Mollusk type. Class Gastropoda

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.

The type of molluscs, numbering about 130,000 species, is second in number of species only to arthropods and represents the second largest type of animal world. Mollusks are predominantly aquatic; only a small number of species live on land.

Mollusks are of various practical importance. Among them there are useful ones, like pearl and barley, which are mined in order to obtain natural pearls and mother-of-pearl. Oysters and some other species are harvested and even bred for food use. Some species are pests of agricultural crops. From a medical point of view, mollusks are of interest as intermediate hosts of helminths.

General characteristics of the type

Animals belonging to the type of molluscs are characterized by:

• three-layer, - i.e. formation of organs from ecto-, ento- and mesoderm
• bilateral symmetry, often distorted due to displacement of organs
• non-segmented body, usually covered by a shell, whole, bivalve, or consisting of several plates
• skin fold - a mantle that fits the whole body
• muscular outgrowth - a leg that serves to move
• poorly defined coelomic cavity
• the presence of the main systems: the apparatus of movement, digestive, respiratory, excretory, circulatory system, nervous and sexual

The body of mollusks has bilateral symmetry, in gastropods (they include, for example, a pond snail), it is asymmetrical. Only the most primitive mollusks retain signs of segmentation of the body and internal organs; in most species, it is not divided into segments. The body cavity is secondary, presented in the form of a pericardial sac and a cavity of the gonads. The space between the organs is filled with connective tissue (parenchyma).

The body of mollusks consists of three sections - the head, trunk and legs. In bivalves, the head is reduced. Leg - a muscular outgrowth of the abdominal wall of the body - serves for movement.

A large skin fold, the mantle, is developed at the base of the body. Between the mantle and the body there is a mantle cavity, in which there are gills, sensory organs, openings of the hindgut, excretory and reproductive systems open here. The mantle gives off a shell that protects the body from the outside. The shell can be solid, bivalve or consist of several plates. The composition of the shell includes calcium carbonate (CaCO 3) and organic matter conchiolin. In many mollusks, the shell is more or less reduced (for example, in some cephalopods, in naked slugs, etc.).

The circulatory system is not closed. The respiratory organs are represented by gills or a lung formed by part of the mantle (for example, in pond snails, grape and garden snails, naked slugs). The excretory organs - the kidneys - are connected by their inner ends to the pericardial sac.

The nervous system consists of several pairs of nerve nodes connected by longitudinal trunks.

The type of molluscs includes 7 classes. The most important of them:

• gastropods (Gastropoda) - slowly crawling snails
• bivalves (Bivalvia) - relatively sedentary molluscs
• cephalopods (Cephalopoda) - mobile molluscs

Class gastropods (Gastropoda)

This class includes molluscs that have a shell (snails). Its height ranges from 0.5 mm to 70 cm. Most often, the gastropod shell has the form of a cap or spiral, only in representatives of one family does a shell develop from 2 valves connected by an elastic ligament. The structure and shape of the shell are of great importance in the taxonomy of molluscs. [show] .

1. A placospiral shell is a highly twisted shell, the whorls of which are located in the same plane.
2. Turbospiral shell - shell whorls lie in different planes
3. Right-handed shell - the spiral of the shell is twisted clockwise
4. Left-handed shell - the spiral is twisted counterclockwise
5. Hidden spiral (involute) shell - the last whorl of the shell is very wide and completely covers all previous ones.
6. Open-spiral (evolute) shell - all whorls of the shell are visible

Sometimes the shell is equipped with a lid located on the dorsal side in the back of the leg (for example, in meadowsweet). When retracting the leg into the shell, the lid tightly covers the mouth.

In some species that have switched to a floating lifestyle (for example, pteropods and keeled legs), the shell is absent. Shell reduction is also characteristic of some terrestrial gastropod mollusks living in the soil and forest litter (eg, slugs).

The body of gastropods consists of a well-separated head, legs and torso - an visceral sac; the latter is placed inside the shell. On the head are a mouth, two tentacles and at their base - two eyes.

Digestive system. At the front end of the head is the mouth. A powerful tongue is developed in it, covered with a hard chitinous grater, or radula. With its help, molluscs scrape algae from the ground or aquatic plants. In predatory species, a long proboscis develops in the front of the body, which can turn out through a hole on the lower surface of the head. In some gastropods (for example, cones), individual teeth of the radula may protrude from the mouth opening and have the form of stylets or hollow harpoons. With their help, the mollusk injects poison into the body of the victim. Some predatory species of gastropods feed on bivalve mollusks. They drill into their shells, releasing saliva containing sulfuric acid.

Through the esophagus, food enters the sac-like stomach, into which the ducts of the liver flow. Then the food enters the intestine, which bends in a loop and ends on the right side of the body with an anus.

The nerve ganglions are collected in the peripharyngeal nerve ring, from which the nerves extend to all organs. On the tentacles are tactile receptors and organs of chemical sense (taste and smell). There are balance organs and eyes.

In most gastropods, the body protrudes above the leg in the form of a large spirally twisted bag. Outside, it is covered with a mantle and closely adheres to the inner surface of the shell.

The respiratory organs of mollusks are represented by gills located in the anterior part of the body and directed with their apex forward (anterior gill mollusks) or located in the right rear part of the body and directed backward with their apex (posterior gills). In some gastropods (for example, nudibranchs), real gills have been reduced. As respiratory organs, they develop the so-called. skin adaptive gills. In addition, in land and secondary aquatic gastropod mollusks, part of the mantle forms a kind of lung, numerous blood vessels develop in its walls, and gas exchange occurs here. The pond snail, for example, breathes atmospheric oxygen, so it often rises to the surface of the water and opens a round breathing hole on the right at the base of the shell. Next to the lung is the heart, which consists of an atrium and a ventricle. The circulatory system is open, the blood is colorless. The excretory organs are represented by one kidney.

Among gastropods, there are both dioecious species and hermaphrodites, the gonad of which produces both spermine and eggs. Fertilization is always cross, development, as a rule, with metamorphosis. All land, freshwater, and some marine gastropods have direct development. The eggs are laid in long slimy filaments attached to moving objects.

belongs to the class of gastropods

• Common pond snail, often found on aquatic plants in ponds, lakes and rivers. Its shell is solid, 4-7 cm long, spirally twisted, with 4-5 whorls, a sharp apex and a large opening - the mouth. The leg and head can protrude through the mouth.

  Intermediate hosts of trematodes also belong to gastropods.

• The intermediate host of the cat's fluke - bithynia (Bithynia leachi) - is widespread in freshwater reservoirs of our country. It lives in the coastal zone of rivers overgrown with vegetation, in lakes and ponds. The shell is dark brown, has 5 convex whorls. Shell height 6-12 mm.
• The intermediate host of the liver fluke - the small pond snail (Limnea truncatula) - is widely distributed in Russia. The shell is small, no more than 10 mm in height, forms 6-7 whorls. It lives in ponds, swamps, ditches and puddles, where it often occurs in large numbers. In some areas, there are more than 1 million pond snails per hectare of swamps. When swamps dry up, pond snails burrow into the ground, experiencing a dry time in the ground.
• Intermediate hosts of the lanceolate fluke are terrestrial mollusks Helicella and Zebrina (Helicella and Zebrina). Distributed in Ukraine, Moldova, Crimea and the Caucasus. Adapted to life in arid conditions; live in the open steppe on the stems of herbaceous plants. During the heat, helicella often accumulate on plants in clusters, escaping in this way from drying out. Helicella has a low-conical shell with 4-6 whorls; the shell is light, with dark spiral stripes and a wide rounded mouth. Zebrina has a highly conical shell with 8-11 whorls; the shell is light, with brown stripes running from the apex to the base; the mouth is irregularly oval.

Class bivalve (Bivalvia)

This class includes mollusks with a shell consisting of two symmetrical halves, or valves. These are sedentary, sometimes completely immobile animals that live at the bottom of the seas and freshwater reservoirs. They often burrow into the ground. The head is reduced. In freshwater reservoirs, toothless or barley are widespread. Of the marine forms, oysters are of the greatest importance. Very large species are found in tropical seas. The shell of a giant tridacna weighs up to 250 kg.

Pearl barley, or toothless lives on the silty and sandy bottom of rivers, lakes and ponds. This inactive animal feeds passively. Toothless food is detritus particles suspended in water (the smallest remains of plants and animals), bacteria, unicellular algae, flagellates, ciliates. The mollusk filters them out of the water passing through the mantle cavity.

The body of the toothless, up to 20 cm long, is covered on the outside with a bivalve shell. Distinguish between an expanded and rounded anterior end of the shell, and a narrowed, pointed posterior end. On the dorsal side, the flaps are connected by a strong elastic ligament, which keeps them in a semi-open state. The shell closes under the action of two closing muscles - anterior and posterior - each of which is attached to both valves.

Three layers are distinguished in the shell - horny, or conchiolin, which gives it a brownish-green color on the outside, a middle thick porcelain-like layer (consists of prisms of carbonic lime; located perpendicular to the surface - shells) and an inner mother-of-pearl layer (in it, between the thinnest calcareous leaves, there are thin layers of conchiolin). The mother-of-pearl layer is underlain on each of the two flaps by a yellowish-pink fold of the mantle. The epithelium of the mantle secretes a shell; in some species of freshwater and marine pearl mussels, it also forms pearls.

The body is located in the dorsal part of the shell, a muscular outgrowth departs from it - the leg. In the mantle cavity on both sides of the body there are a pair of lamellar gills.

In the posterior part, both shell valves and mantle folds do not fit snugly against one another; two openings remain between them - siphons. The lower, introductory, siphon serves to introduce water into the mantle cavity. A continuous directed flow of water is carried out due to the movement of numerous cilia that cover the surface of the body, mantle, gills and other organs of the mantle cavity. Water washes the gills and provides gas exchange, it also contains food particles. Through the upper, output, siphon, the used water, together with excrement, is brought out.

The mouth is at the front end of the body above the base of the leg. On the sides of the mouth are two pairs of triangular oral lobes. The cilia covering them with their movement adjust the food particles to the mouth. Due to the reduction of the head in barley and other bivalve mollusks, the pharynx and associated organs (salivary glands, jaws, etc.)

The digestive system of the barley consists of a short esophagus, a sac-like stomach, a liver, a long loop-shaped curved midgut and a short hindgut. A sac-like outgrowth opening opens into the stomach, inside of which there is a transparent crystalline stalk. With its help, the food is crushed, and the stalk itself gradually dissolves and releases the amylase, lipase and other enzymes contained in it, which provide the primary processing of food.

The circulatory system is not closed; colorless blood flows not only through the vessels, but also in the spaces between the organs. Gas exchange occurs in the gill filaments, from there the blood is sent to the efferent gill vessel and then to the corresponding (right or left) atrium, and from it to the unpaired ventricle, from which two arterial vessels begin - the anterior and posterior aorta. Thus, in bivalves, the heart consists of two atria and one ventricle. The heart is located in the pericardial sac on the dorsal side of the body.

The excretory organs, or kidneys, look like dark green tubular sacs, they start from the pericardial cavity and open into the mantle cavity.

The nervous system consists of three pairs of nerve nodes connected by nerve fibers. The sense organs are poorly developed due to the reduction of the head and a sedentary lifestyle.

Cephalopoda class

unites the most highly organized mollusks leading an active lifestyle. Cephalopods include the largest representatives of invertebrates - octopuses, squids, cuttlefish.

The body shape of cephalopods is very diverse and depends on their lifestyle. The inhabitants of the water column, which include most squids, have an elongated, torpedo-shaped body. For benthic species, among which octopuses predominate, a sac-like body is characteristic. In cuttlefish living in the bottom layer of water, the body is flattened in the dorsal direction. Narrow, spherical or jellyfish-like planktonic species of cephalopods are distinguished by their small size and gelatinous body.

Most modern cephalopods do not have an outer shell. It turns into an element of the internal skeleton. Only nautiluses retain an external, spirally twisted shell, divided into internal chambers. In cuttlefish, the shell, as a rule, looks like a large porous calcareous plate. The spirula retains a spiral shell hidden under the skin. In squids, only a thin horny plate remains from the shell, stretching along the dorsal side of the body. In octopuses, the shell is almost completely reduced and only small crystals of carbonic lime remain from it. Female argonauts (one of the species of octopuses) develop a special brood chamber, shaped very much like an outer shell. However, this is only an apparent resemblance, since it is secreted by the epithelium of the tentacles and is intended only to protect the developing eggs.

One of the distinguishing features of cephalopods is their internal cartilaginous skeleton. Cartilage, similar in structure to the cartilage of vertebrates, surrounds the head cluster of ganglia, forming a cartilaginous capsule. Processes depart from it, reinforcing the eye openings and organs of balance. In addition, supporting cartilage develops in cufflinks, the base of the tentacles, and fins.

The body of cephalopods consists of a head with compound eyes, a crown of tentacles or arms, a funnel, and a torso. Large complex eyes are located on the sides of the head and are not inferior in complexity to the eyes of vertebrates. The eyes have a lens, cornea and iris. Cephalopods have developed not only the ability to see in stronger or weaker light, but also accommodation. True, it is achieved not due to a change in the curvature of the lens, as in humans, but due to its approach or removal from the retina.

On the head around the mouth opening is a crown of very mobile tentacles, which are one part of a modified leg (hence the name). In the vast majority of species, powerful suckers are located on their inner surface. Squids use tentacles to catch prey, in male octopuses one of the tentacles is used to carry sexual products. During the breeding season, this tentacle is modified, and during the mating period it breaks off and, due to its ability to move, penetrates into the mantle cavity of the female.

The other part of the leg turns into a funnel, which plays an important role in movement. It grows to the ventral side of the body, opening at one end into the mantle cavity, and at the other into the external environment. The mantle cavity in cephalopods is located on the ventral side of the body. At the point of transition of the body to the head, it communicates with the external environment through the transverse abdominal opening. For its closure, in most cephalopods, paired semilunar pits are formed on the ventral side of the body. Opposite them, on the inside of the mantle, there are two hard tubercles reinforced with cartilage, the so-called. cufflinks. As a result of muscle contraction, the cufflinks enter the semilunar recesses, tightly fastening the mantle to the body. When the abdominal opening is open, water freely penetrates into the mantle cavity, washing the gills lying in it. After this, the mantle cavity closes and its muscles contract. Water is pushed out with force from the funnel lying between two cufflinks, and the mollusk, receiving a reverse push, moves forward with the rear end of the body. This type of movement is called reactive.

All cephalopods are predators and feed on various crustaceans and fish. They use tentacles to capture prey, and powerful horny jaws to kill. They are located in the muscular pharynx and resemble the beak of a parrot. A radula is also placed here - a chitinous ribbon with 7-11 rows of teeth. 1 or 2 pairs of salivary glands open into the pharynx. Their secret contains hydrolytic enzymes that break down polysaccharides and proteins. Often, the secretions of the second pair of salivary glands are poisonous. The venom also helps to immobilize and kill large prey.

The intestines are branched, with digestive glands. In many species, the duct of the ink gland opens directly in front of the anus into the lumen of the hindgut. It secretes a dark secret (ink) that can cloud a large amount of water. The ink serves as a smoke screen, disorients the enemy, and sometimes paralyzes his sense of smell. Cephalopods use it to escape predators.

The circulatory system is almost closed. Heart with 2 or 4 atria, kidneys also 2 or 4, their number is a multiple of the number of gills.

The nervous system has the highest organization with developed structures of touch, smell, sight and hearing. The ganglia of the nervous system form a common nerve mass - a multifunctional brain, which is located in a protective cartilage capsule. Two large nerves depart from the posterior part of the brain. Cephalopods have complex behavior, have a good memory and show the ability to learn. For the perfection of the brain, cephalopods are called "primates of the sea."

The unique skin photoreceptors of cephalopods react to the slightest changes in illumination. Some cephalopods are able to glow due to the bioluminescence of photophores.

All cephalopods are dioecious animals; some of them have pronounced sexual dimorphism. Males, as a rule, are smaller than females, armed with one or two modified arms - hectocotyls, with the help of which "packages" with seminal fluid - spermatophores - are transferred during the copulation period. Fertilization is external-internal and occurs not in the genital tract of the female, but in her mantle cavity. It consists in the capture of sperm by the gelatinous shell of the eggs. After fertilization, females attach clusters of eggs to bottom objects. Some species take care of the offspring and guard the developing eggs. The female guarding offspring can starve for more than 2 months. In octopuses, cuttlefish and nautiluses, each egg hatches a mini copy of the parents, only in squid development comes with metamorphosis. The young grow rapidly and often reach sexual maturity by the year.

The value of shellfish

Freshwater pearl shells with a mother-of-pearl layer thickness of about 2.5 mm are suitable for making mother-of-pearl buttons and other jewelry. Some bivalves (mussels, oysters, scallops), a grape snail from gastropod mollusks (in some European countries it is bred in snail farms), squids are especially valuable from cephalopods in terms of caloric content and protein composition (more than 600 thousand of them are harvested annually in the world). . T).

River zebra mussel is found in huge numbers in the reservoirs of the Volga, Dnieper, Don, in lakes, estuaries of the Black Sea, desalinated areas of the Azov, Caspian and Aral Seas. It overgrows stones, piles and various hydraulic structures: watercourses, technical and drinking water supply pipes, protective gratings, etc., and its amount can reach 10 thousand copies per 1 m 2 and cover the substrate in several layers. This makes it difficult for the passage of water, so constant cleaning of zebra mussels from fouling is necessary; mechanical, chemical, electrical and biological control methods are used. Some bivalve molluscs make passages in the bottoms of ships, wooden parts of port facilities (shipworm).

Perlovitsa and some other bivalves play an important role in marine and freshwater biocenoses as natural water purifiers - biofilters. One large barley is able to filter 20-40 liters of water per day; mussels inhabiting 1 m 2 of the seabed can filter about 280 m 3 of water per day. At the same time, mollusks extract organic and inorganic substances from polluted water, some of which are used for their own nutrition, and some are concentrated in the form of lumps that are used to feed microorganisms.

Thus, mollusks are one of the most important parts of the self-purification system of the reservoir. Of particular importance in the system of biological self-purification of water bodies are mollusks, which have special mechanisms of resistance to pollution of water bodies with toxic substances and mineral salts, and are also adapted to living in water with a reduced amount of oxygen. The basis of the molecular mechanism of such adaptation is the carotenoids contained in the nerve cells of molluscs. Pearl barley and other filter-feeding mollusks need protection. They can be bred in special containers and used to clean artificial reservoirs from pollution, dispose of waste and obtain additional food.

Shellfish fishing is especially important in Japan, the USA, Korea, China, Indonesia, France, Italy, and England. In 1962, the production of mussels, oysters, scallops and other bivalve mollusks amounted to 1.7 million tons, by now the natural natural reserves of valuable edible mollusks have been depleted. In many countries, marine and freshwater mollusks are bred artificially. Since 1971, mussels have been bred on an experimental farm in the northwestern part of the Black Sea (productivity is 1000 centners of mussels per year), studies on mussel breeding are also being carried out in the basins of other seas washing the shores of our country. Shellfish meat is easily digestible, it contains a lot of vitamins, carotenoids, trace elements (iodine, iron, zinc, copper, cobalt); it is used for food by the population, as well as for fattening domestic animals. Filter-feeding mollusks can also be used in a biomonitoring system for monitoring the chemical composition of water in reservoirs.

Cephalopods, common in all seas, except for desalinated ones, despite the fact that they are predators, often serve as food for many fish and marine mammals (seals, sperm whales, etc.). Some cephalopods are edible and are an object of fishing. In China, Japan and Korea, the use of these animals as food goes back centuries; in the Mediterranean countries it also has a very long history. According to Aristotle and Plutarch, octopuses and cuttlefish were common foods in Ancient Greece. In addition, they were used in medicine, perfumery and in the manufacture of first-class paints. Currently, innate programs of complex behavior are being studied in cephalopods under laboratory conditions.

Theory for preparation for block No. 4 of the Unified State Examination in biology: with system and diversity of the organic world.

Type Mollusca (Mollusca)

Mollusks, or soft-bodied, are a type of three-layer animals that have a coelom (secondary body cavity). Symmetries are bilateral, but in many species during ontogenesis, the organs are displaced, the animals become asymmetric.

A distinctive feature of the type is the presence of a mantle, a skin fold around the body. The space between the mantle and the body is called the mantle cavity. Outside, the mantle is covered with a calcareous shell, which in some species can protect the entire body, while in others it can be reduced to a small plate. The body of animals is divided into head, trunk and leg.

More than 100,000 species of molluscs are known, ranging in size from 1 mm to 10 m (Antarctic giant squid). Mostly they are aquatic animals. Some species lead a terrestrial lifestyle, preferring wet places. There is no consensus on the origin of mollusks, most scientists believe that their ancestors are annelids.

Classification

The type is divided into two subtypes: side nerve And shell. The latter include animals with a solid or bivalve shell, forming five classes. As part of the school curriculum, only three of them are considered: gastropods, laminabranchial And cephalopods.

Class Gastropoda (Gastropoda)

All gastropods have a whole spirally twisted clockwise shell, asymmetrical body, isolated head. On the head are eyes, tentacles, mouth. The foot is usually large, flattened from below, forming a sole. There are many mucous glands on the sole, which helps the mollusk move on various surfaces.

Digestive system

In the digestive system Gastropods are divided into anterior, middle, and hindguts. The anterior intestine includes the oral cavity, pharynx and esophagus. In the oral cavity are powerful horny jaws. The pharynx has thick muscular walls and a muscular "tongue" on which rows of chitinous teeth are located. Such an apparatus is called a radula, which translates as "scraper". Using the radula as a grater, herbivorous molluscs rip off food particles from plants, and acting like a drill, predators bite into the covers of other animals.

The midgut consists of the stomach and several loops of the small intestine. The hindgut opens into the mantle cavity with an anus near the head of the mollusk.

Respiratory system

respiratory system form gills or, in the case of a subclass of pulmonary, an unpaired lung. Gills can be of two types: primary and secondary. Primary gills (knetidia) are preserved in the adult state in a small number of species, they are strands with many feathery outgrowths, where gas exchange occurs. According to the location of the cnetidia, the subclasses of the anterior branchial and posterior branchial gastropods are divided.

Secondary gills have nothing to do with true gills - they are just abundantly supplied protrusions on the body that serve for gas exchange.

The lung is found in terrestrial and freshwater gastropods and is a modified section of the mantle cavity. The surface area of ​​the lung increases significantly due to the many folds.

Circulatory system

Circulatory system open type, consists of a heart and a developed vascular system. Between the bringing and taking away vessels there are not capillaries, but lacunae. From the lacunae, blood is collected first into the venous sinuses, then into the veins.

excretory, nervous and reproductive systems

excretory system consists of two (in many species - one) kidneys. The kidney is turned into the pericardial cavity - a funnel with cilia. Through it, metabolic products enter the mantle cavity.

Nervous system well developed, consists of large nodes (ganglia) and trunks between them. This type of nervous system is called scattered-nodular. On the head are tactile tentacles, eyes, olfactory labial tentacles. Nerves from them depart to the cerebral ganglion.

The organ of balance is statocysts - small fluid-filled vesicles lined with sensitive cells. The liquid contains hard pieces of calcium carbonate, which press against the walls of the statocyst if the mollusk leans.

reproductive system consists of the ovary or testis and the genital ducts. Gastropods can be either dioecious or hermaphrodite. Cross fertilization, internal. The female lays eggs, from which a free-swimming larva emerges - a sailboat.

Class Bivalves or Lamellibranchia (Bivalvia or Lamellibranchia)

This rather large group includes more than 20,000 species, the classic representative is toothless (Anodonta). Sizes vary from 1 mm to 1.5-2 m. They live in fresh and sea water.

A distinctive feature of the structure is the absence of a head. The body consists of a leg and torso, enclosed in a bivalve shell. The valves are connected by an elastic ligament, a ligament, which keeps the shell open at rest.

Powerful closing muscles allow the mollusk to close the shell. Some species (for example, scallops) can move quickly using jet propulsion, opening and quickly closing the valves. However, most species lead an attached or sedentary lifestyle, moving slowly with the help of their legs.

The inside of the shell is covered with a layer of mother-of-pearl. After a foreign body enters the mantle or between the mantle and the shell, the glandular cells around it secrete mother-of-pearl. With a successful combination of circumstances, a pearl is formed.

In bivalves, the edges of the mantle grow together, tubular spaces, siphons, form between them. Toothless has two siphons, along the lower one water enters the mantle cavity, along the upper one it exits the body.

Digestive system simplified, pharynx reduced. The stomach is voluminous, the ducts of the liver flow into it. The stomach is followed by the midgut, then the hindgut. The hindgut passes through the heart and opens into the mantle cavity through the anus.

Bivalves feed mainly by filtration, driving water through siphons. This contributes to the purification of water bodies.

Breath gill. The structure of the gill apparatus is diverse, in some species it is absent and breathing is carried out by the surface of the body.

circulatory system open. The heart is three-chambered, consists of a ventricle and two atria. Primitive species retain two hearts.

excretory system formed by the kidneys. Each kidney opens at one end into the pericardial sac, and at the other end into the mantle cavity. There are also pericardial glands that excrete metabolic products into the pericardial cavity.

Due to a sedentary lifestyle, nervous system poorly developed. Consists of three pairs of ganglia. The head tentacles and eyes are absent, but there may be numerous (up to 100!) eyes scattered along the edge of the mantle. There are also statocysts, organs of touch and organs of chemical sense.

Sexual system in the vast majority of bivalves, it is dioecious. Fertilization occurs in females in the mantle cavity, that is, in the external environment. The eggs hatch into larvae. In marine mollusks, the larvae swim freely, then settle to the bottom and turn into an adult.

Type mollusks 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 20m. Many lead a sedentary or sedentary lifestyle, and only cephalopods are able to actively move in the water. The science of shellfish is called malacology, she is studying 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. The following sections are distinguished: the head part with the organs of vision and tentacles, the body itself and the leg - a muscular formation, serves to move. All bivalves are characterized by the presence of a leg, while in cephalopods it has been transformed into tentacles and a siphon.

The body of the mollusk is surrounded by a shell, serves as a place for attachment of muscles. In gastropods, it has an integral 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 a shell.

From the lateral parts of the body departs the mantle, sent by epithelial cells. 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

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

circulatory system open, in the heart there is 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 inhabitants of land, it is carried out by lungs. The lung tissue is equipped with a dense vascular network, where oxygen and CO 2 are exchanged. The lung communicates with the external environment with a spiracle.

Nervous system of molluscs consists of five pairs of nerve nodes, united by fibrous cords. The unequal development of the sense organs in mollusks indicates a different way of life of representatives of the type.

For example, cephalopods have a fairly developed vision, the structure of the eye is similar to the structure of the eye of vertebrates. The predatory nature forced them to adapt to changing environmental conditions through the complication of the visual apparatus. They formed 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, the rest are direct.

Features of the structure of mollusks compared with annelids

What new organs appeared in molluscs compared to worms?

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

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

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

How are shellfish adapted to their environment?

Representatives of the type inhabit the expanses of water and the surface of the land. For existence outside the reservoir and breathing atmospheric air, soft-bodied animals developed lung tissue. Inhabitants of reservoirs receive O 2 with the help of gill arches.

How do shellfish protect themselves from enemies?

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

Poisonous and chemical substances (ink) serve as protection against predators. Some are able to burrow into the sandy bottom in seconds when threatened, or hide using a springy leg.

What is the function of a mollusk shell?

First of all, it is a support function, it serves as an external skeleton. Also, a strong shell of bivalves and gastropods is needed to protect against adverse factors. So, when danger approaches, they hide in them and become inaccessible to most fish.

Similarities and differences between gastropods and bivalves

The value of mollusks in nature and human life

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

Water passes through the body of the mollusk and is cleansed of pollutants. And the molluscs, in turn, get food particles from filtered water.

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

Widely used in cooking, considered a delicacy in many countries. These are mussel meat, scallops, oysters, cuttlefish and octopuses. Due to the popularity of dishes from exotic animals, they began to be grown on specially equipped farms.

Between the valves of the shell, a valuable jewelry raw material is formed - pearls. A pearl is formed after getting inside a foreign body. 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.

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

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

Pests of agriculture - slugs, destroy crops, garden plants (potatoes, cabbage, tomatoes).

Flatworms that cause diseases in humans and animals use mollusks as intermediate hosts.

You will find a message about class 7 mollusks, one of the most numerous species of animals, in this article.

Report on shellfish

These animals mainly live in the seas, fresh waters and terrestrial wet environments. There are 130,000 species of mollusks. The most common of them are oysters, mussels, octopuses, toothless, slugs, grape. Body size ranges from a couple of millimeters to 20 meters. Most of them lead a sedentary lifestyle. Only a species of cephalopod mollusks moves in a jet way.

General structural features of molluscs

• Body without segmentation. They are bilaterally symmetrical or asymmetrical. The body is divided into the following segments: a head with eyes and a pair of tentacles, a torso with internal organs, and an abdominal muscular part that serves for movement. The body of animals is enclosed in a shell, which protects it from enemies and serves as a support for the muscles. The shell consists of an outer stratum corneum, a middle porcelain layer, and an inner mother-of-pearl layer. In gastropods, the shell has the form of a spirally curled turret or cap. In bivalve mollusks, it consists of two valves connected by an elastic ligament, muscles-contactors, teeth of the “lock”. Most cephalopods do not have a shell.
• The body is covered with a mantle - a skin fold. Its epithelium forms the substance of the shell. Also between the body and the mantle there is a mantle cavity with gills and sensory organs, anus and excretory organs.
• The digestive system is represented by 3 sections - anterior, middle and posterior. Most of them have a developed muscular tongue with teeth, with which molluscs capture food and grind it.
• Animals have an open circulatory system of blood vessels and the heart. The cardiac organ has a ventricle with atria. Aquatic molluscs breathe with gills, while terrestrial molluscs breathe with lungs.
• Excretory system in the form of kidneys.
• The nervous system consists of 5 pairs of large ganglia located in the head, mantle, leg, respiratory organs, and visceral sac. They are connected by nerve trunks.
• They reproduce sexually.

Mollusk classification

There are such types of mollusks - Bivalves, Armored, Gastropods, Cephalopods, Nudibranchs and the Monoplacophora class.

What do molluscs eat?

They eat plant and animal food. The one that is not digested in the stomach is thrown out through the anus.

• In 1956, the largest mollusk weighing 340 kg was caught in Okinawa (Japan).
• The oldest shellfish caught by man was 405 years old.
• The age of these animals is determined by the number of rings placed on the shell valve.
• The main food of molluscs is plankton, which animals filter out of the water.
• More facts about mollusks: some types of scallops have several dozen blue eyes along the edges of the shell. They help scallops to notice predators in time to run away from enemies.

We hope that the presentation about shellfish helped you prepare for the lesson. And you can leave your message about mollusks through the comment form below.