Type Mollusks: general characteristics. Type and classes of mollusks

1. The body is not segmented.

2. Most have a sink.

3. The body is covered on the outside with a special fold of skin - the mantle.

4. Feeding: a) active (scraping, predation); b) passive (filtration)

5. Breathing: lungs (a special pocket of the mantle) or gills.

6. Circulatory system open-ended, have a heart. Blood - hemolymph.

7. Nervous system nodal In sedentary people, the sense organs are poorly developed, while in active ones, they are well developed.

Main aromorphoses:
1. Merger of segments into body parts (increasing the functional significance of organs).
2. Formation of nerve nodes in parts of the body.
3. The appearance of the heart, increasing the speed of blood circulation.
4. The appearance of digestive glands, more complete breakdown of food.

Comparative characteristics main classes of mollusks

Signs	Classes
Signs	Gastropods	Bivalve	Cephalopods
Habitats	Mainly land and fresh water bodies	Fresh waters and seas	Salty warm seas
Body symmetry	Asymmetrical	Bilaterally symmetrical	Bilaterally symmetrical
Body parts	Head, torso, leg	Torso, leg	Head, torso
Organs present on the head	1–2 pairs of tentacles, 1 pair of eyes at the base or at the ends of the upper pair of tentacles	No head	Tentacles formed from part of the leg that moved onto the head and surrounding the mouth; 2 large eyes
Sink	Single in the form of a curl or reduced	Made of two flaps with an elastic ligament on the dorsal side	Remains of a reduced shell under the skin or absent
Leg	Muscular, occupies the entire ventral side of the body	Muscular, wedge-shaped abdominal part of the body	Split into tentacles
Movement	Using your feet	Using the foot or (rarely) in a reactive manner (pushing water out of the mantle cavity)	Using tentacles (arms) and a reactive method (by pushing water out of the mantle cavity through a funnel)
Respiratory organs	“Lung” is a cavity formed by the mantle between the bodies and part of the shell; at marine species may have gills	Lamellar gills on the sides of the body	Gills
Nervous system	Periopharyngeal ganglia	3 pairs of ganglia	Ganglia forming the common peripharyngeal mass ("brain")
Excretory organs	1 pair of buds	1 pair of buds	1 or 2 pairs of buds
Reproduction and development	Usually dioecious, direct development	Most are dioecious, development with transformation (larva - glochidium)	Dioecious. The gonad is unpaired. Development is direct.

Meaning in nature and human life

In nature:

a) a link in food chains (for example: land mollusks serve as food for toads and moles);
b) bivalves filter water (one oyster filters about 10 liters of water per hour).

Interesting facts

The largest pearl, 24 cm in length and 14 cm in diameter, was found in a coral reef mollusk shell tridacni off the Philippine Islands. This mollusk has a shell length of up to 1.4 m, a weight of about 200 kg, and a body weight of about 30 kg. Unfortunately, pearls, unlike precious stones, is not eternal: 50–60 years after the pearl is removed from the mollusk, it begins to become covered with cracks. The maximum "life" of a pearl as jewelry does not exceed 150 years; this is due to the drying out of the organic layers inside it.

Byssus is produced mussels And pins. These are the threads that attach the shell to the substrate. According to their properties, these silky threads are yellowish or brownish in color, elastic, strong, with an unusual shine, and contain a protein substance close to fibroin in silk. The length of the byssal fibers reaches 30 cm. Beautiful lace and fabrics were made from the byssus. The first indications of the use of byssus threads for spinning and weaving date back to the 2nd and 3rd centuries. ADIn the 18th century V European countries Stockings, gloves, wallets, lace, as well as hats, jackets and dresses were made from “shell silk”. Naturally, products made from such an expensive material were highly valued: in Italy at the end of the 18th century. a pair of shell silk gloves cost 20 gold ducats.

Cephalopods are, without a doubt, the most aggressive and warlike inhabitants of the sea. Although they have plenty of enemies. But cephalopods don't give up without a fight. One of the most amazing protective devices, which cephalopods acquired in the process of evolution, this miracle weapon is an ink bomb. Ink is produced by a special outgrowth of the rectum called the ink sac. It consists of two parts: ink is produced in one part, and ink is accumulated in the other. At the moment of danger, the entire contents of the bag are thrown away. But after a few minutes, the mollusk is again ready to repel any attack. But this does not exhaust all the possibilities of ink. In 1956, Dr. D. Hall published interesting observations on the behavior of squid in the English journal Nature. The zoologist put the squid in a tub and tried to catch it with his hand. When his fingers were already a few inches from the target, the squid suddenly darkened and, as it seemed to Hol, froze in place. The next moment, Hall grabbed... an ink model that fell apart in his hands. The deceiver was floating at the other end of the tub. What a subtle maneuver! The squid didn’t just leave its image in its place. No, it's a dress-up scene. First, it attracts the enemy's attention with a sharp change in color. Then he immediately replaces himself with someone else dark spot- the predator automatically fixes his gaze on him - and disappears from the scene, changing his “outfit”. Please note: the squid is now not black, but white.

Go ink molluscs have one more amazing property. American scientist McGinity conducted a series of experiments on the Californian octopus and moray eel. And this is what he found: octopus ink, it turns out, paralyzes the olfactory nerves of predatory fish.

Dangerous Is cephalopod ink good for humans?

Let’s ask an underwater hunting expert like James Aldridge to answer this question. He writes: “I behaved so freely with the octopus that I received a stream of ink right in the face. And since I was without a mask, the liquid got into my eyes and blinded me. The world around me, however, did not darken as a result, but became colored. a wonderful amber color. Everything around me seemed amber in color as long as the film of this ink remained on my eyes. It lasted for about ten minutes or so. This incident did not affect my vision."

However, octopuses and cuttlefish have another amazing adaptation that allows them to avoid meeting the enemy. It turns out that they have an exceptional ability to camouflage, so they can safely be called the “kings of camouflage.”

The ability has long been known cephalopods to the glow. The French naturalist Jean Baptiste Verani loved to come to the seashore when fishermen returned with their catch. One day, near Nice, he saw a crowd of people on the shore. A completely unusual creature was caught in the net. The body is thick - like a sac, like an octopus, but there are ten tentacles, and they are connected by a thin membrane. Verani lowered the bizarre captive into a bucket of sea water; “at that very moment,” he writes, “I was captivated by the amazing spectacle of sparkling spots that appeared on the skin of the animal. Now it was the blue ray of sapphire that blinded me, now the opal ray of topaz, now both richly hued colors mixed in a magnificent radiance , surrounded the mollusk at night, and it seemed one of the most wonderful creations of nature." Thus, in 1834, Jean Baptiste Verani discovered the phenomenon of bioluminescence.

Habitat and external structure. Most mollusks live in the seas, a relatively small number live in brackish and fresh waters, even less on land. Many aquatic mollusks lead a bottom-dwelling lifestyle.

Some mollusks are bilaterally symmetrical animals. However, gastropods developed a twisted shell, and their body became asymmetrical for the second time.

Mollusks are characterized by a hard mineral shell that covers the animal’s body from the dorsal side. The shell consists of crystals of calcium carbonate. On top it is usually covered with a horn-like organic substance, and on the inside it is lined with a hard, shiny calcareous layer - mother-of-pearl. The shell can be solid, bivalve or consisting of several plates (in sea mollusks chitons).

Slowly moving and immobile mollusks have a highly developed shell. However, in some mollusks it is reduced (underdeveloped) or absent altogether. This happens when the mollusk lives in places where it is difficult for predators to reach (for example, when it burrows deep into the sand of the seabed or drills passages in the trunks of trees that have fallen into the sea). Slugs and mollusks that swim well have lost their shells.

The body of mollusks consists of a trunk, head and legs (Fig. 70). Almost all mollusks have a head. It contains a mouth opening, tentacles and eyes. The leg is a muscular unpaired outgrowth of the body. It is located on the ventral side and is used for crawling.

Rice. 70. Various mollusks: A - gastropods: B - bivalve; B - cephalopod: 1 - leg; 2 - sink; 3 - tentacles

In bivalves, due to their sedentary lifestyle, the head is absent and the legs are reduced. In some species, the leg has become a swimming organ (for example, in cephalopods).

Internal structure. The body of the mollusk is surrounded by a fold of skin - the mantle. The space between the walls of the body and the mantle is called the mantle cavity. The openings of the excretory organs, genitals and anus open there. It contains the respiratory organs - gills. The secondary body cavity (coelom) is well defined in the embryonic state, and in adult animals it remains in the form of a pericardial sac and a cavity of the gonad. The spaces between organs are filled with connective tissue.

Digestion. The mouth opening leads into the pharynx. In the pharynx of many species there is a grater (radula) - a special device in the form of a ribbon lying on the protrusion of the floor of the oral cavity. There are teeth on this tape. Using a grater, herbivorous mollusks scrape food from plants, while carnivorous mollusks (which have larger grater teeth) grab prey. Some carnivorous mollusks have salivary glands that open into the oral cavity. The secretion (secreted substance) of the salivary glands contains poison.

The pharynx passes into the esophagus, followed by the stomach, into which the liver ducts open. The secretion of the liver dissolves carbohydrates, and food absorption also occurs in the liver. The stomach passes into the intestine, ending in the anus. In bivalves that feed on microscopic algae and small organic particles suspended in water, the structure oral apparatus simplified: the pharynx, grater and salivary glands are lost.

Breath. In aquatic mollusks, the respiratory organs are paired gills - flat skin outgrowths lying in the mantle cavity. Terrestrial mollusks breathe with with the help of the lung. It is a pocket (fold) of the mantle, which is filled with air and communicates with the external environment through the breathing hole.

Circulatory system. The heart of mollusks usually consists of three sections (one ventricle and two atria). The circulatory system is not closed. Some mollusks have manganese or copper in their blood. Their compounds play the same role as iron in the blood of higher animals - they ensure the transfer of oxygen.

The excretory organs are represented by paired kidneys, which at one end communicate with the cavity of the pericardial sac, which surrounds the heart, and at the other open into the mantle cavity. The pericardial sac is the remnants of the coelom. Therefore, we can talk about the similarity of the excretory systems of mollusks and annelids.

Nervous system. The central nervous system consists of several pairs of ganglia (nodes) connected by nerve trunks, from which nerves extend to the periphery.

Sense organs. Mollusks have well-developed organs of touch, chemical sense and balance. Motile mollusks have visual organs. The eyes of fast-swimming cephalopods are especially well developed.

Reproduction. Most mollusks are dioecious. However, there are also hermaphrodites in whom cross-fertilization occurs. 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 develops, leading a planktonic lifestyle (the so-called swallowtail), or a formed small mollusk.

Meaning. Representatives of some classes of mollusks play an important role in many natural biocenoses. Aquatic mollusks are often the most abundant group in benthic ecosystems. The filtration method of feeding of bivalves leads to the fact that many of them precipitate mineral and organic particles, providing water purification. Fish, birds and animals eat shellfish.

Shellfish serve as food for people and are traditional objects of fishing and farming (oysters, scallops, mussels, hearts, squid, Achatina, grape snail). Very beautiful pearls are formed in the shells of sea mollusks, pearl mussels. Aboriginal people used cowrie shells as coins. Geologists can use the shells of fossil mollusks to determine the age of sedimentary rocks.

Origin. There are several points of view on the origin of mollusks. Some zoologists believe that the ancestors of mollusks were flatworms. Others suggest that mollusks originated from annelids. Still others think that mollusks originate from ancestors common to annelids. Embryological data indicate the relationship of mollusks with annelids.

A typical mollusk larva (sailfish) is very similar to an annelid larva, 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.

Mollusks are non-segmented, bilaterally symmetrical soft-bodied animals (in gastropods the body is asymmetrical), having a shell, a mantle cavity, a reduced coelom, and an open circulatory system). Apparently, they 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.

Exercises based on the material covered

Name the main methods of movement of mollusks.
Name the main similarities and differences between the external and internal structures of mollusks and annelids.
What traits do fast-swimming mollusks have?
What is the influence of a sedentary lifestyle on the organization of mollusks?
What is the role of mollusks in nature and in human life? Give examples.

), fresh water bodies (toothless, pond snails, viviparidae), less often - in wet terrestrial environment(grape snail, slugs). Body sizes of adult mollusks different types vary significantly - from a few millimeters to 20 m. Most of them are sedentary animals, some lead an attached lifestyle (mussels, oysters), and only cephalopods are able to move quickly in a reactive manner.

Basic characteristic features structures of mollusks following blowing:

The body lacks segmentation, has bilateral symmetry (bivalves and cephalopods) or asymmetrical (gastropods). The parts of the body are head with eyes located on it and 1-2 pairs of tentacles, torso, where most of the internal organs are located, and leg- the muscular abdominal part of the body used for movement. In bivalves, the head is reduced.
The body of the mollusk is enclosed sink, protecting the animal and providing support for muscle attachment. The outer layer of the shell is horny, the middle (porcelain) and inner (mother-of-pearl) are calcareous. Gastropods have a solid shell in the form of a cap or a spirally curled turret. In bivalves, it consists of two valves connected elastic ligament, “lock” teeth and closing muscles. Most cephalopods have lost their shells.
The body of the mollusk is covered with a fold of skin - mantle, the epithelium of which secretes the shell substance. Between the mantle and the body is formed mantle cavity, in which the gills, some sensory organs, the anus, and the opening of the excretory organs are located.
Body cavity secondary (in general), however, it is greatly reduced and preserved only in the form of the pericardial cavity and the cavities of the gonads. The rest of the space between internal organs filled with loose tissue - parenchyma.
The digestive system consists of three sections: the foregut, midgut and hindgut. Most mollusks (except bivalves) have a muscular tongue in the pharynx, covered with a horny plate with numerous denticles - grater With it they actively capture and crush plant and animal food. Ducts open into the pharynx salivary glands, and into the stomach - the duct of a special digestive gland - liver. Bivalves feed passively, filtering food suspension (algae, bacteria, detritus) through the gills, which enters the mantle cavity with water through the inlet siphon.
Circulatory system open and consists of hearts And vessels. The heart has a ventricle and 1-2 (less often 4) atria. In addition to the vessels, blood passes part of the way in the slit-like cavities between the organs.
Respiratory organs in aquatic mollusks - gills, for terrestrial - lung, representing a section of the mantle cavity. The wall of the lung contains a dense network of blood vessels through which gas exchange occurs. The lung opens outward with the respiratory opening - spiracle.
The excretory system is represented by 1-2 kidneys. They are modified metanephridia. The funnel of the kidney opens into the pericardial sac, and the excretory opening into the mantle cavity.
Nervous system scattered-node type: five pairs of large ganglia are located in vital organs (head, leg, mantle, respiratory organs and sac) and are connected by nerve trunks. Of the sense organs, the most developed are the organs of chemical sense, touch, balance, and in mobile predators - vision.
Reproduction occurs sexually. Most mollusks are dioecious animals, less often - hermaphrodites (pulmonary gastropods). In dioecious mollusks, fertilization is external, while in hermaphroditic mollusks, fertilization is internal, cross-fertilization. In freshwater and terrestrial pulmonary mollusks, as well as cephalopods, development is direct, in marine bivalves and gastropods - with incomplete metamorphosis, i.e., with a planktonic larval stage, which contributes to their settlement.

1. What are the main features that characterize mollusks? Draw up and draw a diagram of the structure of a gastropod, and label it in the drawing.

The body of mollusks in most cases consists of a head, torso and muscular legs;

The torso is a skin-muscular sac surrounded by an extensive fold of skin - the mantle;

Between the mantle and the body wall, a mantle cavity is formed, in which the respiratory organs, some sensory organs are located, the anus, ducts of the kidneys and gonads open into it;

Digestive system consists of the foregut, midgut and hindgut;

In the pharynx there is usually an organ that grinds food - a grater with horny teeth located on it;

The circulatory system is not closed (except for cephalopods); The movement of blood is ensured by a two-chambered heart;

There are organs of vision, balance, chemical sensitivity, and touch;

The respiratory organs of aquatic forms are gills, and those of terrestrial forms are lungs; represent a specialized area of the skin fold - the mantle.

2. What is the mantle? What is its meaning?

The mantle is a fold of tissue in mollusks surrounding the body. Between the mantle and the body, a mantle cavity is formed, in which some sensory organs are located and into which the anus, ducts of the kidneys and gonads open.

3. What is a sink? What are its functions? Which chemicals form the bulk of the shell?

A shell is a protective formation covering the body of a mollusk. The shell is usually formed by secretions of the mantle. It can be solid, less often bicuspid or consisting of several plates. The outer layer of the shell is formed by an organic horn-like substance, the inner layer is formed by the thinnest plates of lime. The uneven reflection of light from these plates gives the inner surface of the shell a pearlescent shine. In some mollusks, the underdeveloped shell is buried under the skin or disappears completely (slugs, cephalopods).

4. Describe the body structure of bivalves.

The soft body of mollusks in most cases consists of a head, torso and legs. The mouth and sensory organs are located on the head. The body is a bag, its base is surrounded by an extensive fold of skin - the mantle. On the dorsal side, as a rule, there is a protective shell secreted by the mantle. The abdominal side, strongly thickened due to muscles, forms various shapes legs: wide - crawling, wedge-shaped, fin-like - for swimming, rounded - suction, etc.

5. Discuss as a class the similarities and differences between different classes of mollusks.

In the process of settling around the planet, mollusks were divided into a number of groups, united into several classes. Among them are Gastropods, Bivalves and the most highly organized - Cephalopods.

6. Make a table “Comparative characteristics of annelids and mollusks” (work in small groups).

7. What role do you think mollusks play in nature? Give examples of mollusks that live in your area.

Shellfish are important as a source of mother-of-pearl and pearls. The most prized pearls are the sea pearl mussel, found in the Red Sea, Indian and Pacific Oceans. Many marine bivalves are eaten, such as oysters, mussels, scallop, squid. Bivalves are powerful natural water purifiers (biofilters). Feeding on organic substances suspended in water and tiny living organisms (plankton), they pass large amounts of water through the mantle cavity, purifying it.

8. Highlight the structural features of the body of mollusks associated with their way of life.

Main features:

The body lacks segmentation, has bilateral symmetry (bivalves and cephalopods) or asymmetrical (gastropods);

The body of mollusks is enclosed in a shell, which protects the animal and provides support for the attachment of muscles; in gastropods the shell is solid in the form of a cap or spirally curled turret; in bivalves it consists of two valves connected by an elastic ligament; most cephalopods have lost their shell.

9. Make a detailed plan for the paragraph.

General characteristics external structure representatives of the phylum Mollusca;

Structure of the nervous system;

The structure of the sense organs;

Structure of the circulatory system;

The structure of the respiratory organs;

The structure of the digestive system;

The structure of the excretory organs;

Meaning of shellfish;

Class Gastropods (general characteristics of the class);

General characteristics of the class Bivalves;

Economic importance, representatives;

General characteristics of the class Cephalopods;

Structural features of representatives of this class;

Economic importance.

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. In this they are helped by a developed salivary gland. By the way, what is the salivary gland in mollusks? This general concept refers to a fairly wide range of specific organs located in the pharynx and oral cavity. They are intended for secretion 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. 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 its relatives ( sulfuric acid perfectly dissolves their limescale 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 all over 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 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 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 often differ in the most important practical significance and 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 they produced the most valuable purple, 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 relatively small quantity species chose 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.

Features gastropods and bivalves

Feature under consideration	Classes of mollusks
Feature under consideration	Bivalve	Gastropods
Symmetry type	Bilateral.	There is no symmetry, some organs are completely reduced.
Presence or absence of a head	Completely atrophied, like all organ systems that historically belonged to it.	There is, as well as the entire set of organs (oral cavity, eyes).
Respiratory organs		Gills or lung (pond snail, for example).
Sink type	Bivalve.	One-piece, can be twisted into different sides(pond snails, ampularia) or in a spiral (Lake coil).
Sexual dimorphism, reproductive system	Dioecious, males are often smaller.	Hermaphrodites, sometimes dioecious. Dimorphism is weakly expressed.
Power type	Passive (water filtration). In general, these mollusks in nature contribute to excellent water purification, as they filter tons of organic impurities from it.	Active, there are predatory species (Cones (lat. Conidae)).
Habitat	Seas and fresh water bodies.	All types of reservoirs. There are also terrestrial mollusks (Grape snail).

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 skin forms a large fold, a 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. This shell contains many carbon dioxide(of course, in a bound state - CaCO 3), as well as conchiolin, a special organic matter, which 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 organs

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.
Cephalopoda. Mobile mollusks have shells either completely absent or in their infancy.

Who 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, some of the most dangerous and incredible predators on our planet are considered... outwardly 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! Predatory snail this is quite enough. 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.