Type Molluscs: general characteristics, internal and external structure, significance in nature. Type and classes of molluscs. General characteristics of molluscs. What is the salivary gland in molluscs? About methods of hunting snails

Molluscs are among the most ancient invertebrates. They differ in the presence of a secondary body cavity and rather complexly arranged internal organs... Many of them have a limestone shell, which quite well protects their body from the encroachments of numerous enemies.

This is not often remembered, but many species of this type lead a predatory lifestyle. A developed salivary gland helps them in this. By the way, what is the salivary gland in molluscs? This generalizing concept means 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 carnivorous species, the secret they secrete contains from 2.18 to 4.25% of chemically pure sulfuric acid. It helps both to fend off predators and to hunt their congeners (sulfuric acid perfectly dissolves their calcareous shells). This is what the salivary gland is in molluscs.

Other natural value

Many of the slug species, as well as the grape snail, are causing great harm to agriculture around the world. At the same time, it is mollusks that play the most important role in the worldwide purification of water, since they use organics 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 diet food.

V the former USSR 19 representatives of this oldest type... Despite the variety of molluscs, they should be treated with care, as they are extremely important for the proper functioning of many natural biotopes.

In general, molluscs are often of the most important practical value for humans. For example, pearl mussel is massively bred in many coastal countries, as this species is a supplier of natural pearls. Some molluscs are of great value to the medicine, chemical and processing industries.

Want to know Interesting Facts about shellfish? In the Antiquity and the Middle Ages, inconspicuous cephalopods were sometimes the basis of the well-being of entire states, since the most valuable purple was extracted from them, with which the royal robes and clothes of the nobility were painted!

Shellfish type

In total, it has more than 130,000 species (yes, the variety of molluscs is incredible). Molluscs in total number are second only to arthropods, they are the second most common living organisms on the planet. Most of them live in water, and only comparatively not a large number of species chose dry land as their place of residence.

general characteristics

Almost all animals that are part of this type differ in several specific features at once. Here is the general characteristic of molluscs accepted today:

• First, the three-layer structure. Their organ system is formed from ectoderm, endoderm, and mesoderm.
• Symmetry of the bilateral type, caused by a significant displacement of most of their organs.
• The body is non-segmented, 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) serves for movement.
• The coelomic cavity is very poorly expressed.
• There are practically all the same organ systems (in a simplified version, of course) as in higher animals.

Thus, the general characteristics of mollusks indicate that we are faced with rather developed, but still primitive animals. It is not surprising that it is mollusks that many scientists consider the main ancestors of a large number of living organisms on our planet. For clarity, we present a table in which the characteristics of the two most common classes are described in more detail.

Detailed characteristic

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

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

• Head.
• The torso.
• The muscular leg through which movement is carried out.

In all bivalve species, the head is completely reduced. A leg is 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: the gills, as well as the findings of the reproductive and excretory systems. It is the mantle that secretes those substances that, having reacted with water, form a solid shell.

The shell can be either completely solid or consist of two valves or several plates. This shell contains 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. In slugs, only a microscopic plate remained 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 will talk about them below), the tongue has evolved into a flexible and rigid proboscis, which is intended for opening the shells of other mollusks.

In the 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 into 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 to the bulky stomach.

Respiratory system

Most species breathe with gills. There are "front" and "back" 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 apex looks back. Some have lost their gills in the literal sense of the word. These large molluscs breathe directly through their skin.

For this, they have developed a special adaptive skin organ. In terrestrial species and secondary aquatic molluscs (their ancestors returned to the water again), part of the mantle is wrapped up, forming a kind of lung, the walls of which are densely penetrated by blood vessels. In order to breathe, such snails rise to the surface of the water and take in a supply of air with the help of a special spiracle. The heart, located near the simplest "construction", consists of one atrium and a ventricle.

The main classes included in the type

How is the mollusc type subdivided? Classes of mollusks (there are eight of them in total) are "crowned" with the three most numerous:

• Gastropods (Gastropoda). This includes thousands of snail species of all sizes, the main distinguishing feature of which is a low speed of movement and a well-developed muscular leg.
• Bivalves (Bivalvia). Sink with two sashes. As a rule, all species included in the class are sedentary, sedentary. They can move both with the help of a muscular leg, and by means of jet thrust, ejecting water under pressure.
• Cephalopods (Cephalopoda). Motile molluscs, shells are either completely devoid of, or it is in its infancy.

Who else is the type of molluscs? Classes of molluscs are quite diverse: in addition to all of the above, there are also Shovellegs, Carapace and Pit-tailed, Furrow-bellied and Monoplacophores. All of them are living and living.

What fossils does a type of shellfish contain? Classes of molluscs that are already extinct:

• Rostroconchia.
• Tentaculitis.

By the way, the same Monoplacophores were considered completely extinct until 1952, but at that time the Galatea ship with a research expedition on board caught several new organisms that were assigned to the new species Neopilina galatheae. As you can see, this species was named after the research vessel that discovered them. However, in scientific practice this is not uncommon: 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 molluscs: the classes of mollusks, which are now considered extinct, may well remain somewhere in the bottomless depths of the world's oceans.

No matter how strange it may sound, but one of the most dangerous and incredible predators on our planet are ... seemingly harmless gastropods. For example, the Cones snails (Latin Conidae), whose venom is so unusual that it is used by modern pharmacists in the manufacture of certain types of rare medicines. By the way, the name of the mollusks of this family is fully justified. Their shape is, in fact, most of all similar to a truncated cone.

They can be persistent hunters, dealing extremely ruthlessly with floodplain prey. Of course, the role of the latter is often played by colonial, sedentary animal species, since other snails simply cannot keep up. The victim itself can be dozens of times larger than the hunter. Want to know more fun facts about shellfish? Yes please!

About methods of hunting snails

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

Other types of gastropods prefer not to press anything at all, carefully drilling through the shell of prey with the help of a special proboscis. But this process cannot be called simple and quick with all the desire. So, with a shell thickness of only 0.1 mm, drilling can take up to 13 hours! Yes, this way of "hunting" is suitable exclusively for snails ...

Dissolution!

To dissolve someone else's shell and its owner itself, the mollusk uses sulfuric acid (you already know what the salivary gland is in molluscs). So the destruction is much easier and faster. After the hole is made, the predator begins to slowly eat its prey out of the "package", using its proboscis for this. To some extent, this organ can be safely considered an analogue of our hand, since it is directly involved in the capture and retention of prey. In addition, this manipulator can often be extended so that it exceeds the length of the hunter himself.

This is how snails can get their prey even from deep crevices and large shells. Once again, we remind you that it is from the proboscis in the victim's body that a strong poison is injected, the basis of which is chemically pure sulfuric acid (secreted from the "harmless" salivary glands). In a word, from now on you know exactly what the salivary gland is in molluscs and why they need it.

Question 1. Explain how molluscs are adapted to their environment.

Among the mollusks there are inhabitants of the aquatic and terrestrial-air environment.

Many both terrestrial and aquatic molluscs have a shell, which in both of them acts as a passive defense.

The most important adaptation to the terrestrial lifestyle in land mollusks is breathing with the lungs.

Aquatic molluscs are adapted to receive oxygen from water - they breathe with gills or the surface of the mantle.

Cephalopods have developed a special - reactive - mode of movement, very effective in the aquatic environment.

Question 2. What are the similarities and differences between gastropods and bivalve molluscs?

Gastropods and bivalve molluscs are multicellular animals, whose bodies do not have a segmental structure. The body of most species of these classes of molluscs is completely or partially covered with a shell. In gastropods, the shell is efficient, often asymmetric and twisted; in bivalves, it consists of two valves.

The head, torso and leg can be distinguished only in gastropods; in bivalves, the head is absent.

Gastropods have well-developed sense organs - touch, chemical sense, balance and vision. In bivalves, the sense organs are poorly developed.

Bivalves are exclusively aquatic animals, while among gastropods there are inhabitants of both aquatic and terrestrial-air environments.

Question 3. List the methods of protection from the enemies of the shellfish you know.

Some gastropods and almost all bivalve molluscs are able to fully or partially hide in the shell - this is a passive way of protection.

Squids, when rescuing from pursuit, can fly several tens of meters over the water. Material from the site

Some mollusks, for example, kara-katitsa, octopuses, in case of danger, are capable of changing color or emitting an ink substance produced in a special organ - an ink sac. Before using this protective agent, the mollusk first darkens, then ejects ink in a film shaped like its body. The pursuer grabs the ink "bomb" - the film breaks, the ink stains a large volume of water and paralyzes the enemy's sense of smell. This saves the life of the mollusk: after ejection of ink, it quickly turns pale and floats away almost invisibly.

Question 4. What allows cephalopods to move quickly in water and escape from enemies?

The rapid movement of cephalopods in the water is provided by the pulsating ejection of water from the mantle strip ( jet propulsion). This method of movement allows you to develop a significant swimming speed: squid - up to 40 km / h, octopuses - up to 15 km / h.

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On this page material on topics:

• movement in gastropods
• gastropods summary
• What is the difference between gastropods, bivalves and cephalopods
• organs and methods of movement of bivalve molluscs
• class cephalopods short biography

The type of molluscs 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 water. The science of shellfish is called malacology., she is studying the structure, development of soft-bodied, role in the surrounding world.

Structural features of Molluscs

External structure

The trunk is bilaterally symmetrical in bivalves and cephalopods, or asymmetric in gastropods. There are such sections of it: the head part with the organs of vision and tentacles, the body itself and the leg - muscle formation, serves for movement. 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 and serves as a place of muscle attachment. 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 cords of connective tissue. Most of the cephalopods lack shell.

A mantle, expelled by epithelial cells, departs from the lateral parts of the body. 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.

Molluscs 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 hemocele.

Internal structure

The digestive system of shellfish divided into three parts: the anterior, middle and hind intestines. Many representatives have a radula in the throat - a tongue designed for chopping food. It has chitinous plates with teeth. They use the radula to absorb bacteria or food. plant nature... Saliva is secreted into the pharyngeal cavity and glues food particles. The food then goes into the stomach, where the digestive gland (liver) opens up. After digestion, the residues are removed to the outside through anus.

Circulatory system open, in the heart there is a ventricle and usually two (rarely four) atria. From the bloodstream, the blood enters the sinuses and lacunas 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 the gills; in the inhabitants of the land - by the 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.

The 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 lifestyle of the 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 changeable conditions environment through the complication of the visual apparatus. They developed a peculiar type of accommodation, which was carried out by changing the distance between the retina and the lens.

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

Structural features of molluscs in comparison with annelids

What new organs have appeared in molluscs compared to worms?

Molluscs have specialized organs. This is an 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 not closed, annelids- closed.

The nervous system of molluscs looks like nerve ganglia connected with each other by nerve fibers. Annelids only have a nerve chain in the abdominal region, which branches out into segments.

How are molluscs adapted to their habitat?

Representatives of the type inhabit the expanses of water and the surface of the land. For existence outside the reservoir and breathing atmospheric air lung tissue appeared in soft bodies. Inhabitants of reservoirs receive O 2 with the help of gill arches.

How do molluscs protect themselves from enemies?

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

Poisonous and chemical substances(ink). Some are capable of burying themselves in a sandy bottom in seconds in the presence of a threat, or hide using a springy leg.

What is the function of a clam shell?

First of all, it is a supporting 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 bivalve molluscs

The value of molluscs in nature and human life

They are an integral part of the food chain. The soft-bodied ones are used by frogs, fish, birds. Seals eat cephalopods sea ​​stars- bivalve.

Water passes through the body of the mollusk and is purified from polluting factors. And shellfish, in turn, receive food particles from filtered water.

Soft-bodied valves are involved in the formation of sedimentary rocks.

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

A valuable raw material of jewelry - pearls - is formed between the shells. A pearl is formed after a foreign body enters. Since the muscles of the molluscs are underdeveloped, they cannot throw it out. In order 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 opened the flaps, foreign objects are placed under the mantle, and the mollusk is moved into a reservoir with favorable conditions for life and after three years pearls are obtained.

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

Agricultural pests - slugs, destroy grain crops, garden plants (potatoes, cabbage, tomatoes).

The flatworms, which cause disease in humans and animals, use molluscs as intermediate hosts.

Bivalve molluscs include about 20 thousand species. These are bottom sedentary animals. In rivers and lakes live toothless, pearl barley. The well-known sea mollusk mussel. Bivalve molluscs feed on small plankton and particles suspended in water, playing an important role in water purification.

External structure. The body of bivalve molluscs is oblong, bilaterally symmetrical, laterally flattened. There is no head (fig. 76). In the body, the torso is distinguished, and in many, the leg.

Rice. 76. Variety of bivalve molluscs: 1 - pearl barley; 2 - mussel; 3 - oyster; 4 - scallop

In the toothless, the leg is wedge-shaped and serves for movement in sand and silt. At the same time, the mollusk pushes the leg forward, then expands it, fixing it in the ground, and tightens the body (Fig. 77).

Rice. 77. The scheme of movement of the toothless

In a mussel, which leads an immobile lifestyle, the leg has lost its motor function. With special glands, the mussel secretes strong protein threads - byssus (from the Greek byssos - "thin yarn"), with which it is attached to the stones.

The body of bivalves is covered with a mantle, which hangs freely along the sides of the body in the form of two large folds. At the rear end of the body, the mantle often grows together and forms two tubes - siphons.

The outer side of the mantle folds forms a calcareous shell. In the toothless, its length can reach 10 cm, in the mussel - 20 cm. The shell consists of two symmetrical valves, covering the body from the sides. A short transverse ribbon of elastic material connects the valves on the dorsal side. The valves are closed by special muscles-closures. The toothless has two of these muscles, and the mussel has one. When the mollusk relaxes the muscles, the flaps open and remain half open.

In some mollusks, the edges of the valves on the dorsal side form outgrowths - teeth. This is a lock that reinforces the fastening of the sash. The toothless has no such outgrowths, for which it got its name. In toothless and mussels, the inner surface of the shell is lined with a durable shiny mother-of-pearl layer. Foreign particles (for example, grains of sand), falling between the mantle and the shell valve, are enveloped in layers of nacre and turn into pearls (Fig. 78).

Rice. 78. Diagram of pearl formation: 1 - shell; 2 - mantle (outer layer) 3 - grain of sand: 4 - pearl

Digestive system. Reduction of the head in bivalve molluscs led to the disappearance of many digestive organs that are in gastropods: pharynx, graters, jaws, salivary glands (Fig. 79).

Rice. 79. Internal structure of a toothless tooth in a longitudinal (A) and transverse (B) section: 1 - leg; 2 - mouth opening; 3 - esophagus; 4 - liver; 5 - stomach; 6 - intestine; 7 - heart; 8 - kidney; 9 - anus; 10 - gills; 11 - mantle; 12 - sink; 13 - ovary

The mouth, surrounded by two pairs of lobes, is located at the anterior end of the body, at the base of the leg. It leads into the short esophagus, which opens into the saccular stomach. The intestine descends from the stomach into the base of the leg, makes several bends and ends at the posterior end of the body with the anus.

Bivalve molluscs are filter feeding animals. They feed on plankton and small organic particles suspended in water. On the gills of these molluscs there are numerous, very small, constantly oscillating cilia. Their movement creates a flow of water in the mantle cavity: through the inlet siphon, water is sucked into the mantle cavity. Small food particles are brought in with the current of water. They are deposited by secreted mucus and sent to the oral lobes. The mouth blades free the food from inedible particles. Edible particles are sent to the mouth, inedible particles - through the siphon to the outside. Through it, excrement is also removed from the body. Bivalve molluscs can filter a large volume of water in a short time. For example, mussel filters up to 5 liters of water per hour.

Respiratory system. In toothless and mussels, the gills are lamellar. They are located under the mantle on either side of the animal's body. The flow of water brings (due to the work of the cilia) oxygen-rich water to the gills and carries out water rich in carbon dioxide.

Circulatory system in bivalve molluscs it is not closed. The toothless heart has two atria and one ventricle. Two large vessels originate from the ventricle - the anterior and posterior aorta, which split into a series of arteries. From the arteries, blood enters the system of cavities lying in the connective tissue. From them, it travels through the veins to the gills. The gills contain a dense network of the finest blood vessels (capillaries). Here the blood is enriched with oxygen and is directed through the vessels to the atria. The heart beats 3-20 times per minute.

Excretory system consists of two kidneys. The kidneys have the form of two large tubular, double-folded sacs, one side of which communicates with the pericardial sac (the remainder of the cenome), and the other with the mantle cavity. Harmful waste products are released into it and are removed from the body through an outlet siphon.

Nervous system. Consists of three pairs of nerve nodes (nerve ganglia) and numerous nerves extending from them. The ganglia are interconnected by nerve trunks. From the periphery, signals are transmitted along the nerves to the ganglia, and from them to the muscles.

Sense organs poorly developed as a result of the sedentary lifestyle of bivalve molluscs and head reduction. There are organs of balance. Oral blades serve as organs of touch. Tactile cells are also found in the leg, along the edge of the mantle, and in the gills. In some mollusks, the organs of touch are various tentacular appendages that develop along the edge of the mantle. At the base of the gill plates are the organs of the chemical sense. Some molluscs have eyes along the edge of the mantle. Very mobile scallops have over 100 of them.

Reproduction. Toothless and mussel are dioecious animals. Spermatozoa formed in the testes of males, through a siphon, enter the water and penetrate into the mantle cavity of females, where fertilization of eggs takes place. Successful fertilization is possible only with a large accumulation of molluscs.

In the mussel, a small larva emerges from the egg (Fig. 80). After a while, it turns into another larva called a sailboat. The sailboat floats for some time in the water column, then settles on a stone, rock, and other solid objects and gradually turns into a young mollusk.

Rice. 80. Larvae: 1 - mussels: 2 - toothless

Toothless larvae have denticles and sticky threads on the shell, with the help of which they attach to the gills and skin of fish swimming by. At the place of attachment of the larva on the body of the fish, a tumor forms, inside which a mollusk develops. After a while, it comes out and falls to the bottom. So, with the help of fish, the development and settlement of the toothless occurs.

Bivalve molluscs play a huge role in aquatic biocenoses, filtering water. Some aquatic animals feed on toothless.

Bivalve molluscs include animals of various sizes, ranging in length from a few millimeters to 1.5 m. And the mass of the largest bivalve mollusk - tridacna - can exceed 250 kg. Bivalve molluscs are widespread in the oceans. There are especially many of them in coastal shallow areas of warm seas. About 20% of all known species of bivalve molluscs inhabit fresh water, they do not occur on land. Bivalve molluscs, such as oysters, mussels, scallops, heart-shaped, have long been eaten by humans. Some of these molluscs, as well as pearl mussels, form mother-of-pearl and pearls. They are not only mined from the seabed, but also specially grown on sea farms, placing a grain of sand between the shell valve and the mantle.

Laboratory work No. 4

• Theme. The external structure of freshwater and sea ​​molluscs(optionally - item 2 or 3).
• Target. Establish the similarities and differences in the structure of mollusk shells.
• Equipment: tweezers, clam shells: scallop, mussel, pearl barley, toothless, horn coil, large pond snail, etc.

Progress

1. Consider scallop and mussel shells. Find out their similarities and differences. Explain the presence of ridges and depressions on the dorsal side of the shells. Pay attention to the shape and color of the outer and inner pearlescent shells.
2. Consider barley (or toothless) shells, define the front and back. Note the similarities and differences in the external structure. Determine the age of the molluscs by the tree rings located on the shell. Scrape off part of the stratum corneum with a scalpel until it is calcareous. Examine the inner pearlescent layer.
3. Consider the shells big pond snail and horny coil. Note the similarities and differences in the external structure of the shells. Count the number of turns in the curl of each shell.
4. Sketch one shell from each pair. Indicate in the figure the main parts of the external and internal structure shells. Write down the names of these parts.
5. Write the main features the shells of each clam. Explain which of them can be used to determine the habitat, age and lifestyle of the mollusk.

Bivalve molluscs are widespread in the seas. They are water purifiers-filter filters. Their body is enclosed in a bivalve shell. There is no head. Man uses these mollusks for food, extracts pearls and mother-of-pearl from them.

Exercises on the covered material

1. Name the representatives of the bivalve molluscs using Figure 76 (p. 107). What are the hallmarks of their external structure?
2. What are the layers of a clam shell? What substances are they formed by?
3. What are the features of the internal structure and life processes of bivalve molluscs? Explain with the example of toothless and mussels.
4. Describe the importance of bivalve molluscs in nature and human life.

What features of molluscs served as the basis for distinguishing three main classes in the type Molluscs?

Three main classes in the Molluscs type are distinguished based on the characteristics of the external structure of the organism.

What is the significance of molluscs in nature and human life?

Shellfish are an important link in food chains. Many of them are eaten. Among the molluscs there are filter feeders and corpse eaters, so they are orderlies of reservoirs. Bivalve molluscs are pearl producers.

Among the molluscs there are pests of gardens and vegetable gardens.

Questions

1. Explain how the molluscs are adapted to their habitat?

Most molluscs are aquatic, so their respiratory system is adapted to breathing in water. Many have gills. Gastropods and bivalves, which are unable to move quickly, have protective shells. Terrestrial shellfish secrete a large amount of mucus, which protects them from drying out.

2. What are the similarities and differences between gastropods and bivalve molluscs?

Both classes of molluscs are exclusively aquatic animals. Cephalopods, unlike bivalves, do not have a shell; they live only in salt water. Bivalve molluscs are not capable of rapid movement, do not have a head section and are characterized by a less developed nervous system.

3. How do molluscs protect themselves from enemies?

Bivalves and gastropods protect themselves from enemies by hiding in their shells. Cephalopods are able to move quickly, change color and throw ink stains.

4. What allows cephalopods to quickly move in water and escape from enemies?

The cephalopods move quickly through the pulsating ejection of water from the mantle cavity.

5. How do people use clam shells?

The shells of molluscs were used as a material for the manufacture of various tools: fish hooks, incisors, scrapers, hoe attachments. The shells themselves were used as vessels, as well as musical instruments (conch) and ornaments. The mother of pearl obtained from shells is used to make various products, for example, buttons, as well as for inlays. In some areas, shells served as money - for example, cowrie shells in the islands of Oceania.

Tasks

Suggest options for combating gastropods - pests of soda and garden crops, based on the features of the structure and life of these animals.

Mechanical means of control involve the collection of pests by hand, as well as the installation of traps for them. The most convenient way to collect snails and slugs is with tweezers. Since pests prefer dark and humid places during the day, appropriate traps are prepared for them. For example, cabbage leaves, burlap, rags or boards soaked in fruit juice or beer are laid between the beds and on the paths. During the day, pests will crawl into traps, and in the evening they will only have to be collected. In addition, such traps can be organized - shallow containers are dug in at ground level, filling them with strong saline or soapy water, and covered with burlap. Slugs die on contact with soapy or salty liquid. The fact that garden snails and slugs have a very soft body can also be used against them. To do this, it is enough to scatter dry porous material near the plants - crushed eggshells, shells or fine gravel. Since such a surface is unpleasant for molluscs, they are unlikely to get to plants. By the way, in this regard, pests really do not like lime and superphosphate, since these substances absorb mucus and moisture from their bodies, making it difficult to move. However, it should be noted that in rainy weather the effectiveness of such a remedy is reduced. Another insurmountable barrier to slugs and snails is water. You can create such a barrier for them with the help of plastic gutters filled with water. Again, they can be dug into the ground. Gastropods they are unlikely to wish to overcome these obstacles, and if they try, they will simply fall into the water and will not be able to get out of it.

Snails and slugs are deterred by smell spicy herbs- parsley, laurel, lavender, rosemary, thyme, santolin and sage. By planting them around the perimeter of the beds, you thereby protect the crop from many pests. In addition, special phyto-infusions can be prepared from garlic, mustard and bitter pepper, which will also scare away those who want to feast on your harvest.