cephalopodsare the largest representatives of invertebrates. All of you are familiar with squid and octopus, whose relatives are argonaut, cuttlefish, the well-known Nautilus (ship), as well as extinct ammonites.

The most highly organized class of sea. shellfish. They arose in the Cambrian presumably from forms similar to xenoconchs. G.'s evolution of m in the Meso-Cenozoic took place in competition with fish, Ch. arr. bony, which led to the emergence of convergently similar biol. adaptation. The body (length from 1 cm to 5 m) is bilaterally symmetrical, usually markedly divided into a body and a large head. Modified leg turned into a funnel.

The torso is dressed in a mantle, edges, together with a conic. the funnel serves as the main. propulsion organ in jet navigation. Many, in addition, swim. organ - a pair of fins on the cone or sides of the mantle.

Almost all have a crown of 8 arms around their mouths and (in squid and cuttlefish) a pair of tentacles. Limbs with suction cups (in some squids, some of them are turned into hooks). Sink modern. G. m. internal (except for nautilus), often reduced or absent; female argonauts have a special ext. shell for laying eggs. There is a cartilaginous "skull".

Two thick horny jaws, curved like a parrot's beak, play the main. role in capturing and grinding food. Usually there is a radula, 2 pairs of salivary glands, secretions from the posterior pair can be poisonous. The duct of the ink sac usually opens into the hindgut. The brain is complex. On the sides of the head are a pair of large, well-developed eyes.

G. m. are able to quickly change the color of the body (defensive reaction). Often there are organs of luminescence. The circulatory system is usually closed. Dioecious, sometimes with sharp sexual dimorphism. With the help of a peculiarly modified hand (hectocotylus), the male transfers spermatophores to the mantle cavity or the female's spermatheca.

Usually they breed once in a lifetime, after which they die. G.'s eggs are large, rich in yolk. The pelagic hatches. or bottom juveniles. Many species expressed concern for offspring.

7 subclasses, 6 of which include mainly fossil species (including nautiloids and extinct ammonites), 1 modern - two-gill (Dibranchiata) with 7 orders (modern - squid, cuttlefish, octopus, vampiromorphs and extinct - Aulacoceratida, Phragmoteuthida and belemnites).

OK. 650 species, in the seas and oceans from the littoral to the ultraabyssal. The most diverse and numerous in tropical and temperate seas. In the USSR, more than 60 species, in the sowing. and far east. seas.

Scheme of organization of a cephalopod mollusk: 1 - head; 2 - hands; 3 - funnel; 4 - mantle; 5 - sink; 6 - jaws; 7 - radula; 8 - stomach; 9 - liver; 10 - heart and pericardium; 11 - brain and central nervous system; 12 - ink gland; 13 - gonads; 14 - gills.

Cephalopods: 1 - argonaut (Agdonauta argo); 2 - spirula (Spirula), on the right - a diagram of the structure.

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

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

Mollusks are characterized by a hard mineral shell that covers the body of the animal from the dorsal side. The shell is made up of calcium carbonate crystals. From above, it is usually covered with a horn-like organic substance, and from the inside it is lined with a hard, shiny calcareous layer - mother of pearl. The shell can be solid, bivalve, or consisting of several plates (for marine molluscs, chitons).

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

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

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

In bivalve mollusks, due to a sedentary lifestyle, the head is absent, the leg is reduced. In some species, the leg has become a swimming organ (for example, in cephalopods).

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

Digestion. The mouth opening leads into the pharynx. In the pharynx, many species have a grater (radula) - a special apparatus in the form of a tape, lying on the ledge of the bottom of the oral cavity. There are teeth on this tape. With the help of a grater, herbivorous mollusks scrape off food from plants, and predatory mollusks (whose grater teeth are larger) grab prey. Salivary glands open into the oral cavity of some predatory molluscs. The secret (excreted substance) of the salivary glands contains poison.

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

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

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

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

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

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

Reproduction. Most molluscs are dioecious. However, there are also hermaphrodites in which cross-fertilization occurs. Fertilization in mollusks is external (for example, in oysters and toothless) and internal (in snails).

From a fertilized egg, either a larva develops, leading a planktonic lifestyle (the so-called sailboat), or a formed small mollusk.

Meaning. Representatives of some classes of molluscs play an important role in many natural biocenoses. Aquatic mollusks are often the most numerous group in benthic ecosystems. The filtration method of feeding bivalve mollusks leads to the fact that many of them precipitate mineral and organic particles, providing water purification. Mollusks feed on fish, birds and animals.

Mollusks serve as food for people and are traditional objects of fishing and breeding (oysters, scallops, mussels, cockles, squids, Achatina, grape snails). Very beautiful pearls are formed in the shells of sea mollusks of pearl oysters. Aborigines used cowrie shells as coins. From the shells of fossil mollusks, geologists can determine the age of sedimentary rocks.

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

A typical mollusc larva (sailfish) is very similar to annelids larvae, carrying large lobes lined with cilia. The larva leads a planktonic lifestyle, then settles to the bottom and takes on the appearance of a typical gastropod mollusk.

Mollusks are non-segmented, bilaterally symmetrical, soft-bodied animals (in gastropods, the body is asymmetric), having a shell, a mantle cavity, a reduced coelom, an open circulatory system). Apparently, they descended from ancestors common with annelids, which had a poorly developed secondary body cavity, had ciliated integument, and did not yet have a segmentation of the body into segments.

Lesson learned exercises

1. Name the main ways of movement of molluscs.
2. What are the main similarities and differences between the external and internal structure of molluscs and annelids.
3. What are the characteristics of fast swimming molluscs?
4. What is the effect of a sedentary lifestyle on the organization of mollusks?
5. What is the role of mollusks in nature and in human life? Give examples.

1. The body is not segmented.

2. Most have a sink.

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

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

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

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

Comparative characteristics of the main classes of molluscs

• In nature:

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Mollusks: general characteristics of the type

Lesson objectives: To expand knowledge about the diversity of mollusks To study their habitat and lifestyle To study the classification of the type Mollusks To consider the structure of the shell of mollusks To consider the structural features of organisms of representatives of various classes of this type To give a comparative description of the classes of Mollusks.

Mollusks: a general characteristic of the type Mollusks, or soft-bodied (from Latin “molluskus” - soft) Malacology is a branch of zoology that studies mollusks. They form a separate type of invertebrates, leading from the ancient non-specialized polychaete annelids. The number of species reaches 130 thousand.

Variety of molluscs Fossils of molluscs Ammonites Belemnite Phragmokona

Variety of mollusks Scallop Octopus Grape snail Rapana

Variety of molluscs Cuttlefish Nautilus

Variety of mollusks Nudibranch molluscs

Variety of molluscs Angelfish Monkfish

Type classification Molluscs class. Spadefoot cl. Gastropods cl. Bivalve cells. Carapace class. Cephalopods cl. Monoplacophora

Type Molluscs Class Bivalves Class Cephalopods Class Gastropods mussels oysters toothless barley pearl mussels slugs rapana snails reels pond squid octopus nautilus cuttlefish

Mollusk shell structure Horny layer Porcelain layer Mother-of-pearl layer Mantle cells

Class Bivalves

Class Bivalves

Class Gastropoda

Comparative characteristics of classes Character Type class Mollusca Bivalves Gastropods Cephalopods Habitats Symmetry of the body Parts of the body Shell Leg Representatives Significance in nature and for humans

Dorsal fold - mantle. Most have 1 or more. shells. Nervous system diffuse-nodal type. Sense organs - organs of chemical sense and balance, many have eyes. The circulatory system is open, there is a heart from the ventricle and 1-2 atria. Respiratory system: gills or lungs. Digestive system: mouth, pharynx with a grater, stomach, liver, intestine and anus. Excretory system: kidneys. The reproductive system is predominantly dioecious, represented by gonads. Common features in the structure of mollusks:

Mollusks are one of the most ancient invertebrates. They differ in the presence of a secondary body cavity and rather complex internal organs. Many of them have a calcareous shell, which protects their body quite well from the encroachments of numerous enemies.

This is not often remembered, but many species of this type lead a predatory lifestyle. In this they are helped by a developed salivary gland. By the way, what is the salivary gland in 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, molluscs have one or two pairs of such glands, which in some species reach very impressive sizes. In most predatory species, the secret that they secrete contains from 2.18 to 4.25% chemically pure sulfuric acid. It helps both to fend off predators and to hunt their relatives (sulfuric acid perfectly dissolves their calcareous shells). That's what the salivary gland in molluscs is.

Other natural value

Many of the species of slugs, as well as the grape snail, cause great harm to agriculture around the world. At the same time, it is mollusks that play the most important role in the global purification of water, since they use organic matter filtered from it to feed them. In many countries, large ones are bred on marine farms, as they are a valuable food product that contains a lot of protein. These representatives and oysters) are even used in dietary nutrition.

In the former USSR, 19 representatives of this ancient type were considered rare and disappearing at once. Despite the diversity of molluscs, they should be treated with care, as they are extremely important for the proper functioning of many natural biotopes.

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

Want to know interesting facts about shellfish? In the Antique period and the Middle Ages, inconspicuous cephalopods were sometimes the basis of the well-being of entire states, since the most valuable purple was mined from them, which was used to color the royal robes and robes of the nobility!

Type of shellfish

In total, it has more than 130,000 species (yes, the variety of mollusks is incredible). Mollusks 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 a relatively small number of species have chosen land as their place of residence.

general characteristics

Almost all animals that are part of this type are distinguished by several specific features at once. Here is the general characteristic of mollusks accepted today:

• First, three layers. Their organ system is formed from the ectoderm, endoderm, and mesoderm.
• Symmetry of the bilateral type, caused by a significant displacement of most of their organs.
• The body is unsegmented, in most cases protected by a relatively strong calcareous shell.
• There is a skin fold (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 have before us quite developed, but still primitive animals. It is not surprising that many scientists consider mollusks to be 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 has been preserved only in very primitive species. The secondary cavity of the body 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.
• Torso.
• Muscular leg through which movement is carried out.

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

The shell can be either completely solid or consist of two flaps 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 remains 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 a coating of algae or other organic matter from all available surfaces. In predatory species (we will talk about them below), the tongue has degenerated into a flexible and rigid proboscis, which is intended for opening the shells of other mollusks.

In Cones (which will also be discussed separately), individual segments of the radula protrude beyond the oral cavity and form a kind of harpoon. With their help, these representatives of mollusks literally throw their poison at the victim. In some predatory gastropods, the tongue has turned into a special “drill”, with which they literally drill holes in the shell of their prey to inject poison.

Bivalves

In their case, everything is much simpler. They simply lie motionless at the bottom (or hang, tightly attached to the substrate), filtering through their body hundreds of liters of water with organic matter dissolved in it. The filtered particles go directly to the bulky stomach.

Respiratory system

Most species breathe with gills. There are "front" and "rear" views. In the former, the gills are located in front of the body and their tip is directed forward. Accordingly, in the second case, the tip looks back. Some have lost gills in the direct sense of the word. These large clams breathe directly through their skin.

To do this, they have developed a special skin organ of an adaptive type. In terrestrial species and secondary aquatic mollusks (their ancestors returned to the water again), part of the mantle wraps up, forming a kind of lung, the walls of which are densely penetrated by blood vessels. To breathe, such snails rise to the surface of the water and gain air supply with the help of a special spiracle. The heart, located not far from the simplest "design", consists of one atrium and ventricle.

The main classes that make up the type

How is the type of mollusk divided? The classes of mollusks (there are eight in total) are “crowned” by the three most numerous:

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

Who else is included in the type of mollusk? The classes of molluscs are quite diverse: in addition to all of the above, there are also Spadefoot, Armored and Pit-tailed, Furrowed-bellied and Monoplacophores. All of them refer to the living and healthy.

What fossils does the type of mollusk 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 attributed to the new species Neopilina galatheae. As you can see, the name of the molluscs of this species was given by the name of 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 mollusk: the classes of mollusks that are now considered extinct may well be preserved somewhere in the bottomless depths of the oceans.

No matter how strange it may sound, but one of the most dangerous and incredible predators on our planet are ... outwardly harmless gastropods. For example, snails Cones (lat. Conidae), whose poison 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 indeed most similar to a truncated cone.

They can be persistent hunters, dealing with floodplain prey with exceptional ruthlessness. Of course, colonial, sedentary species of animals often act as the latter, since other snails simply cannot keep up. The prey itself can be dozens of times larger than the hunter in size. Want to know more interesting facts about shellfish? Yes please!

About methods of hunting snails

Most often, the insidious mollusk uses its most powerful organ, a strong muscular leg. It can attach itself to prey with the equivalent force of 20kg! This is quite enough for a predatory snail. For example, a "caught" oyster opens up 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 commonly thought ...

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

Dissolution!

To dissolve someone else's shell and its owner himself, the mollusk uses sulfuric acid (you already know what the salivary gland is in mollusks). So the destruction is much easier and faster. After the hole is made, the predator slowly begins to eat its prey out of the "package", using its proboscis for this. To some extent, this body 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's body.

This is how snails can get their prey even from deep crevices and large shells. We remind you once again that it is from the proboscis in the body of the victim 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.