Post organs excreting molluscs. Type and classes of molluscs. General characteristics of molluscs. What is the salivary gland in molluscs? Give definitions of concepts

They will be discussed in this article. There is also a special branch of biology that studies this group of animals. It is called malacology. And the science that studies the shells of molluscs is conchiology.

General characteristics of molluscs

This type is also called soft-bodied. They are quite varied. The number of species is about 200 thousand.

This group of multicellular animals is divided into eight classes:

• Bivalve.
• Armored.
• Grooved belly.
• Pit-tailed.
• Monoplacophores.
• Gastropods.
• Shovellegs.
• Cephalopods.

The body of all these animals is structured according to the same principle. Next, the characteristics of molluscs will be considered in more detail.

Organ systems and organs

Molluscs, like many multicellular animals, are built from different types of tissues that make up organs. The latter, in turn, form

The structure of molluscs includes the following systems:

• circulatory;
• nervous system and sensory organs;
• digestive;
• excretory;
• respiratory;
• genital;
• body integuments.

Let's take a look at them in order.

Circulatory system

In molluscs, it is of an open type. It includes the following bodies:

• heart;
• vessels.

The heart in molluscs consists of two or three chambers. This is one ventricle and one or two atria.

Many soft-bodied blood has an unusual bluish color. This color is given to it by the respiratory pigment hemocyanin, the chemical composition of which includes copper. This substance has the same function as hemoglobin.

The blood in molluscs circulates in this way: from the blood vessels it pours out into the gaps between the organs - lacunae and sinuses. Then it again collects into the vessels and goes to the gills or lung.

Nervous system

In mollusks, it is of two types: ladder and scattered-nodal type.

The first is built in this way: there is a periopharyngeal ring, from which four trunks extend. Two of them innervate the leg, and the other two innervate the insides.

The nervous system of the scattered-nodal type is more complex. It consists of two pairs of nerve circuits. Two abdominal ones are responsible for the innervation of internal organs, and two pedal ones are responsible for the legs. On both pairs of nerve circuits, nodes are present - ganglia. There are usually six pairs of them: buccal, cerebral, pleural, pedal, parietal, and visceral. The first innervate the pharynx, the second - the tentacles and eyes, the third - the mantle, the fourth - the leg, the fifth - the respiratory system, the sixth - other internal organs.

Sense organs

There are such organs of mollusks that allow them to receive information about the environment:

• tentacles;
• eyes;
• statocysts;
• osphradia;
• sensory cells.

The eyes and tentacles are located on the head of the animal. Osphradia are found near the base of the gills. These are the organs of the chemical sense. Statocysts are organs of balance. They are on the leg. Sensory cells are responsible for touch. They are located on the edge of the mantle, on the head and leg.

Digestive system

The structure of molluscs provides for the presence of the following organs of this tract:

• pharynx;
• esophagus;
• stomach;
• midgut;
• hind intestine.

The liver is also present. We also have a pancreas.

In the throat of the soft-bodied there is a special organ for grinding food - the radula. It is covered with chitin teeth, which are renewed as the old ones are worn down.

in shellfish

This system is represented by the kidneys. They are also called metanephridia. The excretory organs of molluscs are similar to those of worms. But they are more complicated.

The excretory organs in molluscs look like a collection of sinuous glandular tubes. One end of metanephridium opens into the coelomic sac, and the other - outward.

The excretory organs in molluscs can be present in different quantities. So, some cephalopods have only one metanephridium, located on the left side. In monoplacophores, as many as 10-12 excretory organs are observed.

Waste products accumulate in the metanephridia of mollusks. They are represented by lumps of uric acid. They are excreted from the body of the animal every two to three weeks.

Also part of the excretory system in molluscs can be called the atria, which are responsible for filtering blood.

Respiratory system

In different mollusks, it is represented by different organs. So, most soft-bodied gills are present. They are also called ctenidia. These are paired bilaterally cirrus organs. They are located in the cavity of the mantle. Molluscs that live on land have a lung instead of gills. It is a modified mantle cavity. Its walls are riddled with blood vessels.

Skin respiration also plays an important role in the gas exchange of molluscs.

Reproductive system

It can be arranged in different ways, since among the mollusks there are both hermaphrodites and dioecious species. In the case of hermaphroditism during fertilization, each individual acts simultaneously as both a male and a female.

So we examined all the organ systems of mollusks.

Shells of the body of molluscs

The structure of this element differs among representatives of different classes.

Let's look at the different types of body integument that molluscs can possess, examples of animals that belong to one class or another.

So, in furrow-bellied and pit-tailed integuments are represented by a mantle, which covers the entire body, with a cuticle consisting of glycoproteins. Also present are spicules - a kind of needles that are composed of lime.

Bivalves, gastropods, cephalopods, monoplacphores and shovellegs lack a cuticle. But there is a shell, which consists of one plate or two in the case of bivalves. In some orders of the gastropod class, this part of the integument is absent.

Features of the structure of the shell

It can be divided into three layers: outer, middle and inner.

The outside of the sink is always built from an organic chemical. Most often it is conchiolin. The only exception to this rule is the mollusk Crysomallon squamiferum from the gastropod class. Its outer shell layer consists of ferrum sulfides.

The middle part of the shell of the molluscs consists of columnar calcite.

The inner one is made of lamellar calcite.

So we examined in detail the structure of mollusks.

Conclusion

As a result, we will briefly consider the main organs and organ systems of soft-bodied bodies in the table. We will also give examples of molluscs belonging to different classes.

The structure of molluscs

System	Organs	Peculiarities
circulatory	vessels, heart	type, the heart is two- or three-chambered.
nervous	nerve circuits and ganglia	Two nerve circuits are responsible for the innervation of the leg, two - for the internal organs. There are five pairs, each of which is attached to specific organs.
digestive	pharynx, esophagus, stomach, intestines, liver, pancreas	There is a radula in the throat, which helps to grind food. The intestine is represented by the middle and hind gut.
excretory	metanephridia	Glandular tubes, one end of which opens outward and the other into a coelomic sac.
respiratory	gills or lung	Located in the cavity of the mantle.
genital	ovaries, testes	Among molluscs there are hermaphrodites, which have both male and female sex glands at the same time. There are also dioecious species.

Now we will consider representatives of various classes of the Molluscs type and the features of their structure.

Class	Examples of	Peculiarities
Bivalve	Mussels, oysters, Japanese scallop, Icelandic scallop	They have a shell of two plates, consisting of calcium carbonate, have well-developed gills, and are filter feeders by the type of feeding.
Gastropods	Pond snails, slugs, coils, snails, bitinia	They have an asymmetrical internal structure due to a swirling shell. On the right side, the organs are reduced. So, many species lack the right ctenidium
Cephalopods	Nautilus, squid, octopus, cuttlefish	They are characterized by bilateral symmetry. The outer shell is absent in these molluscs. The circulatory and nervous systems are the most well developed of all invertebrates. The sense organs are similar to those of vertebrates. The eyes are especially well developed. The excretory organs of mollusks of this class are represented by two or four kidneys (metanephridia).

So we examined the structural features of the main representatives of the Molluscs type.

The type of molluscs, numbering about 130,000 species, is second only to arthropods in terms of the number of species and is the second most abundant type of animal world. Molluscs are predominantly aquatic; only a small number of species live on land.

Molluscs have a variety of practical uses. Among them there are useful ones, like pearl oyster and pearl barley, which are mined for the purpose of obtaining natural pearls and mother-of-pearl. Oysters and some other species are mined and even bred for food. Some species are agricultural pests. From a medical point of view, molluscs are of interest as intermediate hosts of helminths.

General characteristics of the type

Animals belonging to the type of molluscs are characterized by:

• three-layer, i.e. formation of organs from ecto-, ento- and mesoderm
• bilateral symmetry, often distorted due to organ displacement
• unsegmented body, usually covered with a shell, whole, bivalve, or consisting of several plates
• fold of skin - a mantle that fits the whole body
• muscular outgrowth - the leg that serves for movement
• poorly expressed coelomic cavity
• the presence of the main systems: the apparatus of movement, digestive, respiratory, excretory, circulatory system, nervous and reproductive

The body of mollusks has bilateral symmetry; in gastropods (for example, the pond snail belongs to them) it is asymmetric. Only the most primitive mollusks retain the signs of segmentation of the body and internal organs; in most species, it is not divided into segments. The body cavity is secondary, presented in the form of a pericardium and a cavity of the gonads. The space between the organs is filled with connective tissue (parenchyma).

The body of molluscs consists of three sections - head, trunk and legs. In bivalve molluscs, the head is reduced. The leg - a muscular outgrowth of the abdominal wall of the body - is used for movement.

At the base of the body, a large skin fold is developed - the mantle. Between the mantle and the body there is a mantle cavity, in which the gills, sensory organs are located, openings of the hind gut, excretory and reproductive systems open here. The mantle gives off a shell that protects the body from the outside. The shell can be solid, bivalve, or consist of several plates. The shell contains calcium carbonate (CaCO 3) and organic matter conchiolin. In many mollusks, the shell is more or less reduced (for example, in some cephalopods, in naked slugs, etc.).

The circulatory system is not closed. The respiratory organs are represented by gills or lungs formed by part of the mantle (for example, in pond snails, grape and garden snails, naked slugs). The excretory organs - the kidneys - are connected with the inner ends to the pericardial sac.

The nervous system consists of several pairs of nerve nodes connected by longitudinal trunks.

The type of molluscs includes 7 classes. The most important of them are:

• gastropods (Gastropoda) - slowly crawling snails
• bivalves (Bivalvia) - relatively sedentary molluscs
• cephalopods (Cephalopoda) - motile molluscs

Table 1. Characteristic features of bivalve and gastropod molluscs
Sign	Class
	Bivalve	Gastropods
Symmetry type	Bilateral	Asymmetric with reduction of some right organs
Head	Reduced together with related organs	Developed
Respiratory system	Gills	Gills or lung
Sink	Bivalve	Spiral wound or cap-shaped
Reproductive system	Split-sex	Hermaphrodite or dioecious
Nutrition	Passive	Active
Habitat	Marine or freshwater	Marine, freshwater or terrestrial

Class Gastropoda (Gastropoda)

This class includes shellfish (snails). Its height ranges from 0.5 mm to 70 cm. Most often, the shell of gastropods has the form of a cap or a spiral, only representatives of one family develop a shell of 2 valves connected by an elastic ligament. The structure and shape of the shell are of great importance in the taxonomy of mollusks. [show] .

1. Placospiral shell is a strongly twisted shell, the revolutions of which are located in the same plane
2. Turbospiral shell - shell revolutions lie in different planes
3. Right-handed shell - the shell spiral turns clockwise
4. Left-handed shell - the spiral is twisted counterclockwise
5. Hidden spiral (involute) shell - the last turn of the shell is very wide and completely covers all previous ones
6. Open-spiral (evolute) shell - all revolutions of the shell are visible

Sometimes the shell has a lid located on the dorsal side at the back of the leg (for example, in lawns). When the legs are pulled into the shell, the cap tightly covers the orifice.

In some species that have switched to a swimming mode of life (for example, pterygopods and keeled legs), the shell is absent. Shell reduction is also characteristic of some terrestrial gastropods living in soil and forest litter (for example, slugs).

The body of gastropod molluscs consists of a well-separated head, legs and trunk - an internal sac; the latter is placed inside the sink. On the head are a mouth, two tentacles, and at their base there are two eyes.

Digestive system. At the front end of the head is the mouth. It has a developed powerful tongue covered with a hard chitinous grater, or radula. With its help, mollusks scrape off algae from the soil or aquatic plants. In carnivorous species, a long proboscis develops in the front part of the body, which is able to turn out through an opening on the lower surface of the head. In some gastropods (for example, cones), individual teeth of the radula can protrude from the mouth opening and have the form of stylets or hollow harpoons. With their help, the mollusk injects poison into the victim's body. Some of the predatory gastropod species feed on bivalve molluscs. They drill out their shells, releasing saliva containing sulfuric acid.

Through the esophagus, food enters the saccular stomach, into which the liver ducts flow. Then the food enters the intestine, which bends in a loop and ends on the right side of the body with the anal opening - the anus.

Nerve nodes are collected in the periopharyngeal nerve ring, from which nerves extend to all organs. On the tentacles are tactile receptors and organs of the chemical sense (taste and smell). There are organs of balance and eyes.

In most gastropods, the trunk protrudes above the leg in the form of a large, spirally twisted bag. Outside, it is covered with a mantle and closely adheres to the inner surface of the shell.

Respiratory organs of molluscs are represented by gills located in the front part of the body and directed apex forward (prosobranch) or located in the right rear part of the body and directed apex backward (prosobranch). In some gastropods (for example, nudibranchs), the true gills are reduced. As respiratory organs, they develop the so-called. cutaneous adaptive gills. In addition, in terrestrial and secondary-water gastropods, part of the mantle forms a kind of lung, numerous blood vessels develop in its walls, and gas exchange takes place here. A pond snail, for example, breathes atmospheric oxygen, so it often rises to the surface of the water and opens a round breathing hole on the right at the base of the shell. Next to the lung is the heart, which consists of the atrium and ventricle. The circulatory system is not closed, the blood is colorless. The excretory organs are represented by one kidney.

Among gastropods, there are both dioecious species and hermaphrodites, the sex gland of which produces both sperm and eggs. Fertilization is always cross, development, as a rule, with metamorphosis. All terrestrial, freshwater and some marine gastropods show direct development. The eggs are laid in long, slimy filaments attached to moving objects.

Belongs to the class of gastropods

• Common pond snail, often found on aquatic plants in ponds, lakes and rivers. Its shell is solid, 4-7 cm long, spirally twisted, with 4-5 curls, a sharp apex and a large opening - the mouth. A leg and head can protrude through the mouth.

  The intermediate hosts of trematodes also belong to gastropods.

• The intermediate host of the feline fluke, Bithynia leachi, is widespread in the freshwater bodies of our country. Inhabits the coastal zone of rivers, overgrown with vegetation, in lakes and ponds. The shell is dark brown, with 5 convex whorls. Shell height 6-12 mm.
• The intermediate host of the liver fluke, the small pond snail (Limnea truncatula), is widespread in Russia. The shell is small, no more than 10 mm in height, forming 6-7 whorls. It lives in ponds, swamps, ditches and puddles, where it is often found in huge numbers. In some areas, there are more than 1 million pond snails per hectare of bogs. When the bogs dry up, the pond snails burrow into the ground, surviving the dry time in the ground.
• The intermediate hosts of the lanceolate fluke are terrestrial mollusks Helicella and Zebrina. Distributed in Ukraine, Moldova, Crimea and the Caucasus. Adapted to live in dry conditions; live in the open steppe on the stems of herbaceous plants. During the heat, helicellae often accumulate on the plants in clusters, thus saving themselves from drying out. Helicella has a low conical shell with 4-6 curls; the shell is light, with dark spiral stripes and a wide rounded aperture. Zebrina has a highly conical shell with 8-11 curls; shell light, with brown stripes running from apex to base; the mouth is irregularly oval.

Class bivalve (Bivalvia)

This class includes molluscs with a shell consisting of two symmetrical halves, or valves. These are sedentary, sometimes completely immobile animals that live at the bottom of the seas and freshwater reservoirs. They often burrow into the ground. The head is reduced. In freshwater reservoirs, toothless or pearl barley is widespread. Of the marine forms, oysters are of the greatest importance. Very large species are found in tropical seas. The shell of a giant tridacna weighs up to 250 kg.

Pearl barley, or toothless lives on the silted and sandy bottom of rivers, lakes and ponds. This inactive animal feeds passively. Toothless food is suspended in water particles of detritus (the smallest remains of plants and animals), bacteria, unicellular algae, flagellates, ciliates. The mollusk filters them out of the water passing through the mantle cavity.

The body of the toothless, up to 20 cm long, is covered with a bivalve shell outside. Distinguish between an expanded and rounded anterior end of the shell, and a narrowed, pointed posterior end. On the dorsal side, the valves are connected by a strong elastic ligament, which maintains them in a half-open state. The shell is closed under the action of two closure muscles - anterior and posterior, - each of which is attached to both valves.

Three layers are distinguished in the shell - the horny, or conchiolinic layer, which gives it a brownish-green color on the outside, a medium thick porcelain-like layer (consists of prisms of carbonic lime; located perpendicular to the surface - the shell) and the inner mother-of-pearl layer (in it between the thinnest calcareous leaves there are thin conchiolin interlayers). The nacreous layer is underlain by a yellowish-pink fold of the mantle on each of the two valves. The epithelium of the mantle secretes a shell; in some species of freshwater and sea pearl mussels, it also forms pearls.

The body is located in the dorsal part of the shell; a muscular outgrowth - a leg - departs from it. The mantle cavity has a pair of lamellar gills on both sides of the body.

In the posterior part, both shell valves and mantle folds do not adhere tightly to one another; two holes remain between them - siphons. The lower, inlet, siphon serves to introduce water into the mantle cavity. A continuous directed flow of water is carried out due to the movement of numerous cilia that cover the surface of the body, mantle, gills and other organs of the mantle cavity. Water washes the gills and provides gas exchange; it also contains food particles. Through the upper, outlet, siphon, the used water together with excrement is discharged outside.

The mouth is at the front end of the body, above the base of the leg. On the sides of the mouth are two pairs of triangular oral lobes. The cilia covering them with their movement drive food particles to the mouth. Due to the reduction of the head in pearl barley and other bivalve mollusks, the pharynx and associated organs (salivary glands, jaws, etc.)

The digestive system of pearl barley consists of a short esophagus, a saccular stomach, a liver, a long loop-shaped curved midgut and a short hind gut. The opening of the saccular outgrowth opens into the stomach, inside which there is a transparent crystalline stalk. With its help, the food is crushed, and the stalk itself gradually dissolves and releases the amylase, lipase and other enzymes contained in it, which provide the primary processing of food.

The circulatory system is not closed; colorless blood flows not only through the vessels, but also in the spaces between the organs. Gas exchange occurs in the branchial filaments, from there the blood is directed into the outflowing branchial vessel and then into the corresponding (right or left) atrium, and from it into the unpaired ventricle, from which two arterial vessels - the anterior and posterior aorta - begin. Thus, in bivalve molluscs, the heart consists of two atria and one ventricle. The heart is located in the sac on the dorsal side of the body.

The excretory organs, or kidneys, look like dark green tubular sacs, they start from the pericardial cavity and open into the mantle cavity.

The nervous system consists of three pairs of nerve nodes connected by nerve fibers. The sense organs are poorly developed due to the reduction of the head and a sedentary lifestyle.

Class cephalopods (Cephalopoda)

brings together the most highly organized mollusks leading an active lifestyle. The largest representatives of invertebrates are cephalopods - octopuses, squids, cuttlefish.

The body shape of cephalopods is very diverse and depends on their lifestyle. The inhabitants of the water column, to which most squid belong, have an elongated, torpedo-like body. For benthic species, among which octopuses predominate, a saccular body is characteristic. In cuttlefish living in the bottom layer of water, the body is flattened in the spinning direction. Narrow, globular or jellyfish-like planktonic cephalopod species are distinguished by their small size and gelatinous body.

Most modern cephalopods do not have an outer shell. She turns into an element of the internal skeleton. Only the nautilus have an outer, spirally twisted shell, divided into inner chambers. In cuttlefish, the shell, as a rule, looks like a large porous calcareous plate. Spirula retains a spiral-wound shell hidden under the skin. In squid, only a thin corneous plate extending along the dorsal side of the body is preserved from the shell. In octopuses, the shell is almost completely reduced and only small crystals of lime carbonate remain from it. In female argonauts (one of the octopus species), a special brood chamber develops, in shape very much like an outer shell. However, this is only an apparent similarity, since it is secreted by the epithelium of the tentacles and is only intended to protect the developing eggs.

One of the distinguishing features of cephalopods is the presence of an internal cartilaginous skeleton. Cartilage, similar in structure to the cartilage of vertebrates, surrounds the head cluster of ganglia, forming a cartilaginous capsule. From it there are processes that reinforce the eye openings and organs of balance. In addition, supporting cartilage develops in the cufflinks, the base of the tentacles and fins.

The body of cephalopods consists of a head with compound eyes, a crown of tentacles, or arms, a funnel and a torso. Large complex eyes are located on the sides of the head and are not inferior in complexity to the eyes of vertebrates. The eyes have a lens, cornea, and iris. Cephalopods have developed not only the ability to see in stronger or weaker lighting, but also accommodation. True, it is achieved not by changing the curvature of the lens, as in humans, but by moving it closer or moving away from the retina.

On the head, around the mouth opening, there is a crown of very mobile tentacles, which are one part of a modified leg (hence the name). The vast majority of species have powerful suckers on their inner surface. Squids use tentacles to catch prey; in male octopuses, one of the tentacles is used to transfer reproductive products. During the breeding season, this tentacle is modified, and during the mating period it breaks off and, due to its ability to move, penetrates into the mantle cavity of the female.

The other part of the leg turns into a funnel, which plays an important role in movement. It grows to the abdominal side of the body, opening one end into the mantle cavity, and the other into the external environment. The mantle cavity in cephalopods is located on the ventral side of the body. At the place of transition of the body to the head, it communicates with the external environment with the transverse abdominal opening. To close it, in most cephalopods, paired semilunar fossae are formed on the ventral side of the body. Opposite them, on the inner side of the mantle, lie two solid, cartilage-reinforced tubercles, the so-called. cufflinks. As a result of muscle contraction, the cufflinks enter the lunar grooves, tightly fastening the mantle to the body. When the abdominal opening is open, water freely penetrates into the mantle cavity, washing the gills lying in it. After this, the closure of the mantle cavity and contraction of its muscles occurs. Water is forcefully pushed out of a funnel lying between two cufflinks, and the mollusk, receiving a return push, moves forward with the rear end of the body. This method of movement is called reactive.

All cephalopods are predators and feed on various crustaceans and fish. They use tentacles to capture prey, and powerful horny jaws to kill. They are located in the muscular pharynx and resemble a parrot's beak. A radula is also placed here - a chitinous tape with 7-11 rows of denticles. 1 or 2 pairs of salivary glands open into the pharynx. Their secret contains hydrolytic enzymes that break down polysaccharides and proteins. Often, the discharge of the second pair of salivary glands is poisonous. The poison also helps to immobilize and kill large prey.

The intestines are branched, with digestive glands. In many species, a duct of the ink gland opens directly in front of the anus into the lumen of the hind intestine. It secretes a dark secret (ink) that can muddy large amounts of water. The ink serves as a smoke screen, disorienting the enemy, and sometimes paralyzing his sense of smell. Cephalopods use it to escape the pursuit of predators.

The circulatory system is almost closed. A heart with 2 or 4 atria, kidneys also 2 or 4, their number is a multiple of the number of gills.

The nervous system has the highest organization with developed structures of touch, smell, sight and hearing. The ganglia of the nervous system form a common nerve mass - a multifunctional brain, which is located in a protective cartilaginous capsule. Two large nerves branch off from the posterior part of the brain. Cephalopods have complex behaviors, have good memory, and exhibit the ability to learn. For the perfection of the brain, cephalopods are called "primates of the sea."

The unique skin photoreceptors of cephalopods react to the slightest changes in illumination. Some cephalopods are able to glow due to the bioluminescence of photophores.

All cephalopods are dioecious animals; some of them have pronounced sexual dimorphism. Males, as a rule, are smaller than females, armed with one or two modified arms - hectocotyls, with the help of which "packets" with seminal fluid - spermatophores - are transferred during the copulation period. Fertilization is external-internal and does not occur in the genital tract of the female, but in her mantle cavity. It consists in the capture of sperm by the gelatinous membrane of the eggs. After fertilization, females attach clusters of eggs to bottom objects. Some species take care of the offspring and guard the developing eggs. The female guarding the offspring can starve for more than 2 months. In octopuses, cuttlefish and nautilus, a minicopy of parents hatches from each egg, only in squid development goes with metamorphosis. Juveniles grow rapidly and often reach sexual maturity by one year.

The value of shellfish

The shells of freshwater pearl oysters with a thickness of the mother-of-pearl layer of about 2.5 mm are suitable for making mother-of-pearl buttons and other jewelry. They eat some bivalves (mussels, oysters, scallops), a grape snail from gastropods (in some European countries it is bred in snail farms), from cephalopods in terms of calorie content and protein composition, squids are especially valuable (more than 600 thousand of them are harvested every year in the world). . T).

River Dreissena is found in huge quantities in the reservoirs of the Volga, Dnieper, Don, in lakes, estuaries of the Black Sea, desalinated areas of the Azov, Caspian and Aral seas. It grows over stones, piles and various hydraulic structures: watercourses, pipes for technical and drinking water supply, protective gratings, etc., and its number can reach 10 thousand copies per 1 m 2 and cover the substrate in several layers. This impedes the passage of water, therefore, constant cleaning from zebra mussel fouling is necessary; use mechanical, chemical, electrical and biological methods of control. Some bivalve molluscs grind tunnels in the bottoms of ships, wooden parts of port facilities (ship worm).

Pearl barley and some other bivalve molluscs play an important role in marine and freshwater biocenoses as natural water purifiers - biofilters. One large pearl barley is capable of filtering 20-40 liters of water per day; mussels inhabiting 1 m 2 of the seabed can filter about 280 m 3 of water per day. At the same time, mollusks extract organic and inorganic substances from polluted water, some of which are used for their own nutrition, and some are concentrated in the form of lumps, which are used to feed microorganisms.

Thus, molluscs are one of the most important parts of the self-purification system of the reservoir. Of particular importance in the system of biological self-purification of water bodies are mollusks, which have special mechanisms of resistance to pollution of water bodies with toxic substances and mineral salts, as well as adapted to living in water with a reduced amount of oxygen. The molecular mechanism of this adaptation is based on the carotenoids contained in the nerve cells of molluscs. Perlovitsa and other filter-feeding molluscs need protection. They can be bred in special containers and used to clean artificial reservoirs from pollution, waste disposal and obtain additional food.

The shellfish fishing is of particular importance in Japan, the USA, Korea, China, Indonesia, France, Italy, and England. In 1962, the harvest of mussels, oysters, scallops and other bivalve molluscs amounted to 1.7 million tons, by now the natural natural reserves of valuable edible molluscs have been depleted. In many countries, marine and freshwater molluscs are artificially bred. Since 1971, mussels have been bred on an experimental farm in the northwestern part of the Black Sea (productivity is 1000 centners of mussels per year); research on breeding mussels is also being carried out in the basins of other seas washing the shores of our country. Shellfish meat is easily digestible, it contains a lot of vitamins, carotenoids, microelements (iodine, iron, zinc, copper, cobalt); it is used for food by the population, as well as for the fattening of domestic animals. Filter molluscs can also be used in a biomonitoring system to track the chemical composition of water in reservoirs.

Cephalopods, common in all seas, except for desalinated ones, despite the fact that they are predators, often themselves serve as food for many fish and marine mammals (seals, sperm whales, etc.). Some cephalopods are edible and are fished. In China, Japan and Korea, the use of these animals as food goes back centuries; in Mediterranean countries it also has a very long history. According to the testimony of Aristotle and Plutarch, octopuses and cuttlefish were common foods in ancient Greece. In addition, they were used in medicine, perfumery and in the manufacture of first-class paints. At present, congenital programs of complex behavior are being studied in laboratory conditions on cephalopods.

General characteristics of molluscs.

1. The body is not segmented.

2. Most have a shell.

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

4. Food: a) active (scraping, predation); b) passive (filtering)

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

6. The circulatory system is open, they have a heart. Blood is hemolymph.

7. The nervous system is nodal. In sedentary senses are poorly developed, in active ones well

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

Comparative characteristics of the main classes of mollusks

Signs	Classes
	Gastropods	Bivalve	Cephalopods
Habitat	Mainly land and fresh water	Fresh bodies of water and seas	Salty warm seas
Body symmetry	Asymmetrical	Bilaterally symmetric	Bilaterally symmetric
Body parts	Head, torso, leg	Torso, leg	Head, torso
Organs on the head	1―2 pairs of tentacles, 1 pair of eyes at the base or ends of the upper pair of tentacles	No head	Tentacles formed from the part of the leg that has shifted to the head and surrounding the mouth; 2 large eyes
Sink	Single curl or reduced	Two leaflets with elastic ligament on the dorsal side	Remnants of a reduced shell under the skin or absent
Leg	Muscular, occupies the entire abdominal side of the body	Muscular, wedge-shaped abdominal part of the body	Split into tentacles
Movement	Using your leg	Using the foot or (rarely) reactively (pushing water out of the mantle cavity)	Using tentacles (hands) and reactively (pushing water out of the mantle cavity through a funnel)
Respiratory system	"Lung" - the cavity formed by the mantle between the body and part of the shell; marine species may have gills	Lamellar gills on the sides of the body	Gills
Nervous system	Periopharyngeal ganglia	3 pairs of ganglia	Ganglia forming a common periopharyngeal mass ("brain")
Excretory organs	1 pair of kidneys	1 pair of kidneys	1 or 2 pairs of kidneys
Reproduction and development	Usually dioecious, development is direct	Most part-sex, development with transformation (larva - glochidia)	Divided. The reproductive gland is unpaired. Direct development.

Significance in nature and human life

• In nature:

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

Interesting facts

The largest pearl, 24 cm long and 14 cm across, was found in a coral reef clam shell. tridacnae off the Philippine Islands. Such a 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, are not eternal: 50-60 years after a pearl is extracted from a mollusk, it begins to crack. The maximum "life" of a pearl as an adornment does not exceed 150 years; this is due to the drying of the organic layers inside it.

Byssus is produced mussels and pins. These are the threads that attach the shell to the substrate. By their properties, these are silky threads of a yellowish or brownish color, elastic, strong, with an unusual shine, contain a protein substance close to fibroin in the composition of silk. The length of the byssus fibers reaches 30 cm. Beautiful laces and fabrics were made from byssus. The first indications of the use of byssus threads for spinning and weaving production date back to the 2nd and 3rd centuries. AD In the XVIII century. in European countries, stockings, gloves, purses, 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 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 that cephalopods have acquired in the process of evolution, this miracle weapon - the ink bomb. The ink is produced by a special growth of the rectum called the ink sac. It consists of two parts: in one part, ink is produced, and in the other it is accumulated. 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 inches from the target, the squid suddenly darkened and, as it seemed to Hol, froze in place. In the next instant, Hall grabbed ... the ink model, which 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 own image in its place. No, this is a dressing up scene. At first, it attracts the enemy's attention with a sharp change in color. Then it immediately replaces itself with another dark spot - the predator automatically fixes its gaze on it - and disappears from the stage, changing its "outfit". Please note: now the color of the squid is not black, but white.

Go ink catch-footed molluscs have one more amazing property. American scientist McGinity conducted a series of experiments on the California octopus and moray eels. And here's what he found: the ink of the octopus, it turns out, paralyzes the olfactory nerves of predatory fish.

Dangerous is cephalopod ink for humans?

Let's ask an expert in spearfishing like James Aldridge to answer this question. From writes: “I behaved so freely with the octopus that I got a jet of ink right in my face. And since I was without a mask, the liquid got into my eyes and blinded me. a wonderful amber color. Everything around me seemed to be amber as long as the film of this ink was kept in front of my eyes. It lasted 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 disguise, so they can be safely called - "kings of camouflage."

The ability of cephalopods to glow has long been known. French naturalist Jean Baptiste Verany loved to come to the seashore when the fishermen returned with their catch. Once, not far from Nice, he saw a crowd of people on the shore. A completely unusual creature came across in the net. The body is thick - in a bag, like an octopus, but there are ten tentacles, and they are connected by a thin membrane. Verani dropped the bizarre prisoner into a bucket of seawater; “at the same moment,” he writes, “I was captured by the amazing sight of sparkling spots that appeared on the skin of the animal. It was a blue ray of sapphire that blinded me, then an opal one - topaz, then both rich shades of color mixed in a magnificent radiance that surrounded the mollusk at night, and it seemed one of the most wonderful creations of nature. " So in 1834 Jean Baptiste Verani discovered the phenomenon of bioluminescence.

Molluscs are mainly aquatic, but there are also terrestrial representatives. The body of a mollusk consists of a head, torso and legs. On the head are the senses - tentacles and eyes.

The structure of the shell of mollusks

The main feature of the shellfish type is the presence of a shell. The shells of molluscs can be of various shapes - bivalve, as well as in the form of a cap, shell or spiral. The shell consists of two layers - an external organic layer and an internal lime layer. In turn, the calcareous layer consists of a porcelain-like layer consisting of prismatic crystals of calcium carbonate, this layer immediately follows the organic layer. Directly to the body of the mollusk is a nacreous layer consisting of plates of calcium carbonate.

Due to the presence of a shell, the internal structure of mollusks does not have a skin-muscular sac, only bundles of muscles that attach the body of the mollusc to the shell remain. There is also no secondary body cavity, only the cavity of the heart and gonads remains from it. The spaces between the organs of the mollusk are filled with loose connective tissue. The body of the mollusk is surrounded by a skin fold - a mantle. A mantle cavity is located between the body and the mantle.

Digestive system of molluscs

The digestive system of mollusks is an open type, it consists of the anterior, middle and hind intestines. The anterior gut begins with the mouth, passing into the muscular pharynx, then the esophagus, which in many species expands into a goiter. In the throat, some of the representatives of the mollusks have a special organ for grinding food - a grater, which consists of cuticular teeth. The middle intestine begins with the stomach. Molluscs, unlike more simply arranged animals, have a special digestive gland - the liver. The liver performs several functions - this is a secretory function (the liver synthesizes digestive enzymes), an absorption function, a digestive function (in the liver of molluscs there are special amoeba cells, inside which digestion takes place), as well as a storage function (glycogen is deposited in the liver). The liver ducts open into the stomach or midgut. Behind the midgut is the hindgut, which opens with the anus into the mantle cavity.

Respiratory system of molluscs

The respiratory system in primary-aquatic molluscs consists of gills, and in secondary-aquatic and terrestrial ones - of lungs. The lungs are pockets of the mantle, which are densely riddled with blood vessels. The airway opens above the mollusc's head.

Excretory system of molluscs

The excretory system of mollusks consists of kidneys, their channels open into the mantle cavity.

The nervous system of molluscs

The nervous system of molluscs is called scattered-nodular. Nerve nodes are located inside or outside the innervated organs, the nodes are interconnected by nerve trunks. Mollusks have well-developed sense organs, they have organs of sight, touch, balance, chemical feeling.

The circulatory system of molluscs

The circulatory system is of an open type, that is, the blood mixes with the fluid of the body cavities. Molluscs have a heart, which consists of one to two atria and one ventricle. Blood saturated with oxygen enters from the gills or lungs into the atria, then it passes into the ventricle and is pushed out into the arteries, then the blood is distributed to organs and tissues.

Reproduction of molluscs

Molluscs reproduce only sexually. Basically molluscs are dioecious, but there are also hermaphrodites. Development occurs either with metamorphosis (with a planktonic larva), or in a direct way, when a small mollusk emerges from the egg.

The mollusk type includes the classes of gastropods, cephalopods, and bivalve molluscs.

Natalia Popova

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

Different types of molluscs, belonging to different classes, are quite different in structure, and often in life cycle.

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

External structure of molluscs

The body of the mollusk has bilateral (bilateral) symmetry or asymmetry as a result of distortion of bilateral symmetry in the process of individual development.

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

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

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

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

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

On the head of the molluscs there is a mouth opening, tentacles and eyes.

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

Internal structure of molluscs

As well as annelids, molluscs are classified as protostomes, secondary cavities, and three-layered ones.

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

Digestive system

Behind the mouth of the mollusk is the pharynx, in which many species have radula(grater). The radula consists of a tape and denticles located on it, which scrape off plant food or grab animals (protozoa, crustaceans, etc.).

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

In bivalve molluscs, which feed on microorganisms and small organic particles, the esophagus immediately follows the mouth, that is, they do not have a pharynx with a grater.

Respiratory system

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

Circulatory system

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

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

The aorta leaves the heart, then there are arteries that pour blood into the gaps between the organs. Then the venous blood is collected again in other vessels and goes to the gills or lung. From there through the vessels to the heart.

Excretory system

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

Nervous system and senses

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

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

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

Reproduction of molluscs

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

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

The value of shellfish

Bottom bivalve molluscs filter the water, thereby purifying it not only from organic, but also from mineral particles.

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

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

Scientists determine the age of sedimentary rocks from the shells of fossil mollusks.

Some marine bivalves destroy wood, which can cause damage to ships and hydraulic structures.

Ground slugs and snails can harm orchards and vineyards.