Type Mollusks: general characteristics, internal and external structure, meaning in nature. Classes of mollusks and their characteristic features What mollusks live in our area

Shellfish - bilaterally symmetrical soft-bodied animals (in gastropods the body is asymmetrical), having a shell, mantle, mantle cavity, open circulatory system).

The secondary body cavity (coelom) is well defined only in the embryonic state, and in adult animals it remains in the form of the pericardial sac and the gonad cavity. The spaces between organs are filled with connective tissue. Such a body cavity is called mixed or mixocoel.

The phylum Mollusks combines the following classes: Gastropods, Bivalves, Cephalopods.

External structure

Mollusc body unsegmented and consists of head (Bivalves do not have it), torso and legs.

Almost all mollusks have a head, except bivalves. It contains a mouth opening, tentacles and eyes.

Leg- a muscular unpaired outgrowth of the body that serves for crawling.

The body of Mollusks is covered with a fold of skin - the mantle (the substance from which the shell is built is secreted by the cells of the mantle). The space between the walls of the body and the mantle is called mantle cavity. It contains the respiratory organs. The anal, genital and excretory openings open into the mantle cavity.

Mollusk mantle- This is a fold of skin between the body and the shell.

Mantle cavity- this is the space between the walls of the body and the mantle.

The organs of mollusks are united into systems: digestive,respiratory, circulatory, nervous, excretory, reproductive.

Digestive system

The digestive system depends on the type of nutrition of mollusks.

The oral cavity passes into the pharynx, and then into the esophagus, which leads to the stomach and intestines. Channels flow into it digestive gland anal hole.

Respiration in mollusks living in water is carried out by gills, and in terrestrial mollusks it is carried out using the lungs. Some aquatic mollusks (for example, pond snails) also breathe with their lungs, periodically rising to the surface of the water to inhale atmospheric air.

Circulatory system

The circulatory system includes the heart (an organ that ensures the movement of blood through the vessels and cavities of the body) and blood vessels. The heart usually consists of three chambers: one ventricle and two atria (in Gastropods there are two chambers - an atrium and a ventricle).

Mollusks have an open circulatory system (with the exception of cephalopods). This means that blood flows not only through blood vessels, but also through special cavities between organs, and then the blood is collected again in the vessels and enters the gills or lungs to be enriched with oxygen.

Nervous system and sensory organs

The nervous system varies in degree of complexity and is most developed in Cephalopods.

It consists of several pairs of well-developed nerve ganglia located in different parts bodies, and the nerves coming from them. Such nervous system called scattered-knot type.

Excretory system

The excretory organs of mollusks are one or two kidneys, the excretory openings of which open into the mantle cavity.

Reproduction

Shellfish breed only sexually. Most of them are dioecious, but hermaphrodites are also found. Mollusks reproduce by laying fertilized eggs. Fertilization in mollusks can be external (for example, in the oyster and toothless snail) and internal (in the grape snail).
From a fertilized egg, either a larva leading a planktonic lifestyle (sailfish) or a fully formed small mollusk develops.

Origin

Apparently, Mollusks descended from common ancestors with annelids, which had a poorly developed secondary body cavity, had ciliated integuments, and did not yet have the body divided into segments.

In the embryonic (embryo) development of Mollusks, one can observe many similarities with the development of Polychaete Annelids. This indicates ancient historical (evolutionary) connections between them.

Typical larva sea ​​mollusks(sailfish) is very similar to the larva of annelids, bearing large lobes lined with cilia.

The larva leads a planktonic lifestyle, then settles to the bottom and takes on the appearance of a typical gastropod.

Class Gastropods- the most diverse and widespread group of mollusks.

There are about 90 thousand gastropods. modern species, living in the seas (rapana, cones, murexes), fresh water bodies (ponds, coils, meadows), as well as on land (slugs, grape snails).

External structure

Most gastropods have a spirally twisted shell. In some, the shell is underdeveloped or completely absent (for example, in naked slugs).

The body consists of three sections: heads, torso and legs.

On the head are one or two pairs of long soft tentacles and a pair of eyes.

In the body - internal organs.

The leg of gastropods is adapted for crawling and is a muscular outgrowth of the abdominal part of the body (hence the name of the class).

Common pondweed- lives in fresh water bodies and shallow rivers throughout Russia. It feeds on plant foods, scraping the soft tissues of plants with a grater.

Digestive system

IN oral cavity gastropods have a muscular tongue with chitinous teeth that form a “grater” (or radula). In herbivorous mollusks, the grater (radula) is used for scraping plant food, in predators - helps to retain prey.

The salivary glands usually open into the oral cavity.

The oral cavity passes into the pharynx, and then into the esophagus, which leads to the stomach and intestines. Channels flow into it digestive gland. Undigested food remains are thrown out through anal hole.

Nervous system

Nervous system ( shown in yellow in the figure) consists of several pairs of well-developed nerve ganglia located in different parts of the body, and the nerves coming from them.

Gastropods have developed sensory organs, they are located mainly on the head: eyes, tentacles - organs of touch, organs of balance. Gastropods have well-developed olfactory organs - they can recognize odors.

Circulatory system

Gastropods have an open circulatory system consisting of a heart and blood vessels. The heart consists of two chambers: the ventricle and the atrium.

Respiration in mollusks living in water is carried out by gills, and in terrestrial mollusks it is carried out using the lungs.

In the mantle cavity, most aquatic gastropods have one or, less commonly, two gills.

In pond snails, coil snails, and grape snails, the mantle cavity acts as a lung. Oxygen from atmospheric air filling the “lung”, penetrates through the wall of the mantle into the blood vessels branched in it, and carbon dioxide from the blood vessels enters the cavity of the “lung” and comes out.

Excretory system

The excretory organs of mollusks are one or two kidneys.

Metabolic products that are unnecessary for the body come from the blood to the kidney, the duct from which opens into the mantle cavity.

The release of carbon dioxide from the blood and the enrichment of oxygen occurs in the respiratory organs (gills or lungs).

Reproduction

Shellfish breed only sexually.

Ponds, coils, slugs are hermaphrodites.

They usually lay fertilized eggs on plant leaves and various water objects or between lumps of soil. Small snails emerge from the eggs.

Many marine gastropods are dioecious animals; they develop from larval stage- sailboat.

Meaning

Many shellfish serve as food for fish and birds. Terrestrial gastropods are eaten by amphibians, moles, and hedgehogs. Some species of gastropods are also eaten by humans.

Among the gastropods there are pests of gardens and vegetable gardens - slugs, grape snails, etc.

Bivalves Exclusively aquatic animals, they lead a mostly sedentary lifestyle. Most of them live in the seas (mussels, oysters, scallops), and only a small part lives in fresh water bodies (toothless, pearl barley, dreisena).

Characteristic feature of Bivalves - lack of head.

The shell of bivalve mollusks consists of two valves (hence the name of the class).

Representative - common toothless. Her body consists of a torso and legs covered with a mantle. It hangs from the sides in the form of two folds. The cavity between the folds and the body contains the leg and gill plates. The toothless fish, like all bivalves, has no head.

At the posterior end of the body, both folds of the mantle are pressed against each other, forming two siphons: lower (input) and upper (outlet). Through the lower siphon, water enters the mantle cavity and washes the gills, which ensures respiration.

Digestive system

Bivalve mollusks are characterized by a filtration feeding method. They have an inlet siphon through which water with food particles suspended in it (protozoa, unicellular algae, the remains of dead plants) enters the mantle cavity, where this suspension is filtered. Filtered food particles are directed to the mouth opening and pharynx; then goes to esophagus, stomach, intestines and through anal hole enters the outlet siphon.
The toothless has a well developed digestive gland, the ducts of which flow into the stomach.

Bivalves breathe using gills.

Circulatory system

The circulatory system is not closed. It includes the heart and blood vessels.

Reproduction

Toothless is a dioecious animal. Fertilization occurs in the mantle cavity females, where sperm enter through the lower siphon along with water. Larvae develop from fertilized eggs in the gills of the mollusk.

Meaning

Bivalves are water filters, food for animals, used for human food (oysters, scallops, mussels), and producers of mother-of-pearl and natural pearls.

The shell of bivalve mollusks consists of three layers:

• thin outer - horny (organic);
• the thickest medium - porcelain-like (limestone);
• internal - mother-of-pearl.

The best varieties of mother-of-pearl highlight the thick-walled shells of the sea pearl oyster, which lives in warm seas. When certain areas of the mantle are irritated by grains of sand or other objects, pearls form on the surface of the nacreous layer.

Shells and pearls are used to make jewelry, buttons and other products.

Some mollusks, such as the shipworm, so named for its body shape, harm wooden structures in water.

Cephalopods- a small group of highly organized animals, distinguished by the most perfect structure and complex behavior among other mollusks.

Their name - “Cephalopods” - is explained by the fact that the leg of these mollusks has turned into tentacles (usually 8-10 of them), located on the head around the mouth opening.

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. 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 types of slugs, as well as the vine snail, cause great harm agriculture Worldwide. At the same time, it is the mollusks that play vital role in global water purification, since organic matter filtered from it is used to feed them. In many countries, large ones are bred for sea ​​farms because they are valuable food product, which contains a lot of protein. These representatives and oysters) are even used in dietary nutrition.

IN former USSR 19 representatives of this species were considered rare and endangered ancient type. 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 terms of 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 large quantity 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 rather large cavity in which the following organs are located: gills, as well as the genital 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. U 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, mainly hallmark which is low speed 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). Mobile mollusks have shells either completely absent or 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.

Collier's Encyclopedia. - Open Society. 2000 .

See what "MOLLUSCS" is in other dictionaries:

MOLLUSCS, representatives of more than 80,000 species of INVERTEBRATE animals of the phylum Mollusca. These include the well-known snails, bivalves and squids, as well as many less known species. Originally inhabitants of the sea, mollusks are now... ... Scientific and technical encyclopedic Dictionary

Soft-bodied (Mollusca), a type of invertebrate animal. They presumably arose in the Precambrian; From the Lower Cambrian several are already known. classes M. They probably originated from small-segmented worm-like ancestors (annelids) or directly from flat... ... Biological encyclopedic dictionary

SHELLFISH- MOLLUSCS, or soft-bodied ones (Mollusca), are a well-closed type of invertebrate animals. The body is soft, undifferentiated, and typically bears a shell. The skin forms a fold of mantle that covers the body or grows together at the edges with its surface.… … Big medical encyclopedia

- (new Latin mollusca, from Latin mollis soft). Soft-bodied animals, slugs. Dictionary foreign words, included in the Russian language. Chudinov A.N., 1910. MOLLUSCS novolatinsk. mollusca, from dates. mollis, soft. Soft-bodied animals. Explanation... ... Dictionary of foreign words of the Russian language

- (from Latin molluscus soft) (soft-bodied) type of invertebrate animals. The body of most mollusks is covered with a shell. On the ventral side there is a muscular outgrowth of the leg (organ of movement). 2 subtypes: bokonervoe and testate; St. 130 thousand species. They live in... ... Big Encyclopedic Dictionary

Modern encyclopedia

Shellfish- MOLLUSCS, a type of invertebrate animal. The majority of the body is covered with a shell. The head has a mouth, tentacles and often eyes. The muscular outgrowth (leg) on ​​the ventral side is used for crawling or swimming. About 130 thousand species, in the seas (most), ... ... Illustrated Encyclopedic Dictionary

- (Mollusca) a type of animal with a solid, non-segmented body. Most representatives have a calcareous shell, whole or consisting of two, less often several separate parts. The organ of movement is the muscular unpaired... ... Geological encyclopedia

shellfish- The body of most animals is covered with a shell. ▼ side-nervous. armored: chiton tonicella. solenogastra: echinomenia. caudofoveates. shelled monoplacophora: neopilina. Gastropods, snails, gastropods: prosobranchs: cowries. Littorina. abalone. trumpeters... Ideographic Dictionary of the Russian Language

shellfish- A type of soft-bodied, non-segmented invertebrate animals that usually secrete a substance to build a calcareous shell: snails, limpets, bivalves, chitons, squids. … … Technical Translator's Guide

- (Mollusca) (from Latin molluscus soft), soft-bodied, a type of invertebrate animal. 7 classes: Gastropods, Monoplacophorans, Armored molluscs, Gross-bellied molluscs, Bivalve molluscs, Spadefoot molluscs and ... Great Soviet Encyclopedia

SHELLFISH

SHELLFISH, representatives of more than 80,000 species of INVERTEBRATE animals of the phylum Mollusca. These include the well-known snails, bivalves and squids, as well as many lesser-known species. Originally sea dwellers, mollusks are now found in the oceans, fresh water and on land. The classes of mollusks include: primitive Gastropods, monovalves (slugs and snails), Bivalve mollusks, spadefoot mollusks and cephalopods (squids, etc.). The body of a mollusk consists of three parts: head, leg and torso. There is also a fold of skin attached to the body called mantle, producing a calcareous shell (shell), characteristic of most mollusks. The head is well developed only in snails and cephalopods, which have eyes, tentacles and a well-formed mouth. The torso contains the internal organs of circulation (blood vessels and heart), respiration (gills), excretion (kidney) and reproduction (gonads). Molluscs are usually dioecious, but there are many species that are hermaphrodites. Cephalopods, bivalves and gastropods are important fossils - evidence of the geological past. see also HERMAPHRODITES.

Shellfish. Remarkable experts in exploring new habitats, snails previously lived in the sea, but gradually some 22,000 species adapted to life on land, losing gills and developing air-breathing lungs. Most species land snails, such as the snail Helix pomatia pictured here, are ground-dwelling, dull in color, and there are a few tree species, which tend to be brightly colored. Other species have returned to life in the water and must periodically surface to breathe.

Scientific and technical encyclopedic dictionary.

See what "MOLLUSCS" is in other dictionaries:

Soft-bodied (Mollusca), a type of invertebrate animal. They presumably arose in the Precambrian; From the Lower Cambrian several are already known. classes M. They probably originated from small-segmented worm-like ancestors (annelids) or directly from flat... ... Biological encyclopedic dictionary

SHELLFISH- MOLLUSCS, or soft-bodied ones (Mollusca), are a well-closed type of invertebrate animals. The body is soft, undifferentiated, and typically bears a shell. The skin forms a fold of mantle that covers the body or grows together at the edges with its surface.… … Great Medical Encyclopedia

- (new Latin mollusca, from Latin mollis soft). Soft-bodied animals, slugs. Dictionary of foreign words included in the Russian language. Chudinov A.N., 1910. MOLLUSCS novolatinsk. mollusca, from dates. mollis, soft. Soft-bodied animals. Explanation... ... Dictionary of foreign words of the Russian language

- (from Latin molluscus soft) (soft-bodied) type of invertebrate animals. The body of most mollusks is covered with a shell. On the ventral side there is a muscular outgrowth of the leg (organ of movement). 2 subtypes: bokonervoe and testate; St. 130 thousand species. They live in... ... Big Encyclopedic Dictionary

Modern encyclopedia

Shellfish- MOLLUSCS, a type of invertebrate animal. The majority of the body is covered with a shell. The head has a mouth, tentacles and often eyes. The muscular outgrowth (leg) on ​​the ventral side is used for crawling or swimming. About 130 thousand species, in the seas (most), ... ... Illustrated Encyclopedic Dictionary

- (Mollusca) a type of animal with a solid, non-segmented body. Most representatives have a calcareous shell, whole or consisting of two, less often several separate parts. The organ of movement is the muscular unpaired... ... Geological encyclopedia

shellfish- The body of most animals is covered with a shell. ▼ side-nervous. armored: chiton tonicella. solenogastra: echinomenia. caudofoveates. shelled monoplacophora: neopilina. Gastropods, snails, gastropods: prosobranchs: cowries. Littorina. abalone. trumpeters... Ideographic Dictionary of the Russian Language

shellfish- A type of soft-bodied, non-segmented invertebrate animals that usually secrete a substance to build a calcareous shell: snails, limpets, bivalves, chitons, squids. … … Technical Translator's Guide

- (Mollusca) (from Latin molluscus soft), soft-bodied, a type of invertebrate animal. 7 classes: Gastropods, Monoplacophorans, Armored molluscs, Gross-bellied molluscs, Bivalve molluscs, Spadefoot molluscs and ... Great Soviet Encyclopedia

Books

• J.-L. Cuvier. Animal Kingdom. Molluscs, R. Aldonina. This publication introduces the reader to the section “Mollfishes” from the four-volume work of the French naturalist and naturalist Georges-Leopold Cuvier “The Animal Kingdom, Distributed According to...

Which consists of squid, octopus, cuttlefish, nudibranchs, snails, slugs, limpets, mussels, oysters, scallops, as well as many other lesser known animal species. According to scientists, there are more than 100,000 people living on Earth today. known to science species of mollusks. This makes them second in species diversity after .

Mollusks have a soft body that consists of three main parts: the legs, the visceral mass and the mantle with the organ system. Many species also have a protective shell consisting of chitin, proteins and calcium carbonate. Mollusks are so diverse in shape that it is impossible to use representatives of one species for generalization. anatomical features groups. Instead, science books often describe a hypothetical mollusk that has the characteristics of many species.

This hypothetical mollusk has a mantle, shell, leg, and visceral mass. The tunica is a layer of tissue enclosing the visceral mass. Many mollusks have glands that secrete a hard shell.

A leg is a muscular structure located at the bottom of an animal's body. The mollusk secretes mucus from the lower part of the leg to lubricate the underlying surface. Mucus facilitates movement, which is achieved by repeated contraction and stretching of the mollusk leg muscle.

The visceral mass is located above and below the mantle and includes the digestive system, heart and other internal organs. The circulatory system is open. Most species of molluscs use a single pair of gills for breathing, although some species have vestigial lungs, such as land slugs and snails.

Mollusks, unlike vertebrates, transport oxygen throughout the body using other molecules. They use hemocyanin (copper-based respiratory pigment), and vertebrates use hemoglobin (iron-based). Hemocyanin is less efficient at transporting oxygen than hemoglobin. For this reason, clams are more likely to move in quick bursts, but are not able to maintain movement for a long period of time, as they do.

Most marine molluscs begin life as larvae, which later develop into adults. Freshwater and land snails are formed in eggs and hatch miniature but fully formed as adults. Although shellfish are most common in marine environment habitats, they are also found in freshwater and terrestrial environments.

Molluscs are believed to have evolved from segmented, worm-like animals similar to modern ones flatworms. Their closest living relatives are annelids and flatworms.

Classification

Mollusks inhabiting the planet today are divided into the following classes:

• Pit-tailed (Caudofoveata);
• Sulcate-bellied (Solenogastres);
• Armored (Polyplacophora);
• Monoplacophora (Monoplacophora);
• Bivalve (Bivalvia);
• Spadefoot (Scaphopoda);
• Gastropods (Gastropoda);
• Cephalopods (Cephalopoda).