From whom did gastropods originate. Message about gastropods

04.12.2019

This type of animal has about 130 thousand species. Mollusks live in fresh and marine waters, on land. They are known from the first half of the Paleozoic; they arose from polychaete rings as a result of the following aromorphoses:

the appearance of the mantle, mantle cavity and shell;
fusion of all segments of the body;
concentration of the nervous system in the ganglion;
the formation of the heart, consisting of the ventricle and atria.

The following idioadaptations contributed to the biological progress of molluscs:

the appearance of a shell;
the emergence of an apparatus for grinding food - radula;
the emergence of two forms of respiration - gill and pulmonary;
high fertility.

Features of the structure of mollusks:

bilaterally symmetrical animals, many of them have a pronounced asymmetry as a result of displacement of organs;
the body is not divided into segments;
secondary cavity animals; in general - residual, represented by the pericardial sac;
the body consists of a head, torso and legs - a muscular unpaired outgrowth abdominal wall, serving for the movement of the animal;
the body is surrounded by a skin fold-mantle. Between the mantle and the body is the mantle cavity. Gills and some sense organs are located in the cavity. The excretory, genital and anus openings also open here;
on the dorsal side of the body is a shell secreted by the mantle. Shells can be solid and bivalve and have a wide variety of shapes;
part digestive organs the liver enters, the ducts of which flow into the middle intestine;
respiratory organs - primary gills (ctenidia), secondary gills or lungs (under the mantle cavity);
excretory organs are the kidneys, communicating with the inner ends with the pericardial sac, and the outer ends opening into the mantle cavity;
nervous system diffuse-nodular type, four longitudinal nerve trunks depart from the peripharyngeal ring, carrying several pairs of ganglia;
the sense organs are represented by the eyes, the organs of smell, balance and chemical sense;
sexual reproduction; molluscs are mostly dioecious animals; there are hermaphrodites. A larva emerges from the eggs - a trochophore, resembling the larva of annelids. Sometimes a larva develops immediately - a veliger, which turns into adult form. There are forms with direct development, in which a small mollusk emerges from the egg.

Class Gastropoda

These animals live in sea and fresh waters, there are also terrestrial forms. Their sizes are different - from a few millimeters to 60 cm (sea hare).

Structural features of gastropods:

body differentiated into head, trunk, and sole;
the shell is entire, sometimes reduced;
the body is asymmetrical, which is associated with the reduction of the right organs of the mantle complex. Shells are twisted spirally or have the form of a cap;
the shell consists of a thin outer and porcelain-like layer - several systems of calcareous plates crossing each other at right angles, some have a mother-of-pearl layer;
sense organs are represented by tentacles, a pair of eyes, chemical sense organs, statocysts - balance organs;
the nervous system is well developed;
digestive system presented digestive tract and glands (salivary and liver). There is a radula in the throat. AT oral cavity distinguish "jaws" - horny or calcareous thickenings of the cuticle. The anus opens in front of the body;
respiratory system represented by ctenidia (gills), and in terrestrial forms - by lungs;
circulatory system open, formed by the heart and blood vessels. From the ventricle of the heart departs the aorta, which is divided into head and internal. The heart contains arterial blood. Gas exchange takes place in lacunae connective tissue. The blood becomes venous and returns to the gills, where gas exchange between the body and the external environment also takes place. Blood is colorless;
excretory system initially consists of a pair of kidneys, of which one is reduced;
reproduction. There are both dioecious and hermaphrodite. Most fertilization is internal. From the eggs in the lower gastropods, a larva develops - a trochophore, which turns into a veliger (sailboat). An adult mollusk develops from a sailboat. At higher development direct, flowing inside the egg.

Representatives of gastropods: grape snail, small and big pond slugs, slug.

Class Bivalve

This class contains about 1500 species.

The most progressive animals. There are about 700 species. The body of cephalopods is differentiated into the head, torso, and tentacles, into which the leg has turned. Their shell is underdeveloped, preserved only on the dorsal side. There is a radula. The ducts of the ink sac open into the hindgut. Animals breathe with gills. The circulatory and excretory system is similar in structure to the excretory system of bivalves and gastropods. Cephalopods have a well-developed nervous system and vision. The eyes are capable of accommodation. These are dioecious animals with pronounced sexual dimorphism. Their fertilization is internal, development is direct. All representatives of the class are predators. Cephalopods: squid, octopus, cuttlefish.

In zoological classification shellfish, or soft-bodied, belong to the type of invertebrates. The main tissue of their body consists of soft, rather loose cells, among which are located vital important organs these animals. The body of some species of molluscs is protected by a strong shell, sometimes with very beautiful shapes and patterns.

There are a lot of poisonous representatives among mollusks. These are both actively poisonous animals, such as cones, cephalopods, and others, and passively poisonous ones, which include some gastropods and many bivalves.

All cephalopods- inhabitants of the underwater kingdom. These are the most highly organized soft-bodied animals in evolutionary terms. They move along the bottom with the help of tentacles, which perform not only the functions of capturing food and protection from enemies, but also the function of movement.

The group of cephalopods includes nautilus, squid and octopus. These are very cautious and at the same time courageous animals. Typical cephalopods include the well-known cuttlefish. Cephalopods move backwards when moving. The cuttlefish has a special funnel for this purpose, located on the underside of the head. Through this funnel, water is sharply pushed outward, entering the body of the mollusk through another hole, and the animal makes a jerk in the direction opposite to the pushed out water, similar to the movement of a rocket.

cephalopods do not have a shell capable of restraining their movements. Among these truly amazing creatures of the animal world, there are also actively poisonous representatives. The tentacles of cephalopods (from 8 to 10) bear numerous suckers or hooks. In addition, the mouth is surrounded by strong horny jaws with a radula (a long elastic band with numerous teeth) and resembles the shape of a parrot's beak, its dimensions vary depending on the size of the mollusk.

B cephalopod venom both toxic proteins and non-protein toxins have been found. One of the first proteins isolated from posterior salivary glands squid, was Cephalotoxin. It has also been isolated from the venom of certain octopus species. A non-protein toxin, maculotoxin, has been isolated from the posterior salivary glands of small Australian octopuses. Administration of maculotoxin to animals causes their rapid death. It can be considered one of the components of octopus venom.

The main thing that can be wished to explorers is to avoid underwater caves, places chosen by octopuses, where they can hide. In extreme cases, you should put on cloth clothes over the diving suit, to which the octopus will not be able to stick and pull the person towards him. Never handle this animal with bare hands, no matter how small! In the case of a fight with an octopus, even a rather large one, it must be remembered that the most vulnerable spot on its body is the area between the eyes, where, when defending, a strong blow with a knife should be applied. And finally, it is necessary to know exactly what toxins are contained in the venom of cephalopods.

gastropods closed on top with a shell and have a large fleshy "leg". The head of the mollusk is visible in front, behind it the body is a wide fleshy “leg”, which has a flattened “sole” shape that changes from the substrate. With the help of such a “leg”, the mollusk slowly crawls along the substrate. It is enough to recall the well-known grape land snail and pond snails.

Among those actively poisonous to humans, some species of gastropods of the genus cone. There are up to 400 species of them. These animals prefer coral reefs and coastal shallows of the tropical zone of the Indian and Pacific Oceans from Polynesia to the eastern coast of Africa and the Red Sea.

bivalves also differ in appearance, and by the absence of an active poison-producing apparatus. These harmless sedentary animals lie on the bottom of the sea, covered to protect the body from above and below by two shells, which are attached to two muscle ligaments in front and behind.

They do not have a head as such, the front end of the body is covered with shells and has two tentacles in the form of blades, which, moving, drive food into the mouth opening leading through the pharynx to the stomach. Bivalve mollusks feed, as a rule, on plankton - these are small crustacean organisms that live in the seas, often at shallow depths, such as calcareous daphnia and cyclops, which lovers feed their pets.

The shells of molluscs are exceptionally beautiful, for example, mother-of-pearl. From the inside they are lined with a special soft cloth, called the mantle, which secretes a special substance that forms these shells. Under the mantle there is a mantle cavity, where water enters through special holes - siphons along with small crustaceans. It is here that the tentacle-blades begin to work, driving the crustaceans into the mouth opening. The mollusk can slowly move along the bottom. To do this, he has a special device - a thick muscular organ - a "leg", which, if necessary, protrudes from the shell. bivalves are typical secondary venomous animals, since their toxicity, often very strong, depends on the food they receive.

The first thing to mention when discussing class gastropods So this is their diversity. Nearly one hundred thousand various kinds. There are so many of them that these invertebrates live in salty sea waters, having chosen both solid depths and shallow waters, and in fresh rivers, and even on the ground, and you can meet them not only in green thickets, but also in deserts and rocks.

Boast gastropods can also be a variety of sizes. Some individuals do not grow more than 2 millimeters. Others are able to reach up to half a meter. They do not live long: from a couple of months to three years.

These creatures are madly in love with a humid environment, and the air must also be humidified. If they suddenly get too hot or dry, gostropods spew a lot of mucus, so their shell and its contents become protected from drying out. After that, the snails are attached to the vegetation and remain in this state until they again fall into suitable conditions. The favorite places of these creatures are dense grass thickets.

If we consider a typical representative of the class, then this is a snail that has: a body (wider in front and tapering towards the opposite end, a hump-shaped growth on the upper part), a head (a pair of tentacles and eyes on it) and a leg (dense, ends with an extension, foot-like).

All this is covered by a sink. Its shape is variable: from twisted to cone-shaped, and even a flat bowl, but always solid, without flaps. But there are individuals in which this element is not developed, i.e. completely absent, we are talking about slugs. And for example, in marine life, this part has much more modest dimensions.

If the animal is not in danger, it puts only the body in its shell. If the snail has sensed danger, the shell becomes its refuge, capable of accommodating the owner entirely. Another difference from other molluscs is the loss of bilateral symmetry.

Those. if some animals have a pair of kidneys, a pair of gills, etc., then structure of gastropods does not imply this, their organs are quite capable of functioning without a “partner”. All this is the result of the presence of a spiral shell. Invertebrates do not have hearing and voice; they are guided by touch and smell.

Structure

Let's start with the head. It can be called pronounced for mollusks. In addition, there are tentacles on this part of the body, there may be a pair, or two pairs, they have the ability to retract. The eyes sit either on the head itself or on the ends of the "horns". If it is a predator, then the mouth is placed, as a rule, on an elongated proboscis. If necessary, it unwinds outward.

The body of the mollusk is an elongated pouch, on the upper part of which rises a growth twisted in a spiral. The leg is also a part of the body, the animal uses it for movement. Wave-like movements of the sole cause the individual to move. In order to make this process easier, a special is allocated. slime. Keeping balance helps the features of the structure of the legs.

When it was possible to get food, it enters the stomach and intestines. These mollusks are able to break down food with the help of liver enzymes. After the intestine, already processed food exits through the anus, which, as a rule, exits to the right side. The kidneys perform the function excretory system. There can be two of them (if we are talking about the simplest organisms), or one.

A mantle is located above the body of the gastropods. In the cavity between it and the body of the individual are certain organs. Animals that live underwater have gills. Some of them have two, but mostly invertebrates are equipped with one gill (they can be located either in front of the body or in the back).

When such an animal is frightened and drawn into the shell, its mouth is closed with a small lid. If in front of you is a terrestrial creature, or periodically changing its habitat, then the respiratory gastropod system represented by one lung. In this case, when the mollusk hides in the shell, its mouth remains open.

There are also those who live on land, while storing water in the mantle cavity, and using gills for breathing. Such individuals perceive smells and tastes with the help of tentacles. Simpler and smaller varieties have no gills at all. They breathe through their skin. The heart beats next to the lung. There are no blood vessels, the blood circulates freely throughout the body. She is colorless.

From the glands with which the mantle is strewn, a substance is released due to which the shell of animals grows. Her curl can be twisted both to the right and to the left. They cannot reset this “accessory”. It is attached to the body with very strong muscles that allow the mollusk to be drawn in if something happens.

The top of the shell is its oldest part. It consists of lime salts. The shell grows most actively in the warm season, while in winter, active growth slows down. This is due to the fact that the animal does not eat as densely in cold weather, and it does not have enough substances in the body to ensure the increase in the size of its "home".

Annual lines are visible on its surface; they can be used to recognize the age of the mollusk. Sometimes the shell turns into a real underwater flower bed, if the individual does not lead a very mobile lifestyle, it simply becomes overgrown with algae.

In principle, this is beneficial to the invertebrate, because plants contribute to the flow of more oxygen into its body. If the snail has chosen the bare bottom of the reservoir as a refuge, the shell may become overgrown with silt. There are also naked individuals, i.e. those who do not have a "house". These are most often those who have learned to swim in the process of evolution, for example, pteropods, or those who burrow into the ground.

Note that nervous system of gastropods, like the rest of the structure, is in close dependence on torsion. This is the name of a feature of gastropods, which implies that the shell is twisted in the opposite direction, like the body of a mollusk. The nerve ganglions of gostropods are concentrated in the anterior region of the body. And sensitivity is developed over the entire surface of the skin.

And now about reproduction, in invertebrates it occurs only sexually. These creatures reach sexual maturity on average by six months. Among the snails there are hermaphrodites (more often land-based, or living in fresh water) and dioecious (more common in the seas). If we are talking about the former, then, according to the observations of many scientists, during mating, fertilization of both individuals occurs.

After the male's germ cells enter the female's genital opening, new life may not occur immediately. The female is able to delay the process of fertilization, keeping the sperm inside herself.

When this does happen, the invertebrate lays eggs, from which already formed tiny snails, or larvae, are born. However, many publications say that among these animals there are also viviparous. To be precise, the snail does not lay eggs and leaves them inside the body until the cubs hatch from them.

Nutrition

So they call something like a tongue, which is strewn with small chitinous teeth. When this device comes into contact with plants and rubs against them, the sharp protrusions scrape off the top layer from the greenery. The same thing happens when a snail glides over sunken stones, only then it scrapes off various kinds of microorganisms that have stuck to the cobblestones.

Predators have a specific structure of the radula (grater): some of the cloves peep out of the mouth, they are able, like spikes, to stick into the body of the victim, after which they inject poison. A similar scheme works, for example, when their brethren, bivalves, become food for gastropods.

First, the predator makes a hole in their valves, for this he uses saliva, but not ordinary, but with sulfuric acid. Herbivores simply bite on algae and rotting vegetation. This, by the way, is an important role of gastropods in the ecosystem.

Kinds

If we consider types of gastropods, it is worth noting that they are divided into three subclasses:

Anteriobranchial

The most numerous group, with a well-developed, usually spiral shell. But their main distinguishing feature gills in front of the heart. They have an amazing ability to spew pheromones, thereby attracting individuals of the opposite sex. They eat mainly bivalves, do not disdain echinoderms and coelenterates. Next, we will talk about some representatives of the subclass:

Abalone

The mollusk was nicknamed so for its specific shape; its shell is very similar to a real human ear. Medium-sized invertebrate, its "house" grows up to two tens of centimeters. And from the inside it is covered with an iridescent layer of mother-of-pearl.

This feature has turned the sea creature into a craft item, because it makes popular souvenirs. Infrequently, but still, very rare and beautiful pearls are found in multicellular shells, they have an iridescent color, with green and purple tints.

In addition, the ear is actively eaten, like all delicacies, it costs a lot of money. All this leads to a decrease in the population. In some countries, for example, in New Zealand, they have already introduced a limit on the catch of molluscs. This family includes as many as seven dozen various forms individuals.

Prefers warm sea water, and lives there. Desalination of water means certain death for the ear. A solid substrate, for example, stones, is chosen as a permanent place of residence. In order to sit in the right place, they use their powerful leg.

Moreover, such a mount is so strong that the miners of delicacies have to use a knife to tear the mollusk off the base. The gills of an invertebrate are located in the mantle cavity.

The water that enters there supplies oxygen, and then exits through the holes with which the edge of the shell is strewn. Activated at dusk and at nightfall. Fertilization in them occurs outside the body of the individual, i.e. female and male sex cells are found in the water column.

Trumpeter

It has a helical and slightly elongated shell. We often meet such people on the coast. The perfect place for their life - a cool sea. They settle at the bottom, but not at extreme depths, and move slowly as needed. If he is just walking, he overcomes only 10 sentiments of the way in a minute, but if he is looking for food, he can double his speed.

15 centimeters - this is the average height of the "house" of the snail. Inside it is perfectly smooth, without any notches. Previously, people made horns from it, now souvenirs. The mollusk is also valued by man for the taste of its meat. It also has no calories. Most trumpeters are eaten in Asia.

However, if we are talking about a giant trumpeter, then this mollusk is considered the largest among marine life. What can I say if its weight reaches 20 kilograms. An invertebrate has not two gills, but one. She extracts oxygen from a filtered special. water siphon. The same organ is for touch.

Trumpeters are eaten by starfish, crabs and even walruses. The snails themselves feast on carrion and rot. But in which case it can also manifest itself as a predator, eating worms and even small fish. Also among his favorites are bivalves.

For example, with the meat of a whole mussel, this snail cracks down in a couple of hours. A very long proboscis, which ends in a mouth, reaches inaccessible places, and even helps to scrape food from its own shell. Radula. If necessary, protrudes from the throat and grinds food before it enters the goth.

These individuals are dioecious. In the first days of summer, the breeding season is considered open. After mating, the snails lay eggs, which are sealed in a special. capsule. These pouches are attached to something solid, such as corals. If initially there are about a hundred eggs, then over time there will be no more than six of them. To get into open water. A small snail needs to gnaw through the walls of the capsule.

Rapany

Once they could be found only in the Sea of Japan, but now these snails are widespread everywhere, especially in the Black Sea. Rapans are active predators. Their diet is bivalve brethren. They can lead a sedentary lifestyle, or slowly move along the bottom. They usually hibernate buried in the sand.

Their shell is very specific, as it is covered with several cone-shaped protrusions that look like spikes. Most often it is beige with brown stripes, and the inside is a bright carrot color. This makes it more attractive to a person, because the shell goes, usually for souvenirs.

Triton horn (charonia)

A large gastropod, the height of the cone-shaped shell of which reaches as much as 50 cm. On the back side it has a sharp narrowed end, on the front side there is a wide almond-shaped mouth, in front of which there is the largest and convex curl. The yellowish shell is covered with brown spots.

You can meet the mollusk in tropical seas. Deep water is not for him, but the reefs are a favorite place. Unlike its counterparts, which become the prey of starfish, this predator eats them itself. And this is its great value for the biosphere. After all, the stars simply destroy the most beautiful coral reefs, eating everything in their path.

Marisa

It looks like a classic snail with a beige spiral shell with darker veins. They live in fresh and warm water bodies. The dimensions are not large - the shells are about 5 cm in height, but the “horns” are most often even longer. The second pair of shorter tentacles is located below, next to the eyes. The body of an invertebrate is also light, white, or yellowish.

Snails are extremely picky about food: algae, rot, alien caviar and carrion are fed. You can distinguish a female invertebrate from a male by the color of the legs. For "girls" it is dark brown, and for "boys" it is light beige.

In order to lay eggs, the mollusk finds a suitable leaf of some plant, and places the eggs under it. From eggs, not larvae, but small mollusks are born. The older, the more vertically flattened it becomes gastropod shell.

Viviparous (lawn)

These freshwater need cold water and silt at the bottom of a reservoir, whether it be a lake or a river. The shell in the form of a cone twisted to the right has 5-6 curls, a lid and a chocolate color. Invertebrates live up to 6 years.

The female bears three dozen cubs in herself at once, not caviar comes out of her body, but already full-fledged snails. They still have a transparent shell and special. protective shell that disappears over time.

Murex

The intricate shells of these mollusks have not only pimples, spikes and protrusions, but also an interesting color, often ashy white with pinkish lines. 30 centimeters - this is the approximate size of the shells of the most major representatives kind. These invertebrates live in the seas around the globe.

And if now they are mined solely for the purpose of decorating residential premises, but in the old days these snails were destroyed by the millions with one sole purpose - to get purple. To get at least a gram of this dye, you need to ruin thousands of mollusks. They used paint to make clothes for the nobility, paint pictures and as ink.

tilomelania

This bright yellow snail has an almost black, elongated spiral shell. It is small - about 10 centimeters, while the shell can have up to 10 curls. This inhabitant of the lakes is a scavenger.

Belongs to the type of viviparous. A couple of babies are born, up to 1 centimeter in size. If they are gastropods in nature, then lives up to 5 years, but if you put it in an aquarium, life expectancy can increase by 2 times.

Pulmonary

These creatures flooded fresh waters, but are most often found on land. Their shells can be of the classical form of a spiral, and in the form of a flat plate, and even completely absent. On the leg of these snails there is a special gland, from which mucus is secreted in large volumes. The latter is necessary for them to glide smoothly in the process of movement. If the mollusk lives on land, it usually has two pairs of tentacles on its head. If the animal lives in fresh waters - one pair.

Home them distinguishing feature- the free edge of the mantle fuses with the body of the individual from the front side. The mantle cavity, in which there are no gills, but a lung is located (with the help of which the animal breathes), communicates with the environment with the help of a small remaining hole. Through this passage, oxygen enters the lung. This means that aquatic inhabitants have to periodically emerge in order to take in air.

All lung molluscs are hermaphrodites.

Achatinids

Achatina giant is the largest land snail. Its weight reaches a quarter of a kilogram, and the length of the shell grows in some cases up to 30 centimeters. The eyes of the animal sit at the ends of the first pair of tentacles. The mollusk eats everything vegetable - this is both grass and various fruits.

To produce offspring, this snail does not need a partner. She has both male and female sex cells. True, self-fertilization most often occurs if there are few other individuals around. If invertebrates come into contact, then eggs are often born in both individuals. But a lot depends on the size of the mollusks. Such a scheme only works for those who are of the same size.

If the individuals are of different sizes, then the mother is more likely to be the one that is larger. Spermatozoa can live in the animal's body for up to 2 years, and gradually fertilize the eggs. Every year the creeping creature spawns up to 6 times. At one time, this is about a hundred eggs. These white balls are placed in a pre-prepared hole. Mollusks can become sexually mature as early as six months.

This species of snail is popular as a pet.

Prudoviki

If you look at it from above. Then you can see that on one side the shell, which is a twisted cone, is round, and on the other it is thin and sharp. Non-salty water bodies are their favorite homes. At the same time, the pond snail does not like currents, he needs stagnant water. Their age is short - only 9 months, although in captivity they can live up to two years.

Small triangular tentacles are visible on the large head. They cannot boast of a bright color, these are most often marsh and brown shades.

The diet contains plant foods, but they will not refuse, or fish eggs. In 60 seconds, such a snail moves 20 centimeters. Most often, she does not sit still, but is busy with something. And they also have the ability, rare for such mollusks, to swim. To do this, the pond snail turns upside down and bends it.

During the day, the pond snail floats to the surface of the reservoir at least 6 times, all in order to take air into the lung. If suddenly the pond snail is on land, or its pond is covered with a crust of ice, it can perfectly wait for the right conditions by sealing its shell with a special film. Aquarists don't like them very much. types of gastropods, all because of gluttony and fertility.

Posterior branchial

They have a long flattened body. The leg of these individuals can grow peculiar fins (their shape resembles sunflower petals), which help the animal to move not along the bottom, but to swim freely. The posterior gills inhabit mainly the seas. The mantle organs are located on the back side of the body, respectively, and the gills of mollusks are in the same place. The shell can either overgrow and be completely covered with a mantle, or simply be reduced. These are the most unusual-looking gastropods.

Glaucus

It looks more like an exotic fish, it is also called the "blue dragon". A long body, on the sides of which there are several processes similar to fins. By the way, body gastropod has a bright blue, very beautiful color. But this inhabitant of the water does not have a shell. It does not crawl along the bottom, but floats near the surface of the water, taking in air. The animal is small: from a couple of centimeters to five.

Glaucus is very poisonous, it is dangerous not only for those who want to eat it, but also for its victims. It feeds on other types of molluscs. Although this slug is a hermaphrodite, it cannot fertilize itself. By the way, for a person this unusual creature does not pose any danger.

Sea hare (aplysia)

This exotic animal does not have a shell, but there is a dense beige (sometimes purple, brown, circled, or speckled) little body, along the back of which a kind of scallop runs.

Slug horns are very interestingly twisted, resembling rabbit ears. From below, the hare has two blades, thanks to this device, it can easily swim for considerable distances. In his diet only bryozoans. Settles in rocky areas. In case the mollusk is afraid of something, it spews purple ink.

sea slug

To obtain nutrients. This slug does not need food, it is capable of photosynthesis. For this, he needs to say thanks to the special algae that he eats, after which he “steals” their abilities. It looks like a slug green leaf tree, which, moreover, has a snail's head.

Meaning

Without gastropods, there will be a real mess in the reservoirs. notice, that the meaning of gastropods great. Not only do they eat up rotten plants, they also prevent the overgrowth of lakes, rivers, swamps and seas. Land snails are able to enrich the soil with minerals. But some types of molluscs, on the contrary, are harmful. For example, slugs destroy crops.

In addition, these creatures take their place in the food chain; some species of fish and whales cannot live without them. Do not mind eating them and a person. In addition, good crafts and decorations are obtained from shells.

Which live not only in reservoirs, but also on land. That is why gastropods are the most numerous class in terms of the number of species. Gastropod sizes are relatively small - a maximum of large tropical species can reach 60 cm.

A typical representative of the Gastropod class, which almost every one of us has met, is big pond snail. This gastropod lives in stagnant reservoirs, ponds, lakes, backwaters. The body of the Greater Pond Snail can be divided into the head, trunk and leg, which occupies the entire surface of the abdomen, which gave the name to the class.

Body of a pond snail twisted in a spiral sink and covered mantle. The pond snail moves by stretching and contracting the leg muscles. At the bottom of the head, the pond snail has a mouth, and on the sides there are tentacles, at the base of which there are eyes.

The diet of the pond snail is plant food. In the pharynx is a muscular tongue, which has many teeth on the underside. The pond snail uses these teeth to scrape the soft tissues of the plants it eats. After swallowing, food passes through the pharynx and esophagus to the stomach. Digestion of food in the body of a pond snail occurs in three stages: first in stomach, then in liver and the process ends in intestines. Through anus undigested food remains are excreted.

Respiratory system The pond snail consists of a lung - this is the cavity of the mantle, where air enters through the respiratory hole. In the process of breathing, the pond snail rises to the surface to draw air into this cavity. the surface of the lung is covered with many blood vessels, due to which the blood is saturated with oxygen and carbon dioxide is released.

Circulatory system pond snail consists of two-chambered heart(atrium and ventricle) vessels and capillaries. The circulatory system of the pond snail is not closed, since through the capillaries the blood goes directly into the cavity with internal organs. From there, saturated with carbon dioxide, it is collected in a vessel leading to the lung, where carbon dioxide is released and oxygen enters. From there, enriched (arterial) blood enters the atrium, passes into the ventricle and is ejected into the arteries, capillaries and further in the same circle.

Excretory system of the pond snail presented kidney through which blood passes and is cleared of toxic metabolic products. These substances are then excreted through the excretory opening, which is located next to the anal.

Fertilization in pond snails going on cross type, despite the fact that pond snails- hermaphrodites. They lay their eggs on the surface of aquatic plants. Young growth immediately develops from eggs, the type of development of the pond snail is direct.

The gastropods also include slugs, which got their name due to the lack of a shell, which forces them to secrete a huge amount in order to avoid drying out. Slugs live in damp places, but on land. They feed on mushrooms, plants, such as cabbage and other crops, which spoils the crop.

The most notable are marine gastropods, many species of which have beautiful shells used as souvenirs, as well as for buttons (mother-of-pearl layer). Some peoples of Asia and Africa even make money and jewelry from the shells of small kauri mollusks.