shellfish- Bilaterally symmetrical or secondarily asymmetrical three-layered animals. They live in marine and fresh water bodies, on land.

In the body of most species of mollusks, three sections can be distinguished: the head, trunk and leg. On the head are the mouth opening, the sense organs. The strongly thickened ventral side forms various types of legs. The leg, as an organ of movement, can have a different shape: in floating forms it turns into wide lobes or tentacles, in crawling forms it turns into a flat sole.

The torso is surrounded by a skin fold - 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 and chemical sense organs (osphradia). All of the above is called the mantle complex of organs.

Musculature in mollusks is well developed and consists of muscle bundles. They are especially strongly developed in the leg of the animal.

The whole is reduced to the pericardial sac and the cavity in which the gonads are located. The space between 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 - endodermal. In the pharynx of many species there is a specific organ for grinding food - a radula, or grater. The ducts of the salivary glands open into the pharynx, and the ducts of the liver open into the midgut.

Respiratory organs are represented by gills or lungs. Lungs are present not only in terrestrial species, but also in forms that have secondarily switched to an aquatic lifestyle. Gills and lungs are modified parts of the mantle. In water-dwelling species, gas exchange can also occur through the skin.

The circulatory system is open: blood flows not only through the blood vessels, but also through the gaps located in the space between the organs. Mollusks have a heart made up of two or more chambers. The heart is located in the pericardial sac (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 in most molluscs is represented by several pairs of nerve nodes, which are located in different parts of the body. The nervous system of this type is called scattered-nodular. In addition to reflex activity, the nervous system performs the functions of regulating growth and reproduction by secreting various neurohormones. Mollusks have organs of chemical sense (osphradia), balance, numerous tactile receptors are scattered in the skin. Many species have eyes.

The predominant number of species of mollusks are dioecious animals, but there are also bisexual species. The development of all terrestrial species, most freshwater and some marine life is direct. If development proceeds with metamorphosis, then either a larva of the trochophore type or a larva - a veliger (sailboat) comes out of the egg.

Type Mollusks are divided into classes: gastropods (Gastropoda), bivalves (Bivalvia), cephalopods (Cephalopoda), etc.

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

Description of classes, subclasses and units of the Mollusk type:

• Class Gastropoda (Gastropoda)

• Class Cephalopoda (Cephalopoda)

  • Subclass Coleoidea (Coleoidae)

shellfish (lat. Mollusca) soft-bodied belong to the type of primary coelomic organisms with spiral fragmentation. To date, there is no exact data on the total number of these animals, approximate data range from 100 to 200 thousand. This type of animal is divided into 9 (10) classes, including two extinct classes. Mollusks include a wide variety of species of slugs, pond snails, toothless, squid, oysters and other animals. Let's look at the different classes of molluscs in more detail.

Classes of mollusks and their features

All representatives of this class have a soft, non-segmented body, a shell or its remains and a special fold of the skin - the mantle, which forms the mantle cavity.

Their mantle secretes substances from which the shell is formed (horny substances, lime and mother-of-pearl). Some mollusks have a head, a muscular leg, and a body. Many of them have small eyes.

Octopus (lat. Octopus vulgaris) refers to cephalopods

Mollusks differ not only in size, but also in their anatomical structure and behavior. For example, about 80% of the species of these animals belong to the class of gastropods, about 19% are bivalves, and only about 1% belong to the rest of the class of mollusks.

Scallop (lat. Pectinidae) belong to the family of marine bivalves

Mollusk classes: gastropods

Gastropods (snails) are the largest class in the mollusk family (about 90 thousand species). This group includes grape snails, slugs, coils, pond snails. Coils and pond snails live in small fresh water bodies, and slugs live in wet places on land (usually in vegetable gardens and fields), grape snails only in vineyards.

Almost all snails feed on plants, but sometimes they eat small insects. Among them there are predators, for example rapana (they live in the seas - they eat mussels and oysters).

Marine gastropods (lat. Gastropoda)

The structure of gastropods

Gastropods of the class of mollusks have a single shell, which looks like a small curl. And in some of the mollusks (for example, slugs), the shell is reduced or completely hidden under the skin itself. Like all representatives of this species, they have a leg, torso and head. They have a mouth, eyes, and two or one pair of tentacles on their heads. The muscular leg of molluscs occupies almost the entire abdominal part of the body.

In gastropods, the mantle looks like a pocket that forms a "lung" with breathing holes. Oxygen from the atmospheric air fills the “lung” and penetrates through the mantle wall directly into the blood vessels branched in it, and carbon dioxide from the blood vessel comes out.

All gastropods scrape food with the help of a so-called grater - a tongue that is covered with numerous teeth (horny). They have salivary glands - from the duct flow directly into the foreguts, there is a digestive gland that combines the functions of the liver and pancreas.

Mollusk classes: cephalopods

In addition, the order of cephalopods includes cuttlefish, squid, octopus (about 675 modern species). These mollusks inhabit mainly warm salty seas and feed on fish, crabs and other animals. Cuttlefish and squid actively pursue their own prey, while octopuses lie in wait for it.

Nautilus (lat. Nautilus pompilius) - a marine cephalopod that appeared 500 million years ago, is considered the only modern cephalopod that has an external chamber shell

The structure of cephalopods

In addition, they can quickly change the color of their body, which in cephalopods consists of a head and a body. Most animals have a crown around the mouth, consisting of 8 arms (for cuttlefish and squid) of a pair of tentacles with large suckers. The tentacles and arms were formed from the particles of the legs. But the second part of the legs forms a funnel, which is connected with the mantle cavity itself.

The shells of cephalopods are internal, often reduced or completely absent. It is important to note that their mantle cavity functions similarly to a jet engine: through the mantle slots, water is drawn directly into the mantle cavities, and then it is ejected with force through the funnel itself. Cephalopods are crushed with rather thick and powerful horny jaws, and others with a grater. They have two pairs of internal salivary glands.

Squid (lat. Teuthida) - another of the representatives of cephalopods

Origin of shellfish.

Many scientists are of the opinion that all molluscs are descended from ancestors - worm-like marine organisms, or rather, from annelids. As evidence, they cite the similarity of the larvae of many gastropods of marine mollusks, as well as the larvae of polychaete marine worms. In addition, some of the primitive molluscs have a fairly large resemblance directly to the annelids themselves.

However, some scientists believe that molluscs are descended from a genus of giant cephalopods living in the Ordovician period, 470-440 million years ago (Cameroceras lat. cameroceras) fossilized shells have been found in North America, South America and Spain.

Cameroceras (lat. Cameroceras) belongs to the genus of giant cephalopods orthocones

And in more detail with the most interesting representatives of the class of mollusks, you will be introduced to new articles on the pages of the online magazine "Underwater World and All Its Secrets", new and these video stories:

The world of aquatic invertebrates is rich and diverse, and these articles will tell about them:

Type mollusks are soft-bodied animals, predominantly with a bilaterally symmetrical structure, inhabiting both water bodies and land. There are more than 120 thousand species.

The sizes of mature mollusks of different classes differ significantly - from a couple of millimeters to 20m. Many lead a sedentary or sedentary lifestyle, and only cephalopods are able to actively move in the water. The science of shellfish is called malacology, she is studying the structure, development of soft-bodied animals, and their role in the world around them.

Features of the structure of Mollusks

External structure

The body is bilaterally symmetrical in bivalves and cephalopods, or asymmetrical in gastropods. There are such departments of it: the head part with the organs of vision and tentacles, the body itself and the leg - a muscular formation, serves to move. All bivalves are characterized by the presence of a leg, while in cephalopods it has been transformed into tentacles and a siphon.

The body of the mollusk is surrounded by a shell, serves as a place for attachment of muscles. In gastropods, it has an integral structure in the form of a spiral curl. In bivalves, it is represented by two valves, which are connected by flexible strands of connective tissue. Most cephalopods lack a shell.

From the lateral parts of the body departs the mantle, sent by epithelial cells. Together with the body, it forms a cavity where the gill arches, sensory organs, excretory ducts of the glands of the digestive tract, genitourinary system, and anus are located.

Mollusks are coelomic organisms, but their secondary cavity is preserved only near the heart and genitals. The main part of the internal space is represented by the hemocoel.

Internal structure

shellfish digestive system divided into three parts: anterior, middle and hindgut. Many representatives have a radula in the pharynx - a tongue designed to grind food. It has chitinous plates with teeth. With the help of the radula, they absorb bacteria or plant foods. Saliva is secreted into the pharyngeal cavity and sticks together food particles. The food then enters the stomach, where the digestive gland (liver) opens. After digestion, the remains are excreted through the anus.

circulatory system open, in the heart there is a ventricle and usually two (rarely four) atria. From the bloodstream, blood enters the sinuses and lacunae located between the organs, then again passes into the vessels and goes to the respiratory organs.

Breath in aquatic species, it is carried out by gills; in inhabitants of land, it is carried out by lungs. The lung tissue is equipped with a dense vascular network, where oxygen and CO 2 are exchanged. The lung communicates with the external environment with a spiracle.

Nervous system of molluscs consists of five pairs of nerve nodes, united by fibrous cords. The unequal development of the sense organs in mollusks indicates a different way of life of representatives of the type.

For example, cephalopods have a fairly developed vision, the structure of the eye is similar to the structure of the eye of vertebrates. The predatory nature forced them to adapt to changing environmental conditions through the complication of the visual apparatus. They formed a peculiar type of accommodation, which was carried out by changing the distance between the retina and the lens.

Mollusks reproduce sexually. There are both dioecious (with external fertilization) and hermaphrodites (with internal fertilization). In marine bivalves and gastropods, development is indirect, there is a larval stage, the rest are direct.

Features of the structure of mollusks compared with annelids

What new organs appeared in molluscs compared to worms?

Mollusks have specialized organs. This is the excretory, digestive system, which includes a number of departments, there is a heart, a liver. Respiratory organs - gills or lung tissue.

The circulatory system is open, in annelids it is closed.

The nervous system of mollusks has the form of nerve ganglia, united with each other by nerve fibers. Annelids have a nerve chain only in the abdominal region, which branches into segments.

How are shellfish adapted to their environment?

Representatives of the type inhabit the expanses of water and the surface of the land. For existence outside the reservoir and breathing atmospheric air, soft-bodied animals developed lung tissue. Inhabitants of reservoirs receive O 2 with the help of gill arches.

How do shellfish protect themselves from enemies?

To move in the water, cephalopods have adapted to jet locomotion, so they can quickly run away from enemies.

Poisonous and chemical substances (ink) serve as protection against predators. Some are able to burrow into the sandy bottom in seconds when threatened, or hide using a springy leg.

What is the function of a mollusk shell?

First of all, it is a support function, it serves as an external skeleton. Also, a strong shell of bivalves and gastropods is needed to protect against adverse factors. So, when danger approaches, they hide in them and become inaccessible to most fish.

Similarities and differences between gastropods and bivalves

Properties	gastropods	Bivalves
Non-systematic category	Multicellular organisms
Outer coverings	The body is surrounded by a shell (in whole or in part)
Sink	Piecework, asymmetric and twisted	Has two doors
body structure	Head, torso and leg	Trunk, leg
Analyzers	Tactile, chemical reception, balance and vision.	Underdeveloped
Habitat	Water and land	reservoirs

The value of mollusks in nature and human life

They are an integral part of the food chain. Soft-bodied are used by frogs, fish, birds. Seals eat cephalopods, starfish - bivalves.

Water passes through the body of the mollusk and is cleansed of pollutants. And the molluscs, in turn, get food particles from filtered water.

Soft-bodied valves take part in the formation of sedimentary rocks.

Widely used in cooking, considered a delicacy in many countries. These are mussel meat, scallops, oysters, cuttlefish and octopuses. Due to the popularity of dishes from exotic animals, they began to be grown on specially equipped farms.

Between the valves of the shell, a valuable jewelry raw material is formed - pearls. A pearl is formed after getting inside a foreign body. Since the muscles of the mollusks are not sufficiently developed, they cannot throw it out. To neutralize a foreign object, a capsule is formed around it and the mollusk lives with the newly formed pearl all its life.

Now pearls are mined in artificially created conditions. Having slightly opened the valves, foreign objects are placed under the mantle, and the mollusk is transferred to a reservoir with favorable conditions for life, and after three years pearls are obtained.

Cuttlefish and octopuses are used to extract the ink substance from which ink is made.

Pests of agriculture - slugs, destroy crops, garden plants (potatoes, cabbage, tomatoes).

Flatworms that cause diseases in humans and animals use mollusks as intermediate hosts.

Type Mollusks, or Soft-bodied, includes more than 100 thousand species from 7 or 8 (according to different classifications) living class. Moreover, most species belong to the classes Gastropods and Bivalves. Representatives of soft-bodied: snails, slugs, barley, oysters, squids, octopuses, etc.

Various types of mollusks belonging to different classes differ quite strongly in structure, and often in their life cycle.

Body sizes from less than a millimeter to more than 10 m.

The external structure of molluscs

The body of the mollusk has bilateral (bilateral) symmetry or is asymmetrical due to the distortion of bilateral symmetry in the process of individual development.

The body is not divided into segments. However, the most simply arranged mollusks have some signs of segmentation. Therefore, the soft-bodied either could have common ancestors with annelids, or their ancestors were annelids themselves.

The body of many mollusks consists of a head, trunk and legs. In bivalves, the head is absent, the leg is reduced. In cephalopods and a number of others, the leg has become a swimming organ.

The trunk forms a mantle, which is a skin fold that covers the body. Between the body and the mantle, a mantle cavity is formed, where the openings of the excretory organs, sometimes the genitals, and the anus open. Here are the gills (or lung) and some sense organs.

In many mollusks, the body from the dorsal side is covered by a hard shell, which has a mineral nature. It is formed from substances that are released by the mantle. Basically, it is crystalline calcium carbonate (CaCO 3) with an admixture of organic matter. Often, the top of the shell is covered with horn-like organic matter, and the inside has a calcareous layer, which is called mother-of-pearl.

The shell can be solid, bivalve or consisting of several plates. Usually well developed in slow moving and immobile mollusks. Others may have little or none at all. For example, squids, octopuses, and slugs do not have shells.

Mollusks have a mouth, tentacles, and eyes on their heads.

The leg is an unpaired muscular outgrowth of the ventral side of the body. Used for crawling. Can carry organs of balance ( statocysts).

The internal structure of molluscs

Like annelids, molluscs are classified as protostomes, secondary cavities, and three-layered.

Despite the fact that mollusks belong to the secondary cavity, the secondary body cavity (as a whole) is well developed only in their embryos. In adults, the whole remains only in the form of a pericardial sac and the cavity of the gonad, and the gaps between the organs are filled with connective tissue (parenchyma).

Digestive system

Behind the mouth opening of the mollusk is a pharynx, in which many species have radula(grater). The radula consists of a ribbon and teeth located on it, with which vegetable food is scraped off or animal food is seized (protozoa, crustaceans, etc.).

In some predatory mollusks, salivary glands open into the oral cavity, whose secret contains poison.

In bivalves, which feed on microorganisms and small organic particles, the mouth is immediately followed by the esophagus, i.e. they do not have a pharynx with a grater.

Respiratory system

Aquatic mollusks have paired gills ( ctenidia), which are skin outgrowths in the mantle cavity. Terrestrial have a lung, which is a fold (pocket) of the mantle filled with air. Its walls are permeated with blood vessels. Despite the presence of respiratory organs, mollusks also have skin respiration.

Circulatory system

Unlike annelids, molluscs have an open circulatory system. Although in the most complex soft-bodied it is almost closed. In some, the oxygen-carrying pigment contains manganese or copper rather than iron. Therefore, blood can be blue.

There is a heart, which in most species consists of one ventricle and two atria.

The aorta departs from the heart, then arteries follow, which pour blood into the spaces between the organs. Then the venous blood is again collected in other vessels and goes to the gills or the lung. From there through the vessels to the heart.

excretory system

Mollusks have from 1 to 12 kidneys similar to metanephridia. Inside, they open into the cavity of the pericardial sac, the other end into the mantle cavity. Uric acid accumulates in the kidneys.

Nervous system and sense organs

The nervous system of molluscs includes several pairs of ganglia connected by nerve trunks. Nerves come off the trunks.

In different representatives of the type, the degree of development of the nervous system is different. In simpler ones, it is of a ladder type, in the rest it is of a scattered-nodal type.

There are organs of touch, chemical sense, balance. In mobile forms, especially in fast-swimming cephalopods, organs of vision are developed.

Shellfish breeding

Among mollusks there are both dioecious species and hermaphrodites (less often). Fertilization is external or internal. The gonads open as a whole, and the reproductive products are excreted into the mantle cavity through the kidneys.

A planktonic larva (sailfish) or a small mollusk develops from the egg.

The value of shellfish

Bottom bivalve mollusks filter water, which purifies it not only from organic, but also from mineral particles.

Mollusks serve as food for other animals, including birds, mammals, and humans. People breed, for example, oysters.

Pearls are formed in the shells of pearl oysters, which people use as jewelry.

Using the shells of fossil mollusks, scientists determine the age of sedimentary rocks.

Some marine bivalves destroy wood, which can damage ships and waterworks.

Ground slugs and snails can damage orchards and vineyards.

Answer the questions: 1. Why did the molluscs get their name? 1. Why did molluscs get their name? 2. What mollusks live in our area? 2. What mollusks live in our area? 3. What is the significance of molluscs in nature and human life? 3. What is the significance of molluscs in nature and human life?

Type Molluscs Class Armored Class Furrowed-bellied Class Spadeopods Class Bivalves Class Gastropods Class Monoplacophores Class Cephalopods

Gastropods Nudibranch mollusk Grape snail Neptunia despecta Liambis isillipeda (species) Field slug

Bivalves Sea pearl oyster Tridacna large (250 kg) Gray mussel Edible molluscs (species)

Blue-ringed octopus - Lives off the coast of Australia - Length up to 20 cm, weight up to 100 g - The bite is fatal, the poison causes paralysis of the respiratory muscles within 30 minutes Nutritious and tasty dishes from squid, scallops, mussels, cockles and coastal snails

Answer the questions: 1. Explain how molluscs are adapted to their habitat? 1. Explain how molluscs are adapted to their environment? 2. What are the similarities and differences between gastropods and bivalves? 2. What are the similarities and differences between gastropods and bivalves? 3. List the ways to protect the mollusks you know from enemies. 3. List the ways to protect the mollusks you know from enemies. 4. What allows cephalopods to move quickly in water and escape from enemies? 4. What allows cephalopods to move quickly in water and escape from enemies?