Message on the topic of molluscs biology. Mollusks are invertebrate animals. In the research field

Which consists of squid, octopus, cuttlefish, nudibranchs, snails, slugs, limpets, mussels, oysters, scallops, as well as many other lesser known species of animals. According to scientists, today more than 100,000 species of mollusks known to science live on Earth. This makes them the second in species diversity after.

Mollusks have a soft body, which consists of three main parts: the legs, the visceral mass, and the mantle with the organ system. Many species also have a protective shell composed of chitin, proteins, and calcium carbonate. Mollusks are so diverse in form that it is impossible to use representatives of one species to generalize the anatomical features of the group. Instead, science books often describe a hypothetical mollusk that has characteristics of many species.

This hypothetical mollusk has a mantle, a shell, a leg, and a visceral mass. The sheath is a layer of tissue that encloses the visceral mass. Many mollusks have glands that secrete a hard shell.

The leg is a muscular structure located at the bottom of an animal's body. The mollusk secretes mucus from the underside of the leg to lubricate the underlying surface. Mucus facilitates movement, which is achieved by repeated contraction and stretching of the clam's leg muscle.

The visceral mass, located above and below the mantle, includes the digestive system, heart, and other internal organs. The circulatory system is open. Most species of molluscs use a single pair of gills for breathing, although some species have vestigial lungs, such as land slugs and snails.

Mollusks, unlike vertebrates, transport oxygen throughout the body with the help of other molecules. They use hemocyanin (a copper-based respiratory pigment), while vertebrates use hemoglobin (an iron-based one). Hemocyanin is less efficient at transporting oxygen than hemoglobin. For this reason, mollusks are more likely to move in quick jerks, but are not able to maintain movement for a long period of time, as they do.

Most marine molluscs begin their lives as larvae, which later develop into adults. Freshwater and land snails are formed in eggs and hatch as miniature but fully formed adults. Although most common in marine habitats, molluscs are also found in freshwater and terrestrial environments.

Mollusks are believed to have evolved from segmented, worm-like animals similar to modern flatworms. Their closest living relatives are annelids and flatworms.

Classification

Mollusks that inhabit the planet today are divided into the following classes:

• pittails (Caudofoveata);
• furrowed bellies (Solenogastres);
• armored (Polyplacophora);
• Monoplacophora (Monoplacophora);
• Bivalves (Bivalvia);
• Spadefoot (Scaphopoda);
• gastropods (Gastropoda);
• cephalopods (Cephalopoda).

Mollusks are a type of animal that includes several tens of thousands of different species. Yes, mollusks are not fish, not reptiles and not amphibious creatures, namely animals! All mollusks are united by one property - a soft body (hence the word "mollusk" in translation from Latin). And almost all mollusks have a shell - a shell in which they hide their fragile soft body to protect themselves from the outside world. Some mollusks, such as land slugs, have lost their shells and do without them.

Collecting a variety of beautiful shells is a very interesting hobby for many people. Shell collectors, as well as scientists who study mollusks, are called malacologists.

cephalopods

Cephalopods are very rare in nature. At the same time, these are the most "smart" and developed in all respects representatives of this species. Often cephalopods even take care of their offspring, which is not typical for mollusks at all. It can be said that cephalopods are the elite of the entire type of molluscs. These include squids, rare nautiluses (“ships”), cuttlefish, octopuses… Most often, cephalopods are predators.

The body of any mollusk most often consists of a head, torso and legs. The leg in cephalopods in the process of evolution turned into a tentacle.

These are not only the most developed, but also the largest mollusks on Earth. Some species of squid can reach several tons in weight.

gastropods

If you come across a mollusk in nature, then in eight cases out of ten it will turn out to be a gastropod. Gastropods are the same snails. So a snail slowly crawling along the path of the park after the morning rain is a gastropod mollusk on your way. They are called gastropod mollusks due to the fact that their leg, which is typical for mollusks in general, grows from the body.

Interestingly, snails most often have signs of both sexes, that is, each individual is a hermaphrodite. They live, like other mollusks, both in the sea, and on land, and in fresh water. However, most often you will find a gastropod in sea water.

Some gastropods, i.e. slugs and snails, even adapted to breathe with lungs on land.

bivalves

Bivalve mollusks have a shell of two valves, which is why they are named so. These include, for example, sea ​​mussels, sea scallops, oysters, sea dates, shipworms. Such molluscs are most often divided into males and females. Also, bivalve molluscs usually do not have a head, and, accordingly, neither eyes nor ears. Often they also have legs, characteristic of gastropods.

Such creatures live slowly, like snails, they are inactive or do not move at all all their lives, attaching themselves to a rock with the help of algae, for example. They lie at the bottom or burrow into the mud. Sometimes bivalves generally grow into the environment as a shell and can no longer move. Some small and modest bivalves live over a hundred years, and can live up to 500!

These are very tenacious mollusks. Such an amazing bivalve mollusk as the arctic scallop is able to live in water at temperatures below zero, under a layer of ice. And shipworms are able to gnaw passages in wood with a special drilling organ, climb there and live there.

Shellfish in human life

Mollusks for humans are food, pets, and "suppliers" of souvenirs, beautiful shells. Squids, fried mussels, boiled sea scallops are a delicacy for humans, and Achatina snails are used as a cosmetic and medicinal product. With their help, cosmetologists do massage and refresh the face. There are also a number of edible snails that are grown on farms for further consumption. Slugs are also eaten in many Asian countries.

Scallops from the class of gastropods are able to create jet motion with the valves of their shell. Observations of them were useful for scientists in the creation of jet engines. And nautiluses from the class of cephalopods are an example for mechanics and for hydraulic scientists with their complex device. The Nautilus can, by pumping gas into its special chamber, rise and sink like a submarine. Based on its structure, the first submarines were designed.

Mollusks are a large type of animals in terms of the number of species (130 thousand). They live mainly in the seas (mussels, oysters, squids, octopuses), fresh water bodies (toothless, pond snails, livebearers), less often in a humid terrestrial environment (grape snail, slugs). The body sizes of adult mollusks of different species vary significantly - from a few millimeters to 20 m. Most of them are sedentary animals, some lead an attached lifestyle (mussels, oysters), and only cephalopods are able to move quickly in a jet way.

The main characteristic features of the structure of mollusks :

The body is devoid of segmentation, has bilateral symmetry (bivalves and cephalopods) or asymmetrical (gastropods). The divisions of the body are head with eyes located on it and 1 - 2 pairs of tentacles, torso, in which most of the internal organs are located, and leg - muscular abdominal part of the body that serves for movement. In bivalves, the head is reduced.

The body of molluscs is enclosed in sink, protecting the animal and giving support for muscle attachment. The outer layer of the shell is horny, the middle (porcelain) and inner (mother-of-pearl) are calcareous. In gastropods, the shell is integral in the form of a cap or a spirally curled turret. In bivalves, it consists of two valves connected by an elastic ligament, the teeth of the “lock” and the closing muscles. Most cephalopods have lost their shells.

The body of mollusks is covered with a skin fold - mantle, the epithelium of which secretes the substance of the shell. Between the mantle and the body is formed mantle cavity, in which the gills, some sense organs, the anus, the opening of the excretory organs are located.

body cavity secondary (general), however, it is greatly reduced and preserved only in the form of the pericardial cavity and cavities of the gonads. The rest of the space between the internal organs is filled with loose tissue - parenchyma.

The digestive system consists of three sections: the anterior, middle and hindgut. In most molluscs (except for bivalves), a muscular tongue is developed in the pharynx, covered with a horny plate with numerous teeth - grater. With it, they actively capture and grind plant and animal food. The ducts open into the pharynx salivaglands, and in the stomach - a duct of a special digestive gland - liver. Bivalves feed passively, filtering food suspension (algae, bacteria, detritus) through the gills, which enters the mantle cavity with water through the introductory siphon.

Circulatory system open and consists of hearts and cocourts. The heart has a ventricle and 1 - 2 (rarely 4) atria. In addition to the vessels, part of the way the blood passes in the slit-like cavities between the organs.

Respiratory organs in aquatic mollusks - gills, at terrestrial lung, which is part of the mantle cavity. The wall of the lung has a dense network of blood vessels through which gas exchange takes place. The lung opens to the outside through the airway spiracle.

The excretory system is represented by 1 - 2 kidneys. They are modified metanephridia. The funnel of the kidney opens into the pericardial sac, and the excretory opening into the mantle cavity.

Nervous system scattered-nodal type: five pairs of large ganglia are located in the vital organs (head, leg, mantle, respiratory organs and visceral sac) and are interconnected by nerve trunks. Of the sense organs, the most developed are the organs of chemical sense, touch, balance, and in mobile predators - vision.

10. Reproduction occurs sexually. Most mollusks are dioecious animals, less often - hermaphrodites (pulmonary gastropods). In dioecious mollusks, fertilization is external, in hermaphroditic - internal, cross. In freshwater and terrestrial pulmonary, as well as cephalopods, development is direct, in marine bivalves and gastropods, with incomplete metamorphosis, that is, with a planktonic larval stage that contributes to their settling.

shellfish- Bilaterally symmetrical or secondarily asymmetrical three-layered animals. They live in marine and fresh water bodies, on land.

In the body of most species of mollusks, three sections can be distinguished: the head, trunk and leg. On the head are the mouth opening, the sense organs. The strongly thickened ventral side forms various types of legs. The leg, as an organ of movement, can have a different shape: in floating forms it turns into wide lobes or tentacles, in crawling forms it turns into a flat sole.

The torso is surrounded by a skin fold - the mantle. Between the mantle and the body, a mantle cavity is formed, into which the openings of the digestive, excretory and reproductive systems open. The mantle cavity also contains the respiratory and chemical sense organs (osphradia). All of the above is called the mantle complex of organs.

Musculature in mollusks is well developed and consists of muscle bundles. They are especially strongly developed in the leg of the animal.

The whole is reduced to the pericardial sac and the cavity in which the gonads are located. The space between other organs is filled with parenchyma.

The digestive system is divided into three sections: anterior, middle and posterior. The anterior and posterior sections are of ectodermal origin, the middle - endodermal. In the pharynx of many species there is a specific organ for grinding food - a radula, or grater. The ducts of the salivary glands open into the pharynx, and the ducts of the liver open into the midgut.

Respiratory organs are represented by gills or lungs. Lungs are present not only in terrestrial species, but also in forms that have secondarily switched to an aquatic lifestyle. Gills and lungs are modified parts of the mantle. In water-dwelling species, gas exchange can also occur through the skin.

The circulatory system is open: blood flows not only through the blood vessels, but also through the gaps located in the space between the organs. Mollusks have a heart made up of two or more chambers. The heart is located in the pericardial sac (pericardium).

The excretory organs are the kidneys, which are modified metanephridia. The kidney begins as a funnel in the pericardial sac and opens with an excretory opening into the mantle cavity.

The nervous system in most molluscs is represented by several pairs of nerve nodes, which are located in different parts of the body. The nervous system of this type is called scattered-nodular. In addition to reflex activity, the nervous system performs the functions of regulating growth and reproduction by secreting various neurohormones. Mollusks have organs of chemical sense (osphradia), balance, numerous tactile receptors are scattered in the skin. Many species have eyes.

The predominant number of species of mollusks are dioecious animals, but there are also bisexual species. The development of all terrestrial species, most freshwater and some marine life is direct. If development proceeds with metamorphosis, then either a larva of the trochophore type or a larva - a veliger (sailboat) comes out of the egg.

Type Mollusks are divided into classes: gastropods (Gastropoda), bivalves (Bivalvia), cephalopods (Cephalopoda), etc.

The question of the origin of mollusks is still being discussed by zoologists. At present, the hypothesis of the origin of mollusks from primary coelomic trochophore animals, from the same group from which annelids originated, is considered the most proven. The similarity of embryogenesis (spiral fragmentation, metamerism of the rudiments of some organs, teloblastic anlage of the mesoderm) and the presence of a trochophore larva similar to the trochophore of polychaetes in lower mollusks testify to the relationship of mollusks and annelids. It is assumed that the primary molluscs were bilaterally symmetrical animals with a low body, covered with a slightly convex shell, with a muscular flat leg and an almost not isolated head. Two lines of evolutionary development depart from the primary mollusks. The first line leads to the formation of lateral nerve mollusks, this group is not considered in this manual. The second evolutionary line leads to the appearance of shell molluscs. Among shell mollusks, the most primitive are monoplacophores. Bivalves, gastropods, and cephalopods are believed to have originated from ancient monoplacophorans.

Description of classes, subclasses and units of the Mollusk type:

• Class Gastropoda (Gastropoda)

• Class Cephalopoda (Cephalopoda)

  • Subclass Coleoidea (Coleoidae)

They will be discussed in this article. There is also a special branch of biology that studies this group of animals. It's called malacology. And the science that deals with the study of mollusk shells is conchology.

General characteristics of molluscs

Representatives of this type are also called soft-bodied. They are quite varied. The number of species is approximately 200 thousand.

This group of multicellular animals is divided into eight classes:

• Bivalves.
• Armored.
• Furrowed belly.
• Pittails.
• Monoplacophora.
• Gastropods.
• Spadefoot.
• Cephalopods.

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

Organ systems and organs

Mollusks, like many multicellular animals, are built from various types of tissues that are part of the organs. The latter, in turn, form

The structure of molluscs includes the following systems:

• circulatory;
• nervous system and sense organs;
• digestive;
• excretory;
• respiratory;
• sexual;
• body covers.

Let's look at them in order.

Circulatory system

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

• a heart;
• vessels.

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

In many soft-bodied, the 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 performs the same function as hemoglobin.

The blood in molluscs circulates in this way: from the blood vessels it flows into the spaces between the organs - lacunae and sinuses. Then she again gathers in the vessels and enters the gills or lung.

Nervous system

In molluscs, it is of two varieties: ladder and scattered-nodular type.

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

The nervous system of the scattered-nodal type is more complex. It consists of two pairs of nerve circuits. Two abdominal are responsible for the innervation of the internal organs, and two pedal - the legs. On both pairs of nerve circuits there are nodes - ganglia. Usually there are 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 organs, the sixth - other internal organs.

sense organs

There are such organs of molluscs 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 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;
• back gut.

There is also a liver. I also have a pancreas.

In the pharynx of the soft-bodied there is a special organ for grinding food - the radula. It is covered with teeth made of chitin, which are updated 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 mollusks are similar to those of worms. But they are more complex.

The excretory organs in mollusks look like a collection of tortuous glandular tubes. One end of the metanephridium opens into the coelomic sac, while the other end opens outward.

Excretory organs in mollusks can be present in different numbers. So, some cephalopods have only one metanephridium located on the left side. In monoplacophorans, as many as 10-12 excretory organs are observed.

Excretion products accumulate in the metanephridia of molluscs. 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 molluscs, it is represented by different organs. So, most soft-bodied have gills. They are also called ctenidia. These are paired bilaterally pinnate organs. They are located in the cavity of the mantle. Mollusks that live on land have a lung instead of gills. It is a modified mantle cavity. Its walls are permeated 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 molluscs.

Body coverings of molluscs

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

Let's look at the various body coverings that molluscs can have, examples of animals that belong to one class or another.

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

Bivalves, gastropods, cephalopods, monoplacphors, and spadepods lack cuticles. 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 outer part of the shell is always built from an organic chemical. Most often it is conchiolin. The only exception to this rule is the mollusc Crysomallon squamiferum from the class of gastropods. Its outer shell layer consists of ferrum sulfides.

The middle part of the shell of mollusks is composed 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 briefly consider the main organs and systems of soft-bodied organs in the table. We will also give examples of mollusks belonging to different classes.

The structure of molluscs

System	Organs	Peculiarities
circulatory	blood 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 certain organs.
digestive	pharynx, esophagus, stomach, intestines, liver, pancreas	There is a radula in the pharynx, which helps to grind food. The intestine is represented by the middle and hindgut.
excretory	metanephridia	Glandular tubes, one end of which opens outward, and the other - into the coelomic sac.
respiratory	gills or lung	Located in the cavity of the mantle.
sexual	ovaries, testicles	Among mollusks there are hermaphrodites, in which both male and female gonads are present at the same time. There are also separate species.

Now consider the representatives of various classes of the Mollusk type and the features of their structure.

Class	Examples	Peculiarities
Bivalves	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 type of food.
gastropods	Pond snails, slugs, coils, snails, bitinia	They have an asymmetric internal structure due to a twisted 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. These mollusks do not have an external shell. 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 molluscs of this class are represented by two or four kidneys (metanefridia).

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