General features characteristic of molluscs. Excretory organs in molluscs. The structure of molluscs

General signs of molluscs

structural features

Body multicellular unsegmented, observed division of the body into three sections. Head, torso and legs. In most mollusks, the body is binary-symmetrical, however, there is a group of asymmetric mollusks (class Gastropods) that have lost bilateral symmetry as a result of the displacement of a number of organs.

covers . The body is covered with a leathery mantle, which is located under the shell. Mantle- a fold of integument, covers the whole body or part of it. Between the mantle and the body is mantle cavity - the gap between the mantle and the body, connected to environment. The respiratory organs are located in it, and the anus, ducts of the excretory and reproductive systems open into this cavity. The glandular cells of the mantle form sink(shell), which can be solid, bivalve or consist of several plates.

Cavity secondary. Mollusks are cavities with a residual coelom present in most forms. pericardial sac and the cavity of the sex glands - the gonads. The spaces between the organs are filled with loose connective tissue - parenchyma.

Features of life processes

Support ensured exoskeleton in the form of a shell, which consists of three layers: outer - horny, middle - porcelain and inner - mother-of-pearl. The mineral-organic turtle is hallmark mollusks, which in typical cases covers the entire body of the mollusk and performs protective function. In some groups, the turtle is completely (slizuna) or partially reduced (octopuses).

Motion muscular involving the leg. There is a division of muscles into groups: in most muscles consists of smooth muscles, cephalopods already have cross-buttering, the reduction of which ensures rapid movement. The leg in cephalopods turns into tentacles and funnel - special body, which serves for jet propulsion.

Digestion carried out by the digestive system, in which new creations appear. The mouth belongs to the anterior salivary glands and grater, pharynx, esophagus and stomach with ducts of a paired or odd digestive gland, which is called liver. The middle section is represented by the middle intestine. The posterior section is formed by the posterior intestine with anus. In many mollusks in the oral cavity there is a special organ similar to the tongue, called a grater or radula. This is a small protrusion with a cartilaginous base plate, covered with a ribbon with chitinous teeth. Thanks to the muscles, the grater can move back and forth in the oral cavity and move out of the oral cavity. In the oral cavity, except salivary glands, the ducts of other glands also open, for example, poisonous or acid-producing ones.

Transportation of substances It is carried out by an unlocked circulatory system, that is, a system in which blood moves not only by vessels, but also in the spaces between organs (lacunae and sinuses). In the system of molluscs already isolated arteries and veins, as the heart appears. arteries blood vessels that carry blood away from the heart. Vienna - blood vessels that carry blood to the heart. A heart as a muscular organ, the contraction of which ensures the movement of blood in the body, in mollusks it is two-, three- or four-chambered and consists of a ventricle and one or more atria. Oxygen-rich arterial blood moves through the heart. From the organs to the lungs or gills, venous blood moves, saturated carbon dioxide. There it turns into arterial and goes to the heart. In the blood of many mollusks there is a respiratory pigment hemocyanin, which determines its blue color.

Breath with respiratory system. The respiratory organs are in the mantle cavity and presented gills and lungs. Gills are inhabitants of water bodies, lungs are terrestrial mollusks or mollusks whose ancestors lived on land.

Selection carried out by the excretory system, the main organs of which are kidneys. In molluscs, the kidneys are modified metanephridia. The tubule of each kidney begins with a funnel in the cavity of the pericardial sac, and the other end opens into the mantle cavity.

Process regulation occurs with the nervous system scattered-nodal type. The central nervous system, according to the body parts, contains paired nerve nodes: the main, foot and trunk; The PNS is represented by nerve branches. The behavior of most mollusks is instinctive. instincts- a certain sequence of unconditioned reflexes aimed at the implementation of vital important function. Conditioned reflexes and individual manifestations of elementary rational activity are observed in octopuses.

Irritability It is provided in various mollusks by sensory organs that are different in structure and degree of development. Most have eyes, organs of touch (tentacles), chemical sense and balance. In mollusks, chemoreception is well developed, which is carried out nerve cells tentacles, areas around the mouth and chemical sense organs at the base of the gills (osphradia). In the gill molluscs Osphradia, try the water that enters the mantle cavity and brings with it various odors.

reproduction carried out by dioecious and hermaphrodite systems. Fertilization is external or internal.

Development direct or indirect (with transformation).

BIOLOGY +Cephalopods, or "primates of the sea" as I. Akimushkin called, have blue blood - octopuses, squids, cuttlefish. It is also observed in individual gastropod mollusks, for example, in grape snails known to us, in decapod crustaceans (lobsters, lobsters, crabs, shrimps, crayfish) , in many arachnids (horseshoe crab, scorpions) and others. The blue color is due to chemical element like copper (copper) , which contains hemocyanin. It is an important bioelement that is part of organisms and plays a very significant role in physiological processes vital activity. In plants, it is involved in the implementation of photosynthesis and the absorption of nitrogen, in animals it is important for maintaining the structure of bones, cartilage, and the elasticity of blood vessels. It is part of the shells of long nerve processes, which is, quite possibly, one of the reasons that the nervous system in cephalopods is the most of all invertebrates.

Question 1. What are the main features of mollusks?
The main signs of mollusks:
- the body in most cases consists of a head, a trunk and a muscular leg;
- the torso is a skin-muscular sac surrounded by an extensive skin fold - the mantle;
- between the mantle and the wall of the body, a mantle cavity is formed, in which the respiratory organs, some sensory organs are located, the anus, ducts of the kidneys and gonads open into it;
- on the dorsal side, as a rule, there is a protective shell;
- the digestive system consists of the anterior, middle and hindgut;
- in the pharynx there is usually an organ that grinds food - a grater with horn teeth located on it;
- the grater is used for scraping plant food, and in predators - for its active capture;
- the ducts of the digestive gland, which combines the functions of the liver and pancreas, open into the middle intestine; here food is digested and nutrients accumulate;
- the circulatory system is not closed (except for cephalopods);
- the movement of blood provides a two-chambered heart;
- there are organs of vision, balance, chemical sensitivity, touch;
- respiratory organs in aquatic forms - gills, in terrestrial - lungs; - represent a specialized area of ​​​​the skin fold - the mantle.

Question 2. What is a mantle?
The mantle is a fold of fabric that covers the body and hangs over the edges of the leg. Between the mantle and the wall of the body, a mantle cavity is formed, in which some sensory organs are located and where the anus, ducts of the kidneys and sex glands open.

Question 3. What is a sink? What are its functions?
The shell is a protective skeletal formation that covers the body of molluscs. It mainly performs a protective function.

Question 4. How is the shell of mollusks arranged?
The shell is made up of eight individual plates that are often fused. Three layers are distinguished: the outer one is made of organic matter (horny), the middle one is calcareous and the inner one is mother-of-pearl.

Question 5. What is the structure of the nervous system of mollusks?
Nervous system molluscs of scattered-nodular type; it consists of the peripharyngeal nerve ring, in which the supraesophageal ganglion ("brain") receives the greatest development, and the nerve trunks extending from it, connecting the nerve ganglia of different parts of the body.

Question 6. How do mollusks breathe?
The respiratory organs in most species are represented by gills, while in terrestrial representatives - lungs. The role of the respiratory organs is performed by a region of the mantle rich in vessels.

Question 7. What do molluscs eat?
Mollusks feed on organic matter suspended in water and plankton. There are also predators that eat worms, crustaceans or other mollusks.

Question 8. How is the digestive system of mollusks arranged?
Digestive system consists of a mouth, pharynx, esophagus, stomach and intestines, ending with an anus in the mantle cavity. The pharynx usually has an organ that grinds food - a grater (radula) with horny teeth located on it. As a rule, the grater is used for scraping off plant food and only in rare cases (for predators) for its active capture. The ducts of the digestive gland, which combines the functions of the liver and pancreas, open into the midgut.

Question 9. What are the excretory organs of mollusks?
The excretory organs are represented by the kidneys, the ducts of which open into the mantle cavity. There are one, two or four kidneys lying under the heart.

Question 10. Are there hermaphrodites among molluscs?
Most mollusks are dioecious, but there are also hermaphrodites, such as pond snails and coils.

Question 11. What classes are united by the type of molluscs?
The phylum Mollusca includes the classes:
1) Gastropods ( grape snail, patella, abalone, murex, cone, meadow grass, slugs, water ponds, coils, etc.).
2) Bivalves (mussel, oyster, cockle, scallop, freshwater pearl mussel, common toothless, etc.).
3) Cephalopods (cuttlefish, nautilus, argonaut, octopus, etc.).

Question 12. What are the structural features of gastropods.
Structural features of gastropods: asymmetric structure; the shell is spirally twisted, sometimes in the form of a cap; the body is divided into head, torso and leg; on the head are tentacles, eyes and a mouth with a grater;
in most aquatic forms, the respiratory organs are gills; at terrestrial species there is a lung in the mantle cavity.

Question 13. What is the structure of the body bivalves?
Bivalves have the following structural features: bilaterally symmetrical animals; the body is oblong, laterally flattened, consisting of a trunk and a leg; at the front end of the body there is a mouth, at the back there is an anus; the shell consists of two valves connected by an elastic ligament and a lock consisting of tooth-like outgrowths on one valve and corresponding depressions on the other; the body is covered with a mantle surrounding bivalves with two large mantle folds, between which a mantle cavity is formed, where the leg is placed; sense organs are poorly developed.

Question 14. What role do shellfish play in nature?
Bivalves are powerful natural water purifiers (biofilters). One oyster filters about 10 liters of water per hour, clearing it of suspended particles.

Question 15. How does a person use shellfish?
Some shellfish (very few) are important as a source of mother-of-pearl and pearls. For example, mother-of-pearl products are made from barley shells. The pearls of the sea pearl oyster are highly valued. Many marine bivalves are eaten - oysters, mussels, scallops, etc.; edible and squid (cephalopods). Crushed shells are used to feed birds.

With all the external differences between bivalves, gastropods and cephalopods, in all these animals one can catch one general type organization, characteristic of all molluscs and distinguishing them from all other types of the animal world.

The body of mollusks is not segmented. Unlike worms, movement is not carried out by the whole body, but only by a special part of it - a muscular leg. The remaining parts of the body, which themselves do not participate in the mechanism of movement, are either wholly or partially covered by a fold of skin - a mantle, which in the vast majority of forms releases a calcareous shell; in this regard, the integuments of the body remain soft (soft-bodied type).

The shell usually serves as a protective outer shell for the mollusk; it also allowed one order of gastropods to master the conditions of the terrestrial environment, protecting animals from drying out (land snails). In other cases (in most modern cephalopods), the shell is overgrown with soft tissues and is modified, forming no longer an outer shell, but an internal support, or internal skeleton (Fig. 131).

A mantle cavity remains between the mantle and the body itself, in which respiratory gas exchange takes place either with the help of special outgrowths of the body - gills, or with the help of the lungs formed by the walls of the mantle cavity.

In the internal structure of molluscs, there are also a number of common features characteristic of the entire type. In the circulatory system, they have a central organ - the heart. It is located inside the pericardial cavity, which is a modification of the body cavity.

The central nervous system consists of several double ganglions connected by paired nerve cords. In sedentary bivalve mollusks, the head node is not larger than the others, but in gastropods, and especially in cephalopods, leading a more active lifestyle, the head, or brain, node and the sensory organs associated with it are much more developed.

Rice. 131. The scheme of construction of the body of molluscs, highlighting common features this type

A is double-sided. B - gastropod and C - cephalopod. The shaded part of the body is the leg, the convoluted tube is the intestines. blackened nerve nodes and strands. The arrows show the direction of water flow entering and leaving the mantle cavity.

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 kinds tissues that make up 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. The respiratory pigment hemocyanin gives it this color. chemical composition which includes copper. This substance performs the same function as hemoglobin.

The blood in molluscs circulates in this way: from the blood vessels it pours 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.

Excretory organs in molluscs

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 molluscs may be present in different amount. 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 mollusks can have, examples of animals that belong to one class or another.

So, in 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 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.

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

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

The type of molluscs includes invertebrate coelomic animals with bilateral symmetry. This type includes two subtypes (lateral nerve and testate), divided into seven classes: gastropods, cephalopods, spadefoots, bivalves, armored, solenogasters, pit-tails. 98% of mollusk species are included in the two most extensive classes - Gastropods and Bivalves. The type unites more than 130 thousand species of animals that are distributed throughout our planet. In the course of evolution, molluscs appeared about 500 million years ago. Several classes are known from the Lower Cambrian. Many scientists agree that these organisms evolved from primary coelomic animals. This is their common ancestor annelids with which molluscs have common features buildings. Habitat - seas, fresh water. Some of them live on land in a humid environment.

The structure of molluscs . All molluscs can be given the following characteristic: they are secondary cavities and have salient feature structures - bilateral symmetry of the body. However, in some groups, the symmetry of the body may be violated for the second time due to uneven growth or displacement of organs. For example, this happens in gastropods as a result of the presence of a turbospiral shell. The body can be roughly divided into a non-segmented trunk, head, and leg. On the dorsal side, many of them have a shell, while some of them are absent or poorly developed. On the head are tentacles, mouth, eyes. In representatives of the class of bivalves, the head is secondarily reduced. The organ of movement is the leg - a muscular unpaired formation of the abdominal part of the body. In inhabitants of water bodies, the leg can be transformed into a swimming organ, in some species it is completely lost. The trunk is an internal sac with internal organs, outside covered with a mantle - a fold of skin. The mantle takes part in the creation of the shell. In the mantle cavity, formed directly by the mantle and the body, there are respiratory organs, sensory organs, and the ducts of the reproductive and excretory systems and the anus also open into this cavity.

Nutrition shellfish. Nutrition of terrestrial mollusks - plant food, marine - plankton, bacteria, small plants. Many are exclusively herbivores. Sedentary organisms from the class of bivalves are filter feeders by the mechanism of obtaining food by filtering water through the gills, followed by the movement of settling particles into the oral cavity. The digestive system is represented by a through intestine, consisting of three parts: anterior, middle (stomach) and posterior, ending with the anus. Mollusks have a paired digestive gland called the "liver". Breathing is carried out through ctenidia (the prototype of the gills), and in terrestrial forms the mantle cavity turns into a lung. Circulatory system open, the heart consists of one ventricle and has from one to four atria different types. The nervous system is represented by a near-pharyngeal nerve ring and several paired nerve nodes - ganglia. The excretory system consists of two kidneys.

reproduction shellfish. Some species of mollusks are dioecious, but there are also hermaphrodites. A larva or a formed mollusk emerges from a fertilized egg in lower forms, rare higher forms are viviparous.