Lesson "the diversity and significance of mollusks." Type and classes of mollusks

Municipal state educational institution

“Arkhangelsk Secondary School named after A.A. Kudryavtsev"

Arkhangelskoye village, Kamensky district, Tula region

LESSON TOPIC “Diversity of mollusks”

Sindeeva E.A.,

biology teacher

LESSON OBJECTIVES

• 1. Educational:

• learn the structural features and adaptability of mollusks of different classes to living conditions;

Determine the significance of mollusks in nature and in human life.

• 2. Developmental:

• develop speech, logical thinking, memory, attention, independence, interest in the subject.

1. The most numerous class of mollusks.

2. Found in the depths of the ocean, in the coastal zone of oceans and seas, in fresh waters, in mountains and caves, rocky deserts.

3. The shell is solid, often

asymmetrical (twisted to the right or left, cone-shaped)

4. Representatives of gastropods - slugs do not have shells.

5. The body is divided into head, torso and leg.

6. The olfactory organs are well developed.

• Slugs and vine snails are pests of gardens and vegetable gardens. radula snails. WMV
• In many European countries, grape snails are valued as an excellent delicacy, and in some areas they are a common food for the population and are eaten in huge quantities.
• Nowadays, many farms for breeding grape snails have been created in different countries.

1. Exclusively aquatic animals.

2. All bivalve shells consist of two valves , tightly connected to each other elastic ligament .

3. A characteristic feature is the absence of a head.

• The largest

bivalve

mollusk tridacna.

• He reaches

up to 1.5 m in length and

weighs up to 250 kg.

• In the shells of pearl oysters, pearls are formed - a spherical formation.
• It is produced as a protective reaction to irritation of the mantle by some foreign body.
• Pearls are used to make jewelry.

• Cephalopods are the most developed mollusks.
• The majority of the body lacks a shell.
• The leg is transformed into tentacles - legs that surround the head.
• Cephalopods are large mollusks up to 20 m long.
• They actively move in the water column and near the bottom. cephalopods. mp4

CLASS Cephalopods SHELLFISH

• Cephalopods include

squid,

octopuses,

cuttlefish

• A mysterious inhabitant of the deep sea is the cephalopod nautilus, a relative of octopuses.

gastropods

bivalve

cephalopods

• Mandatory: learn point 12, answer questions on page 52, repeat point 11
• Creative (optional):
• 1. Compose a crossword puzzle “Shellfish, their diversity”
• 2. Prepare reports about the classes of Echinoderms

Class Gastropods, or Slugs

The class includes about $85$ thousand species, their size varies from a few millimeters to $15$ cm. In most species, the shell is twisted into a spiral and covers the entire body. Slugs have a small shell or it is completely reduced. The main developed organ is the leg. The back of the leg has a horny operculum that tightly closes the shell in case of danger or unfavorable conditions.

Among gastropods, dioecious and hermaphrodites are distinguished. Fertilization is internal. Their development occurs due to the formation of a larva; a stage without a larva is less commonly observed.

This class has acclimatized to a wide variety of living conditions. They inhabit coastal zones of oceans, seas and other depths, and fresh water bodies. Some species have adapted to a terrestrial existence, even inhabiting deserts and mountain peaks. The respiratory organs of freshwater and terrestrial forms are the so-called lungs. In marine inhabitants, they are represented by gills. Land mollusks have horns on their heads - these are sensory organs. At the ends of the horns there are eyes.

Gastropods feed directly on plankton, aquatic and terrestrial plants, worms, crustaceans and other types of mollusks.

Class Bivalves

This class of mollusks includes about $20,000 species, most of which live in the seas, and some of them inhabit fresh waters. The shell of bivalve mollusks consists of valves connected to each other by an elastic ligament. Each valve in the middle is covered with a mantle. In case of danger, the mollusk simply closes the shell valves. This occurs due to the work of muscles. For a strong closure, the doors have teeth.

Note 1

The head and sensory organs of bivalves are reduced. The pharynx, grater and salivary glands are also absent.

The mantle cavity contains gill sheets, which are washed by a stream of water.

1. Thus, the circulation of water in the middle of the mantle provides the mollusk with food and oxygen.
2. From the mouth, food enters the short esophagus, stomach and intestines.
3. The alimentary canal does not have muscle fibers, so food moves due to the work of the ciliated epithelium.
4. Parts of food that are not digestible are removed using an excretory siphon.

Bivalves are water filters.

Among the representatives of bivalves, both hermaphrodites and dioecious species are distinguished. Development includes both the larval stage and can do without it.

Class Cephalopods

More than $700 species of cephalopods have been described. The shell is presented in a rudimentary manner. The mouth opening has tentacles with suckers. The main part of the body is occupied by the head. The organs of vision are the eyes; accommodation is achieved by moving the crystal away and approaching the retina. Taste buds are located on the tongue and suckers of the tentacles. The balance organs are represented by statocysts.

Note 2

The blood is blue, which proves the presence of the hemocyanin pigment that Cuprum contains. Cephalopods have three hearts.

The main heart consists of an atrium and a ventricle, its function is to ensure the flow of blood throughout the body. The other two hearts push blood through the gills.

The mouth is small, the muscular pharynx has a horny beak.

Cephalopods are exclusively predators. They feed on fish, crabs, turtles and people. Their movement is reactive.

Cephalopods are exclusively dioecious. Fertilization is internal. The male uses a tentacle to transfer the sperm into the female's genital opening. Development is direct.

Note 3

Since the shell is reduced, protection for cephalopods is a change in the color of their body. Mollusks contain various pigments under their skin.

The body is clearly divided into a head with eyes, tentacles and mouth, a leg and a torso. The turtle is whole, spirally twisted, 4-7 cm long.

Living conditions: fresh water bodies, coastal areas.

Movement - on the abdominal side due to contraction of the leg muscles.

Diet - feeds on plants, small animals and animal corpses, from which it scrapes layers of tissue using the radula. The intestine bends in a loop and ends with the anus in the front of the mantle.

Transport of substances is carried out by an unlocked circulatory system. Colorless blood flows through the vessels and between parenchyma cells. Blood movement is ensured by the contraction of the two-chamber heart (20-30 beats per minute).

The secretion of substances is one kidney, the ducts of which open into the mantle cavity.

Response to stimuli - the nervous system has a well-defined main ganglion. There are eyes, an organ of balance (in the leg), organs of touch and organs of chemical sense, with the help of which the quality of water is recognized. In case of danger, it quickly hides in the shell.

Reproduction is hermaphrodite. Internal cross fertilization. The eggs are covered with mucus and are laid on the surface of underwater plants. The larval stage continues in the egg; the young mollusk emerges in a fully formed state.

Class Bivalves (toothless).

The body has bilateral symmetry, distributed over the torso and leg. The shell consists of two valves connected to each other by an elastic ligament and closing muscles; thanks to their contraction, the valves quickly close. In the dorsal part there are inlet and outlet siphons. Through the first, water enters the mantle cavity, and through the second, it is discharged out.

Conditions of existence - lives at the bottom of fresh water bodies, plunging into sand and silt.

Movement - with the help of its legs, it goes deep into sand or silt and moves at a speed of 20-30 cm / hour.

Diet - feeds on small organisms and their remains enter the mantle cavity along with water through the inlet siphon. In the mantle cavity there is a mouth with two blades, through which water and food particles enter the intestines. The intestine forms several loops and ends with the anus in the mantle cavity.

Respiration - breathes oxygen dissolved in water, which it absorbs through gills located in the mantle cavity.

Transport of substances is an open circulatory system with a three-chambered heart.

Excretion of substances – two kidneys.

Response to stimuli - the nervous system consists of three pairs of nerve ganglia located near the mouth, the posterior adductor muscle and in the leg. This placement of the nerve ganglia ensures instant closure of the edentulous valves in case of danger. Along the edges of the mantle there are tactile receptors and chemoreceptors, and at the leg there are balance organs.

Class Cephalopods.

The body is divided into a head, torso and tentacles (branched legs). The body is covered with a mantle, under which, on the dorsal side, there are remains of a shell (only the nautilus has an external shell). Tentacles surround the mouth. Body sizes can reach 2.5 m (giant squid).

Living conditions are exclusively marine animals. They swim with the help of tentacles and in a reactive way - they draw water into the mantle cavity and sharply push it out through the funnel. Some squids can reach speeds of up to 40 km/h.

Food: predators. They catch prey with tentacles, kill it with a chitinous beak and eat it.

Breathing is the same as in bivalves.

Transport of substances - the circulatory system has well-developed capillaries and is practically closed.

Excretion of substances - kidneys.

Reaction to stimuli - they are distinguished by complex forms of behavior, capable of learning and easy formation of conditioned reflexes. The nerve ganglia merge into the brain, surrounded by a cartilaginous membrane. They have large, complexly constructed eyes. They have a delicate sense of smell. A protective form of behavior in some is the release of an ink cloud from a special cavity when an enemy approaches.

Reproduction: dioecious animals. Fertilization is internal. Direct development - the larva has all the organs of an adult, but sometimes differs in body shape. Characteristic concern for one's offspring.

Mollusks are a numerous (about 150 thousand species) and diverse type of animals. The phylum includes about 8 classes, among which the class Gastropods (snails, slugs) is the most numerous. Next come Bivalve mollusks (barley, mussels, oysters, etc.). Among other classes, cephalopods (octopuses, squids, cuttlefish) are of greatest interest.

Despite the apparent difference in the structure of the listed representatives, all mollusks have much in common, especially in the internal structure. This commonality among species can only be observed in the process of embryonic or evolutionary development. For example, the body of a mollusk consists of a head, torso and legs, and is also covered with a mineral shell. However, in bivalves the head is usually reduced (that is, it once was), and many cephalopods do not have a shell at all (it was also reduced in the process of evolution). Thus, the diversity of mollusks was formed as a result of adaptation to various living conditions and methods of life.

Mollusks mainly live in salt waters; among bivalves and gastropods there are those that live in fresh waters. Among gastropods there are terrestrial species (for example, slugs, vine snails), but they live in damp places. Thus, despite their diversity, mollusks are predominantly aquatic animals.

Bivalves are immobile or sedentary. Gastropods (snails) usually crawl slowly, but cephalopods are fast-moving mollusks. Moreover, they move in a reactive manner, pushing water out of the mantle through a special siphon funnel.

The shells of mollusks are also diverse. In snails it is spirally twisted. The body located in it also twists and acquires asymmetry, although all mollusks initially have bilateral (two-sided) symmetry. Usually, snails have only their bodies in the shell, while their heads and legs with a wide sole are outside.

In bivalves, the shell consists of two valves, the entire headless body is located inside. Sometimes only a wedge-shaped leg protrudes outward, with the help of which the shell moves.

Among mollusks there are both herbivores and predators. Of particular note are bivalves, which filter a lot of water and “eat away” various organic contaminants from it, thereby helping to cleanse water bodies. The digestive system of mollusks is characterized by the presence of a radula (grater) in the pharynx. The radula has teeth that are used to scrape or grind food. However, in bivalves, due to the loss of the head, there is no pharynx at all. In cephalopods, the grater rather serves an auxiliary function, since they often have more powerful formations in their throats for tearing apart prey.

Respiration in gastropods can be carried out by the lungs (in terrestrial species or secondary aquatic species) or by gills. Representatives of other classes have only gills.

The diversity of mollusks is also expressed in the reproductive system. Among them there are both hermaphrodites (for example, the pond snail) and dioecious ones (for example, all cephalopods). From fertilized eggs, larvae (sailfish, glochidia) may first develop, or development may be direct, i.e., a small mollusk immediately emerges from the egg.

Mollusks are also called soft-bodied mollusks because there are no bones inside their bodies. However, cephalopods have cartilaginous structures inside their bodies. In general, cephalopods are the most advanced and diverse class of mollusks. In their nervous system, a semblance of a brain is already formed, since the ganglia come together in the head, while other classes have three pairs of ganglia scattered throughout the body (in the head, leg, torso).

The tentacles of snails and cephalopods (octopuses, etc.) have different origins. In snails, they are formed on the head, they are organs of touch, and the eyes are located on them. In cephalopods, the tentacles are formed as a modification of part of the leg of a mollusk, i.e., in essence, it is a leg.

The phylum molluscs are soft-bodied animals, predominantly with a bilaterally symmetrical structure, inhabiting both water bodies and land. There are more than 120 thousand species.

The sizes of mature mollusks of different classes differ significantly - from a couple of millimeters to 20 m. Many lead a sedentary or sedentary lifestyle, and only cephalopods are able to actively move in water. The science of mollusks is called malacology, she studies the structure, development of soft-bodied animals, and their role in the world around them.

Features of the structure of Mollusks

External structure

The body is bilaterally symmetrical in bivalves and cephalopods, or asymmetrical in gastropods. Its sections are distinguished as follows: the head part with the organs of vision and tentacles, the body itself and the leg - a muscular formation, used for movement. All bivalves are characterized by the presence of a leg, but in cephalopods it is transformed into tentacles and a siphon.

The body of the mollusk is surrounded by a shell and serves as a place for muscle attachment. In gastropods, it has a solid structure in the form of a spiral curl. In bivalves, it is represented by two valves, which are connected by flexible strands of connective tissue. Most cephalopods lack shells.

A mantle composed of epithelial cells extends from the lateral parts of the body. Together with the body, it forms a cavity where the gill arches, sensory organs, excretory ducts of the glands of the digestive tract, genitourinary system, and anus are located.

Mollusks are coelomic organisms, but their secondary cavity is preserved only near the heart and genitals. The main part of the internal space is represented by the hemocoel.

Internal structure

Digestive system of shellfish divided into three parts: foregut, midgut and hindgut. Many representatives have a radula in the pharynx - a tongue designed for grinding food. It has chitinous plates with teeth. With the help of the radula, they absorb bacteria or plant food. Saliva is secreted into the pharyngeal cavity and glues food particles together. The food then enters the stomach, where the digestive gland (liver) opens. After digestion, the remains are excreted through the anus.

Circulatory system open, the heart has a ventricle and usually two (rarely four) atria. From the bloodstream, blood enters the sinuses and lacunae located between the organs, then again passes into the vessels and goes to the respiratory organs.

Breath in aquatic species it is carried out by gills; in land dwellers - by the lungs. Lung tissue is equipped with a dense vascular network where oxygen and CO 2 are exchanged. The lung communicates with the external environment through a spiracle.

Nervous system of mollusks consists of five pairs of nerve ganglia, united by fibrous cords. The unequal development of sense organs in mollusks indicates a different lifestyle of representatives of the phylum.

For example, cephalopods have fairly developed vision, the structure of the eye is similar to the structure of the eye of vertebrates. Their predatory nature forced them to adapt to changing environmental conditions through the complication of their visual apparatus. They developed a peculiar type of accommodation, which was carried out by changing the distance between the retina and the lens.

Mollusks reproduce sexually. There are both dioecious (with external fertilization) and hermaphrodites (with internal fertilization). In marine bivalves and gastropods, development is indirect, there is a larval stage, in others it is direct.

Structural features of mollusks compared to annelids

What new organs have appeared in mollusks compared to worms?

Mollusks have specialized organs. This is the excretory, digestive system, which includes a number of departments, including the heart and liver. Respiratory organs - gills or lung tissue.

The circulatory system is open, while in annelids it is closed.

The nervous system of mollusks has the form of nerve ganglia, connected to each other by nerve fibers. Annelids have a nerve cord only in the abdominal region, which branches into segments.

How are mollusks adapted to their environment?

Representatives of the type inhabit water spaces and land surfaces. To exist outside a body of water and breathe atmospheric air, soft-bodied animals developed lung tissue. Inhabitants of reservoirs obtain O2 with the help of gill arches.

How do mollusks protect themselves from enemies?

To move in water, cephalopods have adapted to jet propulsion, so they can quickly run away from enemies.

Toxic and chemical substances (ink) serve as protection against predators. Some are able to bury themselves in the sandy bottom in seconds if there is a threat or hide using a springy leg.

What is the function of a mollusk shell?

First of all, it has a supporting function and serves as an exoskeleton. Also, strong shells of bivalves and gastropods are needed for protection from adverse factors. So, when danger approaches, they hide in them and become inaccessible to most fish.

Similarities and differences between gastropods and bivalves

Properties	Gastropods	Bivalve
Non-systematic category	Multicellular organisms
Outer covers	The body is surrounded by a shell (entirely or partially)
Sink	Piecework, asymmetrical and twisted	Has two doors
Body structure	Head, torso and leg	Torso, leg
Analyzers	Tactile, chemical reception, balance and vision.	Underdeveloped
Habitat	Water and land	Reservoirs

The importance of mollusks in nature and human life

They are an integral part of the food chain. Soft-bodied animals are consumed by frogs, fish, and birds. Seals eat cephalopods, starfish eat bivalves.

Water passes through the body of the mollusk and is purified from pollutants. And the mollusks, in turn, receive food particles from filtered water.

Soft-bodied valves take part in the formation of sedimentary rocks.

They are widely used in cooking and are considered a delicacy in many countries. This includes mussel meat, scallops, oysters, cuttlefish and octopus. Due to the popularity of dishes from exotic animals, they began to be raised on specially equipped farms.

Between the shell flaps, valuable jewelry raw materials—pearls—are formed. A pearl is formed after a foreign body gets inside. Since the muscles of the mollusks are not sufficiently developed, they cannot throw it out. To neutralize a foreign object, a capsule is formed around it and the mollusk lives with the newly formed pearl all its life.

Nowadays pearls are mined in artificially created conditions. Having opened the valves slightly, foreign objects are placed under the mantle, and the mollusk is moved to a reservoir with conditions favorable for life and after three years they obtain pearls.

Cuttlefish and octopuses are used to extract the inky substance from which ink is made.

Agricultural pests - slugs - destroy grain crops and garden plants (potatoes, cabbage, tomatoes).

Flatworms, which cause diseases in humans and animals, use shellfish as intermediate hosts.