General characteristics of gastropods. Key terms and concepts. Basic review questions

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

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

Mollusks are characterized by a hard mineral shell that covers the animal’s body from the dorsal side. The shell consists of crystals of calcium carbonate. On top it is usually covered with a horn-like organic substance, and on 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 (in sea ​​mollusks chitons). Slowly moving and immobile mollusks have a highly developed shell. However, in some mollusks it is reduced (underdeveloped) or absent altogether. This happens when the mollusk lives in places where it is difficult for predators to reach (for example, when it burrows deep into the sand of the seabed or drills passages 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 trunk, head and legs (Fig. 70). Almost all mollusks have a head. It contains a mouth opening, tentacles and eyes. The leg is a muscular unpaired outgrowth of the body. It is located on the ventral side and is used for crawling.

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

U bivalves Due to a sedentary lifestyle, the head is missing and the leg is reduced. In some species, the leg has become a swimming organ (for example, cephalopods).

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

Digestion. The mouth opening leads into the pharynx. In the pharynx of many species there is a grater (radula) - a special device in the form of a ribbon lying on the protrusion of the floor of the oral cavity. There are teeth on this tape. Using a grater, herbivorous mollusks scrape food from plants, while carnivorous mollusks (which have larger grater teeth) grab prey. Some predatory mollusks open into the oral cavity salivary glands. Secretion (secreted substance) salivary glands contains poison.

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

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

Circulatory system. The heart of mollusks 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 ensure the transfer of oxygen.

The excretory organs are represented by paired kidneys, which at one end communicate with the cavity of the pericardial sac, which surrounds the heart, and at the other 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 visual organs. The eyes of fast-swimming cephalopods are especially well developed.

Reproduction. Most mollusks are dioecious. However, there are also hermaphrodites in whom cross fertilization. Fertilization in mollusks can be external (for example, in the oyster and toothless snail) and internal (in the grape snail).

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

Meaning. Representatives of some classes of mollusks play important role in many natural biocenoses. Aquatic mollusks are often the most abundant group in benthic ecosystems. The filtration method of feeding of bivalves leads to the fact that many of them precipitate mineral and organic particles, providing water purification. Fish, birds and animals eat shellfish.

Shellfish serve as food for people and are traditional objects of fishing and farming (oysters, scallops, mussels, hearts, squid, Achatina, grape snail). Very beautiful pearls are formed in the shells of sea mollusks, pearl mussels. Aboriginal people used cowrie shells as coins. Geologists can use fossil mollusk shells to determine the age of sedimentary rocks.

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

A typical mollusc larva (sailfish) is very similar to an annelid larva, bearing 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.

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

Exercises based on the material covered

1. Name the main methods of movement of mollusks.
2. Name the main similarities and differences between the external and internal structures of mollusks and annelids.
3. What traits do fast-swimming mollusks have?
4. What is the influence of a sedentary lifestyle on the organization of mollusks?
5. What is the role of mollusks in nature and in human life? Give examples.

Lecture 6. Type Mollusca (Mollusca)

general characteristics type

Molluscs, or soft-bodied mollusks, comprise about 130 thousand species of animals that live in fresh and salt water; a number of species have adapted to life on land. Protostomes, secondary animals. Animals of the phylum are grouped into several classes: the Gastropoda class, the Bivalvia class, and the Cephalopoda class. Animals of this type are characterized by the following morphophysiological features: External structure . Bilaterally symmetrical animals, but some species become asymmetrical due to the spiral twisting of the body. Metamerism is preserved only in a number of primitive species; in the rest, an unsegmented body is formed, consisting in most of the head, torso and legs; Secondocavity animals , the coelom cavities contain the heart and gonads. The body forms the mantle, the mantle cavity contains the respiratory organs, and the excretory, reproductive and digestive systems open into it. On the dorsal side there is usually a protective shell; in the pharynx of most mollusks there is a radula, a grater for grinding food. Nervous system . The central nervous system is scattered - nodal type. Circulatory system open, there is a heart consisting of a ventricle and atria, sometimes additional hearts appear. Respiratory system . Respiratory organs - gills or lungs. Excretory system . The excretory organs are represented by one or two kidneys of the metanephridial type. Reproduction . Many mollusks are dioecious, but there are also hermaphrodites. Development is direct or with transformation, the larva in the lower ones is a trochophore, in most others it is a veliger larva. Phylogeny . Mollusks appeared at the end of the Proterozoic era from an unspecialized group polychaete worms. The main aromorphoses that led to the appearance of mollusks are as follows: 1. Segments merge into a small number of body sections, each of which provides specific functions. 2. There was a further concentration of the nervous system - the formation of large nerve nodes in various parts of the body. 3. A heart appeared, which increased the speed of blood circulation, which significantly increased the intensity of metabolic processes. 4. Digestive glands appeared, ensuring faster and more complete digestion of food. 5. Shells were formed that act as an external or internal skeleton and protect mollusks.

Structure and life activity

Rice. 123. Cross section of a bivalve mollusk:

1 - kidney, 2 - intestine, 3 - ligament, 4 - atrium, 5 - ventricle of the heart, 6 - coelom, 7 - mantle, 8 - mantle cavity, 9 - leg, 10 - gills, 11 - shell.

Rice. 124. Diagram of the internal structure of bivalve mollusks:

1 - mouth, 2 - anterior closing muscle, 3, 15, 20 - nerve nodes, 4 - stomach, 5 - liver, 6 - anterior aorta, 7 - external opening of the kidney, 8 - kidney, 9 - heart, 10 - pericardium , 11 - posterior aorta, 12 - hindgut, 13 posterior adductor muscle, 14 - anus, 16 - gills, 17 - gonadal opening, 18 - midgut, 19 - gonad.

The excretory organs are represented by two kidneys lying under the heart. Each of them begins in the pericardial cavity with a funnel lined with ciliated epithelium. The ureters open into the mantle cavity. Reproduction. Most bivalves are dioecious. Testes and ovaries are paired. The reproductive ducts open into the mantle cavity. Fertilization is external, in freshwater forms in the mantle cavity, where sperm penetrate through the gill siphon. Development occurs with metamorphosis. The larva of marine mollusks - trochophore, as a result of a number of changes turns into the veliger larva, or sailfish, characteristic of many mollusks. L

Rice. 125. Glochidium toothless

1 - muscle - closure; 2 - cloves; 3 - byssal thread

Rice. 126. Internal structure of a pulmonary mollusk:

1 - mouth, 2 - liver, 3 - anus, 4 - capillary network of the lung, 5 - atrium, 6 - ventricle, 7 - pericardium, 8 - kidney opening.

Key terms and concepts

1. Pericardium. 2. Glochidia. 3. Mantle. 4. Two-chambered heart. 5. Three-chambered heart. 6. Coelomoduct. 7. Mixocel. 8. Trochophore. 9. Veliger.

Basic review questions

Classification of mollusks.

Origin of mollusks.

Body parts of mollusks. What is the name of the fold of skin covering the body of mollusks? What is the secondary body cavity of mollusks? Respiratory organs of mollusks. Heart of Toothless and big pond snail. Pigments contained in the blood of mollusks that are responsible for oxygen transport. Features of the structure of the nervous system of mollusks. Excretory system of mollusks. Toothless larvae, marine mollusks. Origin of mollusks.

Mollusks are a large type of animal in terms of the number of species (130 thousand). They live mainly in the seas (mussels, oysters, squids, octopuses), fresh water bodies (toothless snails, pond snails, livebearers), and less often in a humid terrestrial environment (grape snail, slugs). Body sizes of adult mollusks different types 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 reactive manner.

Basic character traits structures of mollusks following blowing:

1. 
   
   The body is devoid of segmentation, has bilateral symmetry(bivalves and cephalopods) or asymmetrical (gastropods). The parts of the body are head with eyes located on it and 1-2 pairs of tentacles, torso, where most of the internal organs are located, and leg- the muscular abdominal part of the body used for movement. In bivalves, the head is reduced.
2. The body of the mollusk is enclosed sink, protecting the animal and providing support for muscle attachment. Outer layer The shells are horny, middle (porcelain) and inner (mother-of-pearl) - calcareous. Gastropods have a solid shell in the form of a cap or a spirally curled turret. In bivalves, it consists of two valves connected elastic ligament, “lock” teeth and closing muscles. Most cephalopods have lost their shells.
3. The body of the mollusk is covered with a fold of skin - 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 sensory organs, the anus, and the opening of the excretory organs are located.
4. Body cavity secondary (in general), however, it is greatly reduced and preserved only in the form of the pericardial cavity and the cavities of the gonads. The rest of the space between internal organs filled with loose tissue —parenchyma.
5. 
   
   Digestive system consists of three sections: foregut, midgut and hindgut. Most mollusks (except bivalves) have a muscular tongue in the pharynx, covered with a horny plate with numerous denticles - grater With it they actively capture and crush plant and animal food. Ducts open into the pharynx salivary glands, and into the stomach - the 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 inlet siphon.
6. Circulatory system open and consists of hearts And vessels. The heart has a ventricle and 1-2 (less often 4) atria. In addition to the vessels, blood passes part of the way in the slit-like cavities between the organs.
7. Respiratory organs in aquatic mollusks - gills, for terrestrials - lung, representing a section of the mantle cavity. The wall of the lung contains a dense network of blood vessels through which gas exchange occurs. The lung opens outwards with the respiratory opening - spiracle.
8. 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.
9. 
   
   Nervous system scattered-node type: five pairs of large ganglia are located in vital organs (head, leg, mantle, respiratory organs and sac) and are connected 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, while in hermaphroditic mollusks, fertilization is internal, cross-fertilization. In freshwater and terrestrial pulmonary mollusks, as well as cephalopods, development is direct, in marine bivalves and gastropods - with incomplete metamorphosis, i.e., with a planktonic larval stage, which contributes to their settlement.

Mollusks are a large type of animal in terms of the number of species (130 thousand). They live mainly in the seas (mussels, oysters, squids, octopuses), fresh water bodies (toothless snails, pond snails, livebearers), and 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 reactive manner.

The main characteristic features of the structure of mollusks are as follows: blowing:

1. The body lacks segmentation and is bilaterally symmetrical (bivalves and cephalopods) or asymmetrical (gastropods). The parts of the body are head with eyes located on it and 1-2 pairs of tentacles, torso, where most of the internal organs are located, and leg- the muscular abdominal part of the body used for movement. In bivalves, the head is reduced.
2. The body of the mollusk is enclosed sink, protecting the animal and providing support for muscle attachment. The outer layer of the shell is horny, the middle (porcelain) and inner (mother-of-pearl) are calcareous. Gastropods have a solid shell in the form of a cap or a spirally curled turret. In bivalves, it consists of two valves connected elastic ligament, “lock” teeth and closing muscles. Most cephalopods have lost their shells.
3. The body of the mollusk is covered with a fold of skin - 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 sensory organs, the anus, and the opening of the excretory organs are located.
4. Body cavity secondary (in general), however, it is greatly reduced and preserved only in the form of the pericardial cavity and the cavities of the gonads. The remaining space between the internal organs is filled with loose tissue - parenchyma.
5. The digestive system consists of three sections: the foregut, midgut and hindgut. Most mollusks (except bivalves) have a muscular tongue in the pharynx, covered with a horny plate with numerous denticles - grater With it they actively capture and crush plant and animal food. Ducts open into the pharynx salivary glands, and into the stomach - the 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 inlet siphon.
6. Circulatory system open and consists of hearts And vessels. The heart has a ventricle and 1-2 (less often 4) atria. In addition to the vessels, blood passes part of the way in the slit-like cavities between the organs.
7. Respiratory organs in aquatic mollusks - gills, for terrestrial - lung, representing a section of the mantle cavity. The wall of the lung contains a dense network of blood vessels through which gas exchange occurs. The lung opens outward with the respiratory opening - spiracle.
8. 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.
9. Nervous system scattered-node type: five pairs of large ganglia are located in vital organs (head, leg, mantle, respiratory organs and sac) and are connected 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, while in hermaphroditic mollusks, fertilization is internal, cross-fertilization. In freshwater and terrestrial pulmonary mollusks, as well as cephalopods, development is direct, in marine bivalves and gastropods - with incomplete metamorphosis, i.e., with a planktonic larval stage, which contributes to their settlement.