All molluscs are bilaterally symmetrical animals. Classes of mollusks and their characteristic features. The external structure of molluscs

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

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

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

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

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

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

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

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

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

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

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

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

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

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

• Class Gastropoda (Gastropoda)

• Class Cephalopoda (Cephalopoda)

  • Subclass Coleoidea (Coleoidae)

Shovel-footed, or foot-footed (Scaphopoda). The name of the class comes from the Greek skaphe is a boat and pous is a foot. These are bottom marine animals found from shallow waters to a depth of 5 km. Approximately 200 modern species and 350 extinct are known. They are found in all seas, except for the polar ones. The body is bilaterally symmetrical, elongated, covered with a slightly curved tubular shell. The head is reduced to a proboscis with a mouth opening, the heart is also rudimentary - without auricles. Animals are dioecious. This class includes the so-called. marine tooth (Dentalium).
Bivalves, or lamellar-gills (Pelecipoda). The name of the class comes from the Greek pelekys is an ax and pous is a foot. These are symmetrical aquatic, mostly marine, mollusks with a bivalve shell, but without a head. Oysters, pearl oysters, mussels, scallops are all bivalves. The class has approx. 10,000 modern species, of which approximately 80% live in salt waters. They are found mainly at shallow depths. Some, such as oysters, lead a sedentary lifestyle, attaching to hard surfaces with byssal threads or cement, others can slowly crawl along the bottom and even swim (scallops). Many bivalves are able to burrow into the ground, and a small number of species can drill into wood and even stone. Representatives of this class feed mainly on microscopic plankton and detritus particles filtered from the surrounding water. Many bivalves are of great commercial importance. Oyster mining brings a lot of income. Among many other edible species, the most popular are the hard shell (Venus mercenaria) and sand shell (Mya arenaria). Mussels and scallops are also used as food. Most species of this class are dioecious, but hermaphroditism is also quite common in it. Spermatozoa and eggs are usually released into the water, where fertilization occurs, but sometimes, for example, in freshwater toothless and barley, it occurs on the maternal gills, and the larvae begin their development there.
Cephalopods (Cephalopoda). The name of the class comes from the Greek kephale is the head and pous is the foot. These are highly organized marine mollusks, sometimes of very large sizes, with a large head, well-developed eyes and a crown of long tentacles, or arms, surrounding the mouth. Their body plan is the same as that of other molluscs, but the shape and way of life are completely different. This class includes squid, cuttlefish, octopuses and nautiluses (ships). Cephalopods are predators that feed on vertebrates, mollusks and crustaceans. Many species are able to swim quickly by pushing water out of the mantle cavity through a tubular siphon, and sometimes with the help of fins. Octopuses usually crawl along the bottom, using their long tentacles for this. The cephalopods include the largest modern invertebrate - the giant squid Architeuthis princeps, whose length reaches 15 m. In many countries, representatives of this class are used as food, and small squids often serve as bait for fishermen. The shells of some species, such as nautilus, are used to make jewelry. About 400 modern species belong to cephalopods and approx. 5000 fossils.

Collier Encyclopedia. - Open society. 2000 .

See what "SHELLS" are in other dictionaries:

SHELLS, representatives of more than 80,000 species of invertebrates of the Mollusca phylum. These include well-known snails, bivalves and squids, as well as many lesser-known species. Originally inhabitants of the sea, mollusks are now ... ... Scientific and technical encyclopedic dictionary

Soft-bodied (Mollusca), a type of invertebrate. Arise presumably in the Precambrian; from the Lower Cambrian already known several. classes M. Probably descended from low-segment worm-like ancestors (annelids) or directly from flat ones ... ... Biological encyclopedic dictionary

SHELLS- SHELLS, or soft-bodied (Mollusca), a well-closed type of invertebrates. The body is soft, undivided, typically bears a shell. The skin covers form a mantle fold covering the body or fusing along the edges with its surface. ... ... Big Medical Encyclopedia

- (new lat. mollusca, from lat. mollis soft). Soft-bodied animals, slugs. Dictionary of foreign words included in the Russian language. Chudinov A.N., 1910. SHELLS Novolatinsk. mollusca, from dates. mollis, soft. Soft-bodied animals. Explanation… … Dictionary of foreign words of the Russian language

- (from lat. molluscus soft) (soft-bodied) type of invertebrates. The body of most mollusks is covered with a shell. On the ventral side there is a muscular outgrowth of the leg (organ of movement). 2 subtypes: lateral nerve and testate; St. 130 thousand species. They live in…… Big Encyclopedic Dictionary

Modern Encyclopedia

shellfish- SHELLS, a type of invertebrate animal. Most of the body is covered with a shell. The head has a mouth, tentacles, and often eyes. The muscular outgrowth (leg) on ​​the ventral side is used for crawling or swimming. About 130 thousand species, in the seas (most), ... ... Illustrated Encyclopedic Dictionary

- (Mollusca) a type of animal with a solid, non-segmented body. Most representatives have a calcareous shell, one-piece or consisting of two, less often several separate parts. The organ of movement is a muscular unpaired ... ... Geological Encyclopedia

shellfish- the body of most m. is covered with a shell. ▼ side nerves. armored: chiton tonicella. solenogaster: echinomenia. caudofoveates. shell. monoplacophores: neopilin. gastropods, snails, gastropods: anterior gills: cowries. littorinas. sea ​​ears. trumpeters ... Ideographic Dictionary of the Russian Language

shellfish- A type of soft-bodied non-segmented invertebrates that usually secrete a substance to build a calcareous shell: snails, saucers, bivalves, chitons, squids. … … Technical Translator's Handbook

- (Mollusca) (from lat. molluscus soft), soft-bodied, type of invertebrates. 7 classes: Gastropods, Monoplacophores, Shellfish, Chute-bellied mollusks, Bivalves, Spadefoot mollusks and ... Great Soviet Encyclopedia

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 entire 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

Table 1. Characteristic features of bivalves and gastropods
sign	Class
	Bivalves	gastropods
Symmetry type	Bilateral	Asymmetric with reduction of some right organs
Head	Reduced together with related organs	Developed
Respiratory system	Gills	gills or lung
Sink	Bivalve	Spiral twisted or cap-shaped
reproductive system	Dioecious	Hermaphroditic or dioecious
Nutrition	passive	Active
Habitat	Marine or freshwater	Marine, freshwater or land

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 sink. 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 band, 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 sea 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, outlet, siphon, the used water, together with excrement, is discharged outside.

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 mollusks. Pearl barley and other filter-feeding mollusks need protection. They can be bred in special containers and used to clean artificial reservoirs of 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.

Mollusks are one of the most ancient invertebrates. They differ in the presence of a secondary body cavity and rather complex internal organs. Many of them have a calcareous shell, which protects their body quite well from the encroachments of numerous enemies.

This is not often remembered, but many species of this type lead a predatory lifestyle. In this they are helped by a developed salivary gland. By the way, what is the salivary gland in molluscs? This generalizing concept means a fairly wide range of specific organs located in the pharynx and oral cavity. They are intended for the secretion of various substances, the characteristics of which can be very different from our understanding of the word "saliva".

As a rule, molluscs have one or two pairs of such glands, which in some species reach very impressive sizes. In most predatory species, the secret that they secrete contains from 2.18 to 4.25% chemically pure sulfuric acid. It helps both to fend off predators and to hunt their relatives (sulfuric acid perfectly dissolves their calcareous shells). That's what the salivary gland in molluscs is.

Other natural value

Many of the species of slugs, as well as the grape snail, cause great harm to agriculture around the world. At the same time, it is mollusks that play the most important role in the global purification of water, since they use organic matter filtered from it to feed them. In many countries, large ones are bred on marine farms, as they are a valuable food product that contains a lot of protein. These representatives and oysters) are even used in dietary nutrition.

In the former USSR, 19 representatives of this ancient type were considered rare and disappearing at once. Despite the diversity of mollusks, they should be treated with care, as they are extremely important for the proper functioning of many natural biotopes.

In general, mollusks are often of great practical importance for humans. For example, the pearl oyster is massively bred in many coastal countries, as this species is a supplier of natural pearls. Some shellfish are of great value for medicine, chemical and processing industries.

Want to know interesting facts about shellfish? In the Antique period and the Middle Ages, inconspicuous cephalopods were sometimes the basis of the well-being of entire states, since the most valuable purple was extracted from them, which was used to color royal robes and robes of the nobility!

Type of shellfish

In total, it has more than 130,000 species (yes, the variety of mollusks is incredible). Mollusks in total number are second only to arthropods, they are the second most common living organisms on the planet. Most of them live in water, and only a relatively small number of species have chosen land as their place of residence.

general characteristics

Almost all animals that are part of this type are distinguished by several specific features at once. Here is the general characteristic of mollusks accepted today:

• First, three layers. Their organ system is formed from the ectoderm, endoderm, and mesoderm.
• Symmetry of the bilateral type, caused by a significant displacement of most of their organs.
• The body is unsegmented, in most cases protected by a relatively strong calcareous shell.
• There is a skin fold (mantle) that envelops their entire body.
• A well-defined muscular outgrowth (leg) serves for movement.
• The coelomic cavity is very poorly expressed.
• There are practically all the same organ systems (in a simplified version, of course), as in higher animals.

Thus, the general characteristics of mollusks indicate that we have before us quite developed, but still primitive animals. It is not surprising that many scientists consider mollusks to be the main ancestors of a large number of living organisms on our planet. For clarity, we present a table in which the characteristics of the two most common classes are described in more detail.

Characteristic features of gastropods and bivalves

Feature under consideration	Mollusk classes
	Bivalves	gastropods
Symmetry type	Bilateral.	Symmetry is absent, some organs are completely reduced.
The presence or absence of a head	It is completely atrophied, like all organ systems that historically belonged to it.	There is, like the whole set of organs (oral cavity, eyes).
Respiratory system		Gills or lung (pond snail, for example).
sink type	Bivalve.	One-piece, can be twisted in different directions (pond snails, ampoule) or in a spiral (lake coil).
Sexual dimorphism, reproductive system	Dioecious, males are often smaller.	Hermaphrodites, sometimes dioecious. Dimorphism is weakly expressed.
Power type	Passive (water filtration). In general, these mollusks in nature contribute to excellent water purification, as they filter out tons of organic impurities from it.	Active, there are predatory species (Cones (lat. Conidae)).
Habitat	Seas and fresh waters.	All types of reservoirs. There are also terrestrial mollusks (Grape snail).

Detailed characteristic

The body is still symmetrical, although this is not observed in bivalve species. The division of the body into segments has been preserved only in very primitive species. The secondary cavity of the body is represented by a bag surrounding the heart muscle and genitals. The entire space between the organs is completely filled with parenchyma.

The body of the majority can be divided into the following sections:

• Head.
• Torso.
• Muscular leg through which movement is carried out.

In all bivalve species, the head is completely reduced. The leg is a massive muscular process that develops from the base of the abdominal wall. At the very base of the body, the skin forms a large fold, the mantle. Between it and the body there is a rather large cavity in which the following organs are located: gills, as well as the conclusions of the reproductive and excretory systems. It is the mantle that secretes those substances that, having reacted with water, form a strong shell.

The shell can be either completely solid or consist of two flaps or several plates. This shell contains a lot of carbon dioxide (of course, in a bound state - CaCO 3), as well as conchiolin, a special organic substance that is synthesized by the body of the mollusk. However, in many species of mollusks, the shell is completely or partially reduced. In slugs, only a microscopic plate remains from it.

Characteristics of the digestive system

gastropods

There is a mouth at the front end of the head. The main organ in it is a powerful muscular tongue, which is covered with a particularly strong chitinous grater (radula). With its help, snails scrape off a coating of algae or other organic matter from all available surfaces. In predatory species (we will talk about them below), the tongue has degenerated into a flexible and rigid proboscis, which is intended for opening the shells of other mollusks.

In Cones (which will also be discussed separately), individual segments of the radula protrude beyond the oral cavity and form a kind of harpoon. With their help, these representatives of mollusks literally throw their poison at the victim. In some predatory gastropods, the tongue has turned into a special “drill”, with which they literally drill holes in the shell of their prey to inject poison.

Bivalves

In their case, everything is much simpler. They simply lie motionless at the bottom (or hang, tightly attached to the substrate), filtering through their body hundreds of liters of water with organic matter dissolved in it. The filtered particles go directly to the bulky stomach.

Respiratory system

Most species breathe with gills. There are "front" and "rear" views. In the former, the gills are located in front of the body and their tip is directed forward. Accordingly, in the second case, the tip looks back. Some have lost gills in the direct sense of the word. These large clams breathe directly through their skin.

To do this, they have developed a special skin organ of an adaptive type. In terrestrial species and secondary aquatic mollusks (their ancestors returned to the water again), part of the mantle wraps up, forming a kind of lung, the walls of which are densely penetrated by blood vessels. To breathe, such snails rise to the surface of the water and gain air supply with the help of a special spiracle. The heart, located not far from the simplest "design", consists of one atrium and ventricle.

The main classes that make up the type

How is the type of mollusk divided? The classes of mollusks (there are eight in total) are “crowned” by the three most numerous:

• Gastropods (Gastropoda). This includes thousands of species of snails of all sizes, the main distinguishing feature of which is a low speed of movement and a well-developed muscular leg.
• Bivalves (Bivalvia). Sink with two doors. As a rule, all species included in the class are sedentary, inactive. They can move both with the help of a muscular leg, and by means of jet thrust, throwing out water under pressure.
• Cephalopods (Cephalopoda). Mobile molluscs, shells are either completely devoid of, or it is in its infancy.

Who else is included in the type of mollusk? The classes of molluscs are quite diverse: in addition to all of the above, there are also Spadefoot, Armored and Pit-tailed, Furrowed-bellied and Monoplacophores. All of them refer to the living and healthy.

What fossils does the type of mollusk contain? Classes of molluscs that are already extinct:

• Rostroconchia.
• Tentaculitis.

By the way, the same Monoplacophores were considered completely extinct until 1952, but at that time the Galatea ship with a research expedition on board caught several new organisms that were attributed to the new species Neopilina galatheae. As you can see, the name of the molluscs of this species was given by the name of the research vessel that discovered them. However, in scientific practice this is not uncommon: species are much more often designated in honor of the researcher who discovered them.

So it is possible that all subsequent years and new research missions will be able to enrich the type of mollusk: the classes of mollusks that are now considered extinct may well be preserved somewhere in the bottomless depths of the oceans.

No matter how strange it may sound, but one of the most dangerous and incredible predators on our planet are ... outwardly harmless gastropods. For example, snails Cones (lat. Conidae), whose poison is so unusual that it is used by modern pharmacists in the manufacture of certain types of rare medicines. By the way, the name of the mollusks of this family is fully justified. Their shape is indeed most similar to a truncated cone.

They can be persistent hunters, dealing with floodplain prey with exceptional ruthlessness. Of course, colonial, sedentary species of animals often act as the latter, since other snails simply cannot keep up. The prey itself can be dozens of times larger than the hunter in size. Want to know more interesting facts about shellfish? Yes please!

About methods of hunting snails

Most often, the insidious mollusk uses its most powerful organ, a strong muscular leg. It can attach itself to prey with the equivalent force of 20kg! This is quite enough for a predatory snail. For example, a "caught" oyster opens up in less than an hour with an effort of only ten kilograms! In a word, the life of mollusks is much more dangerous than it is commonly thought ...

Other species of gastropods prefer not to press anything at all, carefully drilling the prey shell with a special proboscis. But this process cannot be called simple and fast with all desire. So, with a shell thickness of only 0.1 mm, drilling can take up to 13 hours! Yes, this way of "hunting" is only suitable for snails...

Dissolution!

To dissolve someone else's shell and its owner himself, the mollusk uses sulfuric acid (you already know what the salivary gland is in mollusks). So the destruction is much easier and faster. After the hole is made, the predator slowly begins to eat its prey out of the "package", using its proboscis for this. To some extent, this body can be safely considered an analogue of our hand, since it is directly involved in the capture and retention of prey. In addition, this manipulator can often be extended so that it exceeds the length of the hunter's body.

This is how snails can get their prey even from deep crevices and large shells. We remind you once again that it is from the proboscis in the body of the victim that a strong poison is injected, the basis of which is chemically pure sulfuric acid (secreted from the "harmless" salivary glands). In a word, from now on you know exactly what the salivary gland is in molluscs and why they need it.

Carp fish, catfish, yazyatniki, bream fish catch large fish for mollusks and are especially good in the second half of summer and early autumn, using them both as bait and bait.

According to scientists, almost all fish would eat mollusks with pleasure, if not for the hard protective shell. But it’s easy for fishermen to remove the shell ...

Shellfish nutrition

Mollusks have a varied diet: they can suck on the bacterial film from various underwater surfaces, they can gnaw, they can grind detritus, they can simply filter organic suspensions ... But they also participate in the food chain and fish eat them with pleasure. The meat of these mollusks is very nutritious, it is rich in vitamins and proteins and can be eaten by almost any fish. Mollusks, like other creatures, live where there is more food, where it is calmer.

Where do molluscs live

shellfish are very widely distributed in our reservoirs: they live in large and small lakes, rivers, in ponds and streams, in stagnant and running water. Probably there is no such place where they would not live. Most of all mollusks are near the coast with thickets of grass, where there are snags, all kinds of shelters. A large number of coils, bitiny, barley can collect in places where wastewater is discharged, where organic matter coming out of the sewer settles to the bottom, which turns this bottom into a real table with “delicacy” products.

It turns out that mollusks from all the surroundings crawl to this place in order to feast in plenty. There is also a fish that eats these mollusks. According to scientists, the biomass of mollusks is much greater than the biomass of other benthic organisms taken together. And this is especially true for our rivers. Under these conditions, mollusks develop very well, sometimes covering the bottom of the reservoir with a continuous carpet.

Types of shellfish

According to scientists, in our reservoirs, there are from 40 to 60 species of "shells". Very small, and these are peas and balls have a shell of only 5-10 mm.

In larger ones, and these are coils, bitinia, zebrafish, the size is even larger. Toothless and barley can grow up to 25 cm.

How do mollusks move

“Snails”, “shells” - all these mollusks can move, that is, move, crawl both along the bottom and along aquatic vegetation. bivalves they have a very muscular leg (a kind of triangle of muscles), with the help of which they can move tens of meters, leaving behind a characteristic trace - a path.

Who eats shellfish

Almost all fish eat the most delicious shellfish meat, with the exception of the podust, a perephyton-sucker, probably this water cow, and the silver carp, a filter feeder. Not every fish is capable of reaching the fish to this delicacy, since the mollusk has strong armor - a “shell”. The strength of the armor is different - some are thicker, some are thinner. Soft, thin shells of peas and balls are able to gnaw through many fish, especially carp, bream, silver bream, and crucian love them. In reservoirs where there is little grass and a lot of fish, there are very few such mollusks, since they are eaten by everyone who is not lazy.

Coils and bitinia live easier: their shells are more massive and larger, it is more difficult for most fish to crush them. Large specimens of ide, bream, and roach easily cope with them. Large whitefish are very fond of bitinia and can eat a couple of hundred shells during the day. The roach is very fond of the zebrafish shell, which is the only one of its kind to eat them. Barley shell, toothless, not every fish can crush.

Catfish, large bream, carp - eat these large mollusks, which have not yet strengthened the shell. Adult toothless and barley fish are eaten only after their death, when the muscle-contactor is no longer active. In this case, the valves at the sink diverge and the fish is able to suck out the soft and tasty contents.

Trout and eel feed on small molluscs. Such a fish as a tench, with pleasure eats all kinds of living creatures that are found in water thickets. But due to the fact that his mouth is soft, he cannot crush a large shell, so he has to eat young mollusks. Most of all tench loves balls and coils.

Growing burbots and catfish eat mollusks often and with great pleasure. Having matured, they, of course, switch to feeding on fish, but they will not swim past a dead or crushed toothless, whose shell is open.

Black carp, having pharyngeal teeth like millstones, is a real thunderstorm of molluscs. He is able to crush almost any shell, as long as it fits in his mouth. Well, a large carp can put anything in its mouth ... Black carp easily copes with zebra mussel - which has a powerful shell.

shellfish are food for fish all year round, regardless of the season, but most of all in autumn. Autumn shellfish are much tastier than spring or summer ones. But it's not that. It turns out that the aquatic vegetation dies off and the shells have nowhere to hide, and accordingly it is easier for the fish to detect them. Much depends on the rhythm of nutrition of each type of fish. If at this time of the year the fish does not feed or feeds poorly, then it will not be happy with mollusks either.

What are the diseases of shellfish

Many have seen inky black spots ( diplostomosis) on such fish as roach, bream… Intermediate hosts here are reels and fish, final tree frogs and herons.

Tetracotylosis- here the causative agent of the disease must undergo development in the gastropod mollusk before infecting the fish.

And it turns out that our fish shellfish bring both benefit and harm.