex shell, with a muscular flat leg and an almost distinct head. Two lines of evolutionary development extend from the primordial mollusks. The first line leads to the formation of bokonervny mollusks; this group is not considered in this manual. The second evolutionary line leads to the appearance of shell mollusks. Among shell mollusks, the most primitive are monoplacophorans. It is believed that bivalves, gastropods and cephalopods evolved from ancient monoplacophorans.

Description of classes, subclasses and orders of the Mollusc type:

• Class Gastropoda

• Class Cephalopods (Cephalopoda)

  • Subclass Coleoidae

Mollusks are one of the most ancient invertebrate animals. They are distinguished by the presence of a secondary body cavity and rather complex internal organs. Many of them have a calcareous shell, which protects their body quite well from the attacks of numerous enemies.

This is not often remembered, but many species of this type lead a predatory lifestyle. The developed salivary gland helps them in this. By the way, what is the salivary gland in mollusks? This generalizing concept refers to a fairly wide range of specific organs located in the pharynx and oral cavity. They are intended for the secretion of various substances, the characteristics of which can be very different from our understanding of the word “saliva”.

As a rule, mollusks have one or two pairs of such glands, which in some species reach very impressive sizes. In most predatory species, the secretion they secrete contains from 2.18 to 4.25% chemically pure sulfuric acid. It helps both fight off predators and hunt their relatives (sulfuric acid perfectly dissolves their calcareous shells). This is what the salivary gland is in mollusks.

Other natural value

Many species of slugs, as well as the vine snail, cause enormous damage to agriculture around the world. At the same time, it is mollusks that play a crucial role in global water purification, since they use organic matter filtered from it to feed them. In many countries, large ones are bred on sea farms, as they are a valuable food product that contains a lot of protein. These representatives and oysters) are even used in dietary nutrition.

In the former USSR, 19 representatives of this ancient type were considered rare and endangered. Despite the diversity of mollusks, they should be treated with care, as they are extremely important for the proper functioning of many natural biotopes.

In general, mollusks are often distinguished by their most important practical significance for humans. For example, the pearl mussel is bred en masse in many coastal countries, since this species is a supplier of natural pearls. Some shellfish are of great value for medicine, the chemical and processing industries.

Want to know interesting facts about shellfish? In the Ancient period and the Middle Ages, inconspicuous cephalopods were sometimes the basis of the well-being of entire states, since the most valuable purple was obtained from them, which was used to dye the royal robes and robes of the nobility!

Shellfish type

In total, it has more than 130,000 species (yes, the variety of mollusks is incredible). Mollusks are second only to arthropods in total numbers and are the second most common living organisms on the planet. Most of them live in water, and only a relatively small number of species choose land as their place of residence.

general characteristics

Almost all animals that are part of this type are distinguished by several specific features. Here is the currently accepted general characteristic of mollusks:

• Firstly, three layers. Their organ system is formed from ectoderm, endoderm and mesoderm.
• The symmetry is of a bilateral type, caused by a significant displacement of most of their organs.
• The body is unsegmented, in most cases protected by a relatively strong calcareous shell.
• There is a fold of skin (mantle) that envelops their entire body.
• A well-defined muscular outgrowth (leg) is used for movement.
• The coelomic cavity is very poorly defined.
• There are almost all the same organ systems (in a simplified version, of course) as in higher animals.

Thus, the general characteristics of mollusks indicate that we have before us quite developed, but still primitive animals. It is not surprising that many scientists consider mollusks to be the main ancestors of a large number of living organisms on our planet. For clarity, we present a table that describes in more detail the characteristics of the two most common classes.

Detailed characteristics

The body is still symmetrical, although this is not observed in bivalves. The division of the body into segments was preserved only in very primitive species. The secondary body cavity is represented by a bursa surrounding the heart muscle and genitals. The entire space between the organs is completely filled with parenchyma.

The majority of the body can be divided into the following sections:

• Head.
• Torso.
• A muscular leg through which movement is carried out.

In all bivalve species the head is completely reduced. The leg refers to a massive muscular process that develops from the base of the abdominal wall. At the very base of the body, the skin forms a large fold, the mantle. Between it and the body there is a fairly large cavity in which the following organs are located: gills, as well as the conclusions of the reproductive and excretory systems. It is the mantle that secretes those substances that, when reacting with water, form a durable shell.

The shell can be either completely solid or consist of two valves or several plates. The composition of this shell includes a lot of carbon dioxide (of course, in a bound state - CaCO 3), as well as conchiolin, a special organic substance that is synthesized by the body of the mollusk. However, in many species of mollusks the shell is completely or partially reduced. The slugs have only a microscopic-sized plate left from it.

Characteristics of the digestive system

Gastropods

There is a mouth at the front end of the head. The main organ in it is a powerful muscular tongue, which is covered with a particularly strong chitinous grater (radula). With its help, snails scrape off algae or other organic matter from all accessible surfaces. In predatory species (we'll talk about them below), the tongue has degenerated into a flexible and hard proboscis, which is intended for opening the shells of other mollusks.

In Cones (they will also be discussed separately), individual segments of the radula protrude beyond the oral cavity and form a kind of harpoon. With their help, these representatives of mollusks literally throw their poison at the victim. In some predatory gastropods, the tongue has turned into a special “drill”, with which they literally drill holes in the shell of their prey for injecting poison.

Bivalve

In their case, everything is much simpler. They simply lie motionless on the bottom (or hang, tightly attached to the substrate), filtering hundreds of liters of water with organic matter dissolved in it through their body. The filtered particles go directly into the large stomach.

Respiratory system

Most species breathe through gills. There are “front” and “rear” views. In the former, the gills are located in the front of the body and their apex is directed forward. Accordingly, in the second case the top looks back. Some have lost their gills in the truest sense of the word. These large mollusks breathe directly through their skin.

To do this, they developed a special skin organ of an adaptive type. In land species and secondary aquatic mollusks (their ancestors returned to the water again), part of the mantle is wrapped, forming a kind of lung, the walls of which are densely penetrated with blood vessels. To breathe, such snails rise to the surface of the water and collect air using a special spiracle. The heart, located not far from the simplest “structure,” consists of one atrium and a ventricle.

The main classes included in the type

How is the type of mollusk divided? The classes of mollusks (there are eight in total) are “crowned” by the three most numerous:

• Gastropods (Gastropoda). This includes thousands of species of snails of all sizes, the main distinguishing feature of which is their low speed of movement and well-developed muscular legs.
• Bivalves (Bivalvia). Sink with two doors. As a rule, all species included in the class are sedentary and sedentary. They can move both with the help of a muscular leg and by means of jet propulsion, throwing out water under pressure.
• Cephalopods (Cephalopoda). Motile mollusks, shells are either completely absent, or they are in their infancy.

What else is included in the phylum molluscs? The classes of mollusks are quite diverse: in addition to all of the above, there are also Spade-footed, Armored and Pit-tailed, Grooved-bellied and Monoplacophora. All of them are living and well.

What fossils does this type of mollusk contain? Classes of mollusks that are already extinct:

• Rostroconchia.
• Tentaculitis.

By the way, the same Monoplacophorans were considered completely extinct until 1952, but at that time the ship “Galatea” with a research expedition on board caught several new organisms that were classified as a new species Neopilina galatheae. As you can see, the name of this species of mollusks was given by the name of the research vessel that discovered them. However, this is not uncommon in scientific practice: species are much more often designated in honor of the researcher who discovered them.

So it is possible that all subsequent years and new research missions will be able to enrich the type of mollusks: classes of mollusks that are now considered extinct may well survive somewhere in the bottomless depths of the world's oceans.

No matter how strange it may sound, one of the most dangerous and incredible predators on our planet is... seemingly harmless gastropods. For example, cone snails (lat. Conidae), the poison of which is so unusual that modern pharmacists use it in the manufacture of certain types of rare medicines. By the way, the name of mollusks of this family is completely justified. Their shape is indeed most similar to a truncated cone.

They can be persistent hunters, being extremely ruthless in dealing with floodplain prey. Of course, the role of the latter is often played by colonial, sedentary species of animals, since it is simply impossible for snails to keep up with other snails. The prey itself can be tens of times larger than the hunter. Want to know more interesting facts about shellfish? Yes please!

About snail hunting methods

Most often, the insidious mollusk uses its most powerful organ, a strong muscular leg. It can attach to prey with the equivalent of 20kg of force! This is quite enough for a predatory snail. For example, a “caught” oyster opens in less than an hour with only ten kilograms of force! In a word, the life of mollusks is much more dangerous than is commonly thought...

Other species of gastropods prefer not to press anything at all, carefully drilling into the shell of their prey using a special proboscis. But this process cannot be called simple and fast, even if one wants to. So, with a shell thickness of only 0.1 mm, drilling can take up to 13 hours! Yes, this method of “hunting” is only suitable for snails...

Dissolution!

To dissolve someone else's shell and its owner, the mollusk uses sulfuric acid (you already know what the salivary gland is in mollusks). This makes destruction much easier and faster. After the hole is made, the predator begins to slowly eat its prey from the “package”, using its proboscis for this. To some extent, this organ can safely be considered an analogue of our hand, since it is directly involved in capturing and holding prey. In addition, this manipulator can often extend so that it exceeds the length of the hunter’s body.

This is how snails can get their prey even from deep crevices and large shells. We remind you once again that it is from the proboscis that a strong poison is injected into the victim’s body, the basis of which is chemically pure sulfuric acid (released from the “harmless” salivary glands). In a word, from now on you know exactly what the salivary gland is in mollusks and why they need it.

With about 130,000 species, molluscs are second only to arthropods in number of species and represent the second largest phylum of the animal kingdom. Mollusks are predominantly aquatic inhabitants; only a small number of species live on land.

Mollusks have a variety of practical meanings. Among them there are useful ones, such as pearl mussel and mother-of-pearl, which are mined to obtain natural pearls and mother-of-pearl. Oysters and some other species are harvested and even farmed for food. Some species are pests of agricultural crops. From a medical point of view, mollusks are of interest as intermediate hosts of helminths.

General characteristics of the type

Animals belonging to the mollusc type are characterized by:

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

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

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

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

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

The nervous system consists of several pairs of nerve ganglia, interconnected by longitudinal trunks.

The phylum of mollusks includes 7 classes. The most important of them:

• gastropods (Gastropoda) - slow crawling snails
• bivalves (Bivalvia) - relatively sedentary mollusks
• cephalopods (Cephalopoda) - mobile mollusks

Class Gastropoda

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

1. Placospiral shell - a strongly twisted shell, the whorls of which are located in the same plane
2. Turbo-spiral shell – the revolutions of the shell lie in different planes
3. Right-handed shell - the spiral of the shell twists clockwise
4. Left-handed shell - the spiral twists counterclockwise
5. Cryptospiral (involute) shell - the last whorl of the shell is very wide and completely covers all previous ones
6. Open spiral (evolute) shell – all whorls of the shell are visible

Sometimes the shell is equipped with a lid located on the dorsal side at the back of the leg (for example, in lawns). When you pull your leg into the sink, the lid tightly covers the mouth.

In some species that have switched to a swimming lifestyle (for example, pteropods and keelenopods), there is no shell. Shell reduction is also characteristic of some land gastropods living in the soil and forest litter (for example, slugs).

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

Digestive system. At the front end of the head is the mouth. It has a powerful tongue covered with a hard chitinous grater, or radula. With its help, mollusks scrape algae from the ground or aquatic plants. In predatory species, a long proboscis develops in the front part of the body, capable of turning out through an opening on the lower surface of the head. In some gastropods (e.g., cones), individual teeth of the radula can protrude from the mouth opening and are shaped like stylets or hollow harpoons. With their help, the mollusk injects poison into the victim’s body. Some predatory gastropod species feed on bivalves. They drill into their shells, secreting saliva containing sulfuric acid.

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

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

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

The respiratory organs of mollusks are represented by gills located in the front part of the body and directed with their apex forward (prosobranchial mollusks) or located in the right rear part of the body and directed with their apex backward (opisthobranchial). In some gastropods (for example, nudibranchs), true gills were reduced. They develop so-called respiratory organs. cutaneous adaptive gills. In addition, in terrestrial and secondary aquatic gastropods, part of the mantle forms a kind of lung, numerous blood vessels develop in its walls, and gas exchange occurs here. The pond snail, for example, breathes atmospheric oxygen, so it often rises to the surface of the water and opens a round breathing hole on the right at the base of the shell. Next to the lung is the heart, which consists of an atrium and a ventricle. The circulatory system is not closed, the blood is colorless. The excretory organs are represented by one kidney.

Among gastropods there are both dioecious species and hermaphrodites, the gonads of which produce both spermine and eggs. Fertilization is always cross-fertilization, development, as a rule, with metamorphosis. Direct development is observed in all land, freshwater and some marine gastropods. Eggs are laid in long mucous threads attached to movable objects.

Belongs to the class of gastropods

• The common pond snail is often found on aquatic plants in ponds, lakes and rivers. Its shell is solid, 4-7 cm long, spirally twisted, with 4-5 curls, a sharp apex and a large opening - the mouth. A leg and head can protrude through the mouth.

  Gastropods also include intermediate hosts of trematodes.

• The intermediate host of the cat fluke, Bithynia leachi, is widespread in freshwater bodies of our country. It lives in the coastal zone of rivers, lakes and ponds overgrown with vegetation. The shell is dark brown and has 5 convex whorls. Shell height 6-12 mm.
• The intermediate host of the liver fluke, the small pond snail (Limnea truncatula), is widespread in Russia. The shell is small, no more than 10 mm in height, forms 6-7 whorls. It lives in ponds, swamps, ditches and puddles, where it is often found in huge quantities. In some areas there are more than 1 million pond snails per hectare of swamps. When swamps dry out, pond snails burrow into the ground, surviving the dry time in the ground.
• The intermediate hosts of the lancet fluke are the terrestrial mollusks Helicella and Zebrina. Distributed in Ukraine, Moldova, Crimea and the Caucasus. Adapted to life in arid conditions; live in the open steppe on the stems of herbaceous plants. During hot weather, helicella often accumulate on plants in clusters, thus saving themselves from drying out. Helicella has a low-conical shell with 4-6 whorls; the shell is light, with dark spiral stripes and a wide rounded mouth. Zebrina has a highly conical shell with 8-11 whorls; the shell is light, with brown stripes running from the top to the base; the mouth is irregularly oval.

Class bivalve (Bivalvia)

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

Perlovitsa, or toothless lives on the silted and sandy bottoms of rivers, lakes and ponds. This inactive animal feeds passively. The toothless food consists of detritus particles suspended in water (the smallest remains of plants and animals), bacteria, unicellular algae, flagellates, and ciliates. The mollusk filters them from the water passing through the mantle cavity.

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

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

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

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

The mouth is located at the front end of the body above the base of the leg. On the sides of the mouth there are two pairs of triangular oral lobes. The cilia covering them move food particles toward the mouth. Due to the reduction of the head in pearl barley and other bivalves, the pharynx and associated organs (salivary glands, jaws, etc.) are reduced.

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

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

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

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

Class Cephalopoda

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

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

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

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

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

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

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

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

The intestine is branched, with digestive glands. In many species, just before the anus, the duct of the ink gland opens into the lumen of the hindgut. It secretes a dark secretion (ink) that can cloud a large amount of water. The ink serves as a smoke screen, disorienting the enemy, and sometimes paralyzing his sense of smell. Cephalopods use it to escape from predators.

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

The nervous system has the highest organization with developed structures of touch, smell, vision and hearing. The ganglia of the nervous system form a common nervous mass - a multifunctional brain, which is located in a protective cartilaginous capsule. Two large nerves arise from the posterior part of the brain. Cephalopods have complex behavior, have good memory and exhibit the ability to learn. Because of the perfection of their brains, cephalopods are called “primates of the sea.”

The unique skin photoreceptors of cephalopods respond to the slightest changes in light. Some cephalopods are capable of glowing due to the bioluminescence of photophores.

All cephalopods are dioecious animals; Some of them have well-defined sexual dimorphism. Males, as a rule, are smaller than females, armed with one or two modified arms - hectocotyls, with the help of which "packets" with seminal fluid - spermatophores - are transferred during the copulation period. Fertilization is external-internal and occurs not in the female’s reproductive tract, but in her mantle cavity. It involves the capture of sperm by the gelatinous membrane of the eggs. After fertilization, females attach clusters of eggs to bottom objects. Some species take care of their offspring and protect developing eggs. The female protecting the offspring can starve for more than 2 months. In octopuses, cuttlefish and nautiluses, a minicopy of the parents hatches from each egg, only in squids development occurs with metamorphosis. The young grow quickly and often reach sexual maturity by one year.

The meaning of shellfish

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

River zebra mussels are found in huge quantities in the reservoirs of the Volga, Dnieper, Don, in lakes, estuaries of the Black Sea, and desalinated areas of the Azov, Caspian and Aral seas. It grows over stones, piles and various hydraulic structures: watercourses, technical and drinking water supply pipes, protective gratings, etc., and its quantity can reach 10 thousand copies per 1 m2 and cover the substrate in several layers. This makes it difficult for water to pass through, so constant cleaning of zebra mussel fouling is necessary; use mechanical, chemical, electrical and biological control methods. Some bivalves bore passages in the bottoms of ships and wooden parts of port facilities (shipworm).

Pearl barley and some other bivalves play an important role in marine and freshwater biocenoses as natural water purifiers - biofilters. One large pearl barley can filter 20-40 liters of water per day; mussels inhabiting 1 m2 of the seabed can filter about 280 m3 of water per day. In this case, mollusks extract organic and inorganic substances from contaminated water, some of which are used for their own nutrition, and some are concentrated in the form of lumps that are used to feed microorganisms.

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

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

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