Brief characteristics of mollusks. Notes for state exams for biology students

structural features

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

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

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

Features of life processes

Support provided exoskeleton in the form of a shell, which consists of three layers: the outer one is horny, the middle one is porcelain and the inner one is mother-of-pearl. Mineral-organic turtle is characteristic feature shellfish, which in typical cases covers the entire body of the mollusk and performs a protective function. In some groups, the turtle is completely (slug) or partially reduced (octopuses).

Movement muscular involving the legs. There is a division of muscles into groups: in the majority the muscles consist of smooth muscles, cephalopods already have transversely darkened reduction of which ensures fast travel. leg cephalopods turns into tentacles and funnel- a special organ that serves for jet propulsion.

Digestion carried out by the digestive system, in which new creations appear. The anterior section includes the mouth with salivary glands And grater, pharynx, esophagus and stomach with ducts of the paired or odd digestive gland, which is called liver. The middle section is represented by the midgut. The posterior section is formed by the hindgut with the anus. In many mollusks in the oral cavity there is a special tongue-like organ called a grater or radula. It is a small protrusion with a cartilaginous supporting plate, covered with a band of chitinous teeth. The muscles allow the grater to move back and forth in the mouth and out of the mouth. In the oral cavity, except salivary glands, the ducts of other glands open, for example, poisonous or acid-secreting ones.

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

Breath with respiratory system. Respiratory organs are contained in the mantle cavity and presented gills and lungs. Residents of water bodies have gills, while lungs belong to terrestrial mollusks or mollusks whose ancestors lived on land.

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

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

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

Reproduction carried out by dioecious and hermaphroditic systems. Fertilization is external or internal.

Development direct or indirect (with transformation).

BIOLOGY +Cephalopods, or “primates of the sea” as I. Akimushkin called them, have blue blood - octopuses, squids, cuttlefish. It is also observed in some gastropods, for example, in the grape snails known to us, in decapod crustaceans (lobster, lobster, crab, shrimp, crayfish) , in many arachnids (horseshoe crab, scorpions) and others. The blue color is due to this chemical element like copper (copper) , containing hemocyanin. It is an important bioelement that is part of organisms and plays a very significant role in the physiological processes of life. In plants, it is involved in photosynthesis and nitrogen absorption; in animals, it is important for maintaining the structure of bones, cartilage, and the elasticity of blood vessels. It is part of the sheaths of long nerve processes, which is quite possibly one of the reasons that the nervous system of cephalopods is the largest of all invertebrate animals.

Let's read the information .

Shellfish- multicellular, three-layered animals with bilateral body symmetry, having a mantle (large fold of skin) surrounding the base of the body.

Type There are about 130 thousand species of mollusks.

Modern scientists distinguish in the type Mollusks classes : pit-tailed, groove-bellied, armored (chitons), monoplacophorans, bivalves, spatulate, gastropods (snails), cephalopods (octopuses, squids, cuttlefish).

Malacology(from the Greek malakion - mollusk and logos - word, doctrine) - a section of zoology that studies mollusks.

Conchology(conchiliology) (from the Greek konche, konchylion - shell and logos - word, doctrine) - a branch of zoology that studies shells (mainly mollusks).

Features of the external structure

• have a soft body enclosed in a shell
• have a bilaterally symmetrical body structure, i.e. folded according to the principle of mirror reflection - the left half of the body completely corresponds to the right half. However, during the process, some species experience displacement or uneven growth of organs, resulting in asymmetry. Asymmetry is especially pronounced among gastropods.
• body doesn't have . Consists of three sections: head, legs, torso.
• the torso contains all the main internal organs.
• have a mantle - an epithelial fold that completely or partially covers the body and connects it with the external environment. The mantle cavity contains the mantle complex of organs: the excretory tract of the reproductive system, the excretory tract of the digestive system, and the hypobranchial gland. The complex also includes kidneys and.
• the secondary cavity (in general) is represented by the cavity of the heart sac (pericardium) and the cavity.

Peculiarities internal structure

Organ system	Characteristic
Digestive	Unclosed. Consists of the pharynx, esophagus, stomach, midgut and hindgut (). The hindgut opens through the anus into the mantle cavity. Most mollusks are characterized by the presence in the throat of a special apparatus for grinding food -.
	Available in young individuals. Absent in adults. They are reduced (disappear), because lead a sedentary lifestyle.
Type of development	With metamorphosis, i.e. with the passage of the larval stage - glochidia.

Footnotes

1. Idiomatic adaptation- particular adaptation of organisms to a certain image life in specific environmental conditions.

2. Aromorphosis- a progressive evolutionary change in structure, leading to a general increase in the level of organization of organisms.

3.Bilaterally symmetrical(bilateral) animals are multicellular animals in which the left side of the body mirrors the right half of the body.

4.Gonads- organs of animals that produce sex cells - gametes. Female gonads are ovaries, male gonads are testes. They produce sex hormones - androgens and estrogens.

5.Ctenidia- primary organs of gas exchange in mollusks.

6.Radula(grater) - a device used for scraping and grinding food from mollusks. Located in the oral cavity.

7.Hemocyanin- a respiratory pigment from the group of metalloproteins, contains copper and is an analogue of hemoglobin.

8.Filters- mollusks that use a passive method of feeding, in which organic particles and microorganisms enter the gill cavity through a siphon and are swallowed using two pairs of oral lobes located at the anterior end of the body.

9.Reduction in biology - reduction, simplification of structure or disappearance of organs due to the loss of their functions in the process of evolution.

10.Statocysts- mechanoreceptor organs of balance in invertebrates, which have the appearance of vesicles submerged under the body cover, or straps or flask-shaped protrusions of the cover (in jellyfish and sea urchins).

11.Osphradius- receptor organ of mollusks, formed by specialized sensitive epithelium.

12. Ontogenesis - individual development organism from fertilization (for sexual reproduction) or from the moment of separation from the mother (for asexual reproduction) until death.

13. Segmentation in morphology: the same as metametry: the division of the body or individual organs into repeating segments (body parts).

14. Pericardium(pericardial sac) - the outer connective tissue membrane of the heart, normally separated from the epicardium by a gap filled with serous fluid - the pericardial cavity.

15. Hermaphrodite- an organism that has characteristics of male and female sex, including having both male and female genital organs.

16. Siphon- an organ of bivalve mollusks, which is an outgrowth of the siphonal (posterior) edge of the mantle.

17. Receptors(from the Latin receptor - receiving), in physiology - the endings of sensory nerve fibers or specialized cells (retina of the eye, inner ear, etc.), transforming stimuli perceived from the outside (exteroceptors) or from internal environment body (interoceptors), into nervous excitation transmitted to the central nervous system.

18. Epithelium in animals and humans (epithelial tissue) - a layer of closely spaced cells covering the surface of the body (for example, skin), lining all its cavities and performing mainly protective, excretory and absorption functions. Most glands also consist of epithelium. In plants, cells lining the cavities of organs or their parts (for example, resin ducts in conifers).

The phylum mollusc includes three classes: slowly crawling snails (gastropods), relatively sedentary bivalves, and mobile cephalopods. The phylum has about 130,000 species.

The presence of a protective shell, mechanisms of external and internal fertilization, lungs and gills allowed mollusks to conquer both land and water. In mobile mollusks, the shell, which interferes with movement, is reduced.

The body of mollusks is soft, undivided, in most it is divided into a head, torso and leg. Under the shell there is a fold of skin - the mantle. Circulatory system unclosed, blood pours into body cavities (lacunae). The capillary network is lost.

The respiratory organs of most aquatic mollusks are represented by gills. Terrestrial forms breathe using the lung. The excretory system has a structure similar to that of annelids. Nervous system nodal type, nerve nodes, are located in different parts of the body: head, leg, torso and interconnected nerve trunks. The sense organs of mollusks are diverse and often complexly arranged.

Most mollusks are dioecious, but some are hermaphrodites. For many, fertilization is internal. The development of the most ancient mollusks is carried out with transformation - they retain a planktonic dispersal larva, swimming with the help of cilia. In more advanced forms, development is direct (cephalopods).

Class Gastropods

Gastropods (snails), class of mollusks. The length varies from 1-3 mm to 30-60 cm. The body is clearly divided into three sections: the head, leg and torso, which is enclosed in a solid shell. Its height ranges from 0.5 mm to 70 cm. Most often, the shell of gastropods has the form of a cap or spiral; only representatives of the 1st family develop a shell of 2 valves connected by an elastic ligament. If in a spirally twisted shell the whorls are located in the same plane, then it is called placospiral. If the revolutions lie in different planes, then a turbo-spiral shell is formed. In addition, shells differ in the direction of the spiral turns. If, when looking at the top of the shell, they twist clockwise, then it is a right-handed shell; if it is counterclockwise, then it is a left-handed one. In some gastropods, such as cyprians and volutes, the last whorl of the shell is very wide and completely covers all previous ones. In this case, the shell is called cryptospiral, or involute. If all whorls of the shell are visible, then it is called open-spiral, or evolute. Sometimes the shell is equipped with a lid located on the dorsal side at the back of the leg (for example, in lawns). When you pull your leg into the sink, the lid tightly covers the mouth. In some species that have switched to a swimming lifestyle (for example, pteropods and keelenopods), there is no shell. Shell reduction is also characteristic of some land gastropods living in the soil and forest litter (for example, slugs).

On the head of gastropods there are usually 1-2 pairs of tentacles and eyes. The mouth is on the underside of the head. It has a powerful tongue covered with a hard chitinous grater, or radula. With its help, mollusks scrape algae from the ground or aquatic plants. U predatory species develops in the front part of the body long proboscis, capable of turning out through a hole on the lower surface of the head. In some gastropods (e.g., cones), individual teeth of the radula can protrude from the mouth opening and are shaped like stylets or hollow harpoons. With their help, the mollusk injects poison into the victim’s body. Some predatory gastropod species feed on bivalves. They drill into their shells, secreting saliva containing sulfuric acid.

The respiratory organs of the vast majority of gastropods are gills. In some species they are located in the front part of the body and directed with their apex forward, in others - in the right rear part of the body and directed with their apex backward. In some gastropods (for example, nudibranchs), true gills were reduced. As respiratory organs they develop so-called. cutaneous adaptive gills. In land and secondary aquatic gastropods, a special organ of air respiration is formed - the “lung”. It is a section of the mantle cavity, the walls of which are penetrated by a dense network of blood vessels. Oxygen from the air entering the lung diffuses through its thin walls, entering the blood. For all pulmonary mollusks, skin respiration is of great importance.

Among gastropods there are both dioecious and hermaphrodite species. Fertilization is always cross-fertilization, development, as a rule, with metamorphosis. Direct development is observed in all land, freshwater and some marine gastropods.

A significant portion of gastropods are hunted for their meat, beautiful shells and mother of pearl.

Class Bivalve

Bivalves, a class of mollusks of the testate subtype. Dimensions range from a few mm to 1.4 m, weight can reach 200 kg (for example, in tridacna). The shell consists of two valves connected on the dorsal side by a flexible conchiolin strand - a ligament. The valves are most often symmetrical, but in some species their asymmetry is observed. On the back side of the valves, most bicuspid valves have teeth and grooves that form the so-called lock. Depending on the shape and size of the teeth, the lock can be equal-toothed or hetero-toothed. Some species (eg, toothless) are characterized by a lockless shell. The closing and closing of the valves of a living mollusk occurs as a result of the work of special closing muscles. In rare cases, the shell may be partially (for example, in shipworms) or completely reduced. The shell is secreted by the mantle and consists of three layers. The outer, or conchiolin, layer is formed by the organic substance conchiolin, the middle, or porcelain, by crystals of calcium carbonate, oriented perpendicular to the longitudinal, and the inner, or pearlescent, by crystals of calcium carbonate, oriented parallel to the longitudinal axis of the mollusk's body. Shell growth occurs along the edge of the mantle, which is why concentric lines are visible on the valves. From them, as from the annual rings of trees, one can judge the growth rate and living conditions of specific individuals. If a foreign particle gets between the mantle and the shell valve, then nacre begins to be deposited on its surface in concentric layers. As a result, a pearl is formed, which grows as the mollusk itself grows.

Unlike other mollusks, bivalves have a completely reduced head. This is due to a sedentary or sedentary lifestyle. The wedge-shaped leg is used for digging into the ground. Some bivalves living in the coastal or intertidal zone (for example, mussels) develop a special byssal gland in the leg. It secretes a secretion that hardens in water in thin threads. With their help, the mollusks are securely fixed to the substrate. Sometimes (for example, in oysters) the leg is completely reduced. Some species are able to move using legs or shell flaps (for example, scallops).

The inner surface of the mantle is covered ciliated epithelium. The movement of its cilia creates a flow of water in the mantle cavity. The edges of the mantle can be free, or they can grow together, forming openings for the leg and two siphons located in the back of the shell. Through the lower, or gill siphon, water penetrates into the mantle cavity, and through the upper, or cloacal siphon, it is removed from the body. The respiratory organs are the gills located in the mantle cavity. Only in septatebranchs did they lose their respiratory function and turned into small transverse partitions separating the area of ​​the mantle cavity in which gas exchange occurs.

About 20 thousand species, divided into three superorders: protobranchials, branchials and septiobranchs. Marine and freshwater forms, widely distributed in the World Ocean and various fresh waters. They live on the bottom from shallow waters to maximum depths. By type of nutrition, they are biofilters that play an important role in biological treatment water Some species are indicators of the cleanliness of water bodies. Many bivalves (mussels, oysters, scallops, pearl mussels, etc.) are objects of fishing and aquaculture. Their extraction and breeding is carried out for the sake of valuable food products, as well as pearls and mother-of-pearl.

Cephalopods

Cephalopods- squids and octopuses are the most highly organized mollusks. The length with tentacles ranges from 1 cm to 5 m, and giant squid reaches 18 m. The body shape is very diverse and depends on the lifestyle of the mollusks. Inhabitants of the water column, which include most squids, have an elongated, torpedo-shaped body. Benthic species, among which octopuses predominate, are characterized by a sac-like body. In cuttlefish living in the bottom layer of water, the body is flattened in the dorsoventral direction. Narrow, spherical or jellyfish-like planktonic species of cephalopods are distinguished by their small size and gelatinous body.

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 junction of the body and the head, it communicates with the external environment through a transverse abdominal opening. To close it, in most cephalopods, paired semilunar fossae are formed on the ventral side of the body. Opposite them, on the inner side of the mantle, lie two hard tubercles reinforced with cartilage, the so-called. cufflinks As a result of muscle contraction, the cufflinks fit into the semilunar recesses, tightly fastening the robe 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. The water is forcefully pushed out of the funnel lying between the two cufflinks, and the mollusk, receiving a reverse push, moves forward with the rear end of the body. This method of movement is called reactive.

In the vast majority of modern cephalopods, the shell is vestigial and hidden under the skin. Only nautiluses retain an external, spirally twisted shell, divided into internal chambers. In cuttlefish, the shell, as a rule, has the appearance of a large porous calcareous plate. Only Spirula retains a spirally twisted shell hidden under the skin. In squids, only a thin horny plate is retained from the shell, stretching along the dorsal side of the body. In octopuses, the shell is almost completely reduced and only small crystals of lime carbonate remain. Female argonauts (one of the species of octopuses) develop a special brood chamber, shaped very much like an external shell. However, this is only an apparent similarity, since it is secreted by the epithelium of the tentacles and is intended only to protect developing eggs.

All cephalopods are predators and feed on a variety of 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. 1 or 2 pairs of salivary glands open into the pharynx. Their secretion 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 immobilize and kill large prey. Immediately before the anus, the duct of the ink gland opens into the lumen of the hindgut. She secretes a dark secret a small amount of which can muddy a large amount of water. Cephalopods use it to escape from predators.

One of distinctive features cephalopods is the presence of an internal cartilaginous skeleton. Cartilage, similar in structure to the cartilage of vertebrates, surrounds the head cluster of ganglia, forming a cartilaginous capsule. Branches extend from it, reinforcing the eye openings and balance organs. In addition, supporting cartilage develops in the cufflinks, base of the tentacles and fins. All cephalopods are dioecious animals; Some of them have well-defined sexual dimorphism. Fertilization is external-internal and occurs in the female’s genital tract and in her mantle cavity. Some species take care of their offspring by carrying and protecting developing eggs. Development is direct. About 650 modern species, belonging to 2 subclasses: nautiloids and coleoids. There are much more extinct species - about 11 thousand. They belong to 3 subclasses: ammonites, belemnites and bactrites. Modern cephalopods are widespread in all seas (except desalinated ones). They live in the water column and at the bottom. Despite the fact that they are all predators, they often serve as food for many fish and marine mammals. Some cephalopods are edible and are subject to commercial fishing.

Most general characteristics mollusks can be considered a lack of segmentation and bilateral symmetry. It should, at the same time, be noted that in different groups the body becomes asymmetrical as a result of displacement or uneven growth of various organs. The asymmetry is especially pronounced in gastropods due to torsion and the appearance of a turbospiral shell. More specific unifying features are the presence of the mantle and the mantle cavity, which performs respiratory and excretory functions, and in addition the structure of the nervous system. The wide variety of body structure observed in mollusks makes it difficult to find synapomorphies (characters common to them, but absent in their ancestors) that would unite all of their modern classes.

General plan of the building

The body of mollusks does not bear traces of true segmentation, despite the fact that some organs (for example, the gills of chitons and monoplacophorans) may have a metameric arrangement.

The body of mollusks usually consists of three sections: the head, legs and trunk, which is divided into the visceral mass (internal sac) and the mantle with the mantle complex of organs. Among class representatives Caudofoveata the leg is missing. Bivalve mollusks lose their heads for the second time.

The leg is a muscular unpaired outgrowth of the abdominal wall of the body and, usually, serves for movement, but at the same time it can also perform other functions. The leg also carries a pair of statocysts - organs of balance. In gastropods, it secretes mucus as a lubricant to facilitate movement. In species with a shell covering the body only on top (for example, limpet), the leg attaches the mollusk to a hard surface using vertical muscles. In other mollusks, vertical muscles pull the leg and other soft parts of the body inside the shell. In bivalves, the leg is adapted for burrowing into the ground (however, some bivalves, such as mussels, have lost it). In cephalopods, the leg is transformed into tentacles and is involved in jet propulsion.

The torso contains all the major internal organs. In Group Conchifera it grows strongly on the dorsal side during embryonic development, as a result of which the so-called visceral sac (visceral mass) is formed, the mouth and anus come together, and the intestine forms an anopedic bend.

Extends from the sides of the body mantle- a fold of the body wall, covered, like the whole body, with the epidermis and forming mantle cavity which communicates with the external environment. In chitons and monoplacophorans, the mantle and shell are formed not only from the body, but also from the head. The mantle cavity contains the so-called mantle complex of organs: the excretory tracts of the reproductive, digestive and excretory systems, ctenidia, osphradia and the hypobranchial gland. In addition, the mantle complex of organs includes the kidneys and pericardium, located next to the mantle cavity. In early mollusks, the mantle cavity was located closer to the back of the body, but in modern groups its location varies widely. In bivalves, all soft parts of the body lie within the mantle cavity.

Veils

There is another article: Clam shell

It is believed that in the hypothetical ancestor of mollusks, the integument was represented by a cuticle with aragonite spicules (needles). A similar structure of integument is typical for representatives of the classes Caudofoveata And Solenogastres. At the same time, in all classes of mollusks, except Caudofoveata, a ciliated crawling surface appears - the leg (on this basis they are combined into a group Adenopoda). U Solenogastres the leg is represented by the pedal groove.

Chitons ( Polyplacophora) also have cuticular coverings, but only on the lateral surfaces, called perinotal folds. The dorsal surface is covered with eight shell plates.

In Group Conchifera(including classes Gastropoda, Cephalopoda, Bivalvia, Scaphopoda And Monoplacophora) there are no cuticular covers, and the shell consists of one plate or two (in bivalves, and also gastropods from the family Juliidae).

the shell is secreted by the mantle (some groups, such as nudibranchs ( Nudibranchia), it is secondarily deprived) and consists mainly of chitin and conchiolin (protein strengthened with calcium carbonate). Most upper layer shells ( periostracum) in almost all cases consists only of conchiolin. Mollusks never use phosphates to strengthen their integuments (a possible exception is the chiton Cobcrephora). Despite the fact that most mollusks encrust their shells with aragonite, those gastropods that lay eggs with a hard shell use calcite (in some cases with traces of aragonite) to strengthen their daughter shells.

Three layers can be distinguished in the shell: the outer layer (periostracum), consisting of organic matter, the middle layer, composed of columnar calcite, and the inner layer, consisting of lamellar calcite, often mother-of-pearl. The only mollusk in the world whose outer shell layer is formed by iron sulfides is a deep-sea gastropod. Crysomallon squamiferum, living among “black smokers.”

An interesting mechanism occurs when the direction of twisting of the shell in the pond snail is inherited (right- and left-handed shells are known in the pond snail). It is determined not by the genotype of the mollusk itself, but by the properties of the cytoplasm of the egg, and, consequently, by the genotype of the maternal organism. Thus, in this case, cytoplasmic inheritance itself takes place.

Overall

Despite the fact that mollusks are classified as coelomic animals, the whole is given a rather modest place in them. Coelomic sacs in mollusks are represented by the pericardium (the cavity of the heart sac) and the cavity of the gonads. Together they form gonopericardial system. The main body cavity of mollusks is the hemocoel, through which blood and coelomic fluid circulate; the spaces between the organs are partially filled with parenchyma. The kidneys are in fact coelomoducts associated with the pericardium. The atria perform some functions excretory system, filtering metabolic waste from the blood and discharging it as a whole in the form of urine. The coelomoducts that open into the cavity of the gonads are the reproductive ducts (gonoducts).

Nervous system

For lower groups of mollusks - Caudofoveata, Solenogastres And Polyplacophora- characterized by a ladder-type nervous system, similar to that of some annelids. It consists of a peripharyngeal ring and four trunks: two pedal (innervate the leg) and two visceral (innervate the visceral sac).

In most other representatives of mollusks, the formation of ganglia and their displacement to the anterior end of the body are observed, with the suprapharyngeal ganglion (“brain”) receiving the greatest development. As a result, it is formed scattered-nodular nervous system.

In the nervous system of the scattered-nodular type there are two (in bivalve - three) pairs of nerve chains: two abdominal chains innervate the internal organs, and two pedal ones innervate the leg. Both pairs of circuits contain ganglia as local control centers for important parts of the body. Most of the pairs of corresponding ganglia, located on both sides of the body, are connected to each other by commissures. There are usually 5 pairs of ganglia: cerebral(innervates the eyes and tentacles), pedal(leg), pleural(mantle), parietal(respiratory organs and osphradia) and visceral(internal organs). In some cases, they also distinguish buccal ganglia, innervating the pharynx. They are removed from the peripharyngeal ring and are located on the dorsal side of the pharynx at the point where it passes into the esophagus. The cerebral, pedal and visceral ganglia are connected by transverse nerve cords - commissures. Almost all ganglia are located below the intestine, with the only exception being the cerebral ganglia, located above the esophagus. The pedal ganglia are located just below the esophagus, and their commissures and connectives connecting them with the cerebral ganglia form a nerve ring around the esophagus. In species that have a brain, it surrounds the esophagus in a ring.

In many gastropods, due to the twisting of the body, a decussation is formed between the pleural and parietal ganglia. This intersection was named chiastoneuria. The nervous system without decussation is called epineural, and with a cross - chiastoneural.

Besides reflex activity, the nervous system also regulates growth and reproduction through various neurohormones.

Sense organs

The sensory organs of mollusks include eyes and tentacles located on the head, chemical sense organs - osphradia, located near the base of the gills, and statocysts on the leg. Accommodation of the eye (in species that are capable of it) occurs due to a change in its shape - moving away or bringing the retina and lens closer together. The structure of the eye in cephalopods is very similar to that of vertebrates, but its accommodation occurs differently, and they develop differently during ontogenesis. Tactile sensory cells are concentrated mainly on the head, leg and edge of the mantle.

Circulatory system

Dissected heart and pericardium of a marine slime mold Fiona pinnata. The oval structure on top is the ventricle, part of the aorta extending from it is visible, the atrium is in the center, a small tubular structure on the right is the “portal heart”. In the lower part of the picture you can see the vessels merging into the main bloodstream

Mollusks have an open circulatory system. It includes the heart (an organ that ensures the movement of blood through the vessels and cavities of the body) and blood vessels. The heart consists of a ventricle and one or more often two atria (the nautilus has 4 atria). Blood vessels pour blood into the spaces between organs - into the sinuses and lacunae. Afterwards, the blood collects again in the vessels and enters the gills or lung. The blood of cephalopods and some gastropods has an unusual bluish color when exposed to air. This color is given to it by hemocyanin, a copper-containing respiratory pigment that performs functions similar to those of hemoglobin in the blood of chordates and annelids, therefore, when oxidized, the blood turns blue.

In cephalopods, the circulatory system is almost closed: blood is found outside the vessels only when it partially flows out of the capillaries of the veins and arteries into small lacunae.

Digestive system

Micrograph of a radula Armina maculata

In mollusks digestive system begins with the oral opening leading into the oral cavity, into which they usually open salivary glands. The digestive system consists of the pharynx, esophagus, stomach, midgut and hindgut (rectum). There is also a digestive gland (liver), which is involved in the digestion, absorption and accumulation of nutrients (liver cells of mollusks are distinguished by their ability to phagocytose). Cephalopods also have a pancreas (in other mollusks its functions are performed by the digestive gland).

Most species have a radula (“grater”) in the pharynx - special apparatus for grinding food. The radula is covered with chitinous teeth, which change as they wear. The main function of the radula is to scrape bacteria and algae from stones and other surfaces. The radula is associated with the odontophore, a cartilaginous supporting organ. The radula is unique to mollusks and has no equivalent in other animal groups. In addition to the radula, chitinous jaws are also quite often developed.

Food entering the mouth sticks to viscous saliva, which, thanks to the beating of the cilia, is directed to the stomach. At the pointed end of the stomach, near the border with the intestine, there is prostyle- a cone-shaped, posteriorly pointed formation, consisting of various sedimentary particles. The beating of additional cilia directs saliva to the prostyle, so that it acts as a kind of bobbin. Even before the saliva reaches the prostyle, the acidity of the stomach makes the saliva less sticky, and food particles are separated from it.

Next, food particles are sorted by another group of cilia. Smaller particles, mainly minerals, are guided by cilia towards the prostyle, so that they are eventually excreted, and larger particles, mainly food itself, are sent to the cecum for digestion. The sorting process cannot be called well-coordinated.

Periodically, the mollusk secretes fragments of the prostyle to prevent its excessive growth. The hindgut opens through the anus into the mantle cavity. The anus is washed by streams of water leaving the gills.

Carnivorous mollusks have a simpler digestive system. Aquatic mollusks have a special organ - a siphon, which is part of the mantle. Through the siphon, the mollusk carries out a flow of water (less often air), which is used for one or more purposes: movement, nutrition, respiration, reproduction.

In some Solemia, the digestive system is atrophied to the point of complete reduction; they are expected to learn nutrients due to chemosynthetic bacteria.

Respiratory system

The respiratory system is represented by feathery skin adaptive gills - ctenidia. Skin breathing is also important, for some it is even the only one. Land mollusks, instead of ctenidia, have a special organ of air respiration - the lung, which is a modified mantle cavity, the walls of which are penetrated by blood vessels.

Excretory system

The excretory system of mollusks consists of kidneys (metanephridia), in which excretory products accumulate in the form of lumps of uric acid. They are released every 14-20 days. Many gastropods have only one, left kidney, and greatest number representatives of monoplacophorans have buds (5-6 pairs). The renal funnels face the pericardium, and the excretory openings open into the mantle cavity. As mentioned above, the atria of mollusks, filtering blood, are in fact part of the excretory system.

Osmoregulation

Marine mollusks are poikiloosmotic animals, that is, they are unable to maintain constant osmotic pressure (OP) in tissues when the salinity of water changes, and their blood OP changes following its change in environment(in other words, OD sea ​​mollusks equals OD sea ​​water, that is, they isotonic the environment in which they live). The constant content of water and salts in the cell is ensured by cellular osmoregulation: with an increase or decrease in the OD of the environment by the same amount, the concentration of osmotically active substances changes organic matter(mostly amino acids). Thus, OD in the cell and in external environment leveled out.

Freshwater shellfish hypertensive their habitat, since their AP is greater than AP fresh water. In this regard, the problem of osmoregulation arises more acutely than in marine mollusks. Common feature freshwater mollusks is that the level of salinity of their tissues is much lower than that of marine, as well as other freshwater animals; in addition, in freshwater bivalves this indicator is the lowest among all animals. So the difference between the OD of the mollusk and the environment is not very large, but the need for osmoregulation remains. This function is performed by metanephridia, releasing excess water and salts along with uric acid.

Reproductive system

Grape snail ( Helix pomatia), laying eggs

Mollusks can be either hermaphrodites (snails) or dioecious (most bivalves). However, bivalve mollusk Arca noae protandric hermaphroditism was established (individuals first function as males, then as females). In the case of hermaphroditism, each individual acts as both a male and a female during fertilization. Gonadal ducts - gonoducts- as mentioned above, they are coelomoducts. The germ cells are directed along them as a whole, from where they are filtered by the kidneys and sent to the mantle cavity. The described mechanism occurs in dioecious mollusks with external fertilization (it occurs in water). In more developed cephalopods and most gastropods, internal fertilization occurs. In octopuses, a specialized modified tentacle, the hectocotylus, is used to transfer reproductive products into the mantle cavity of the female.

Molluscs, or soft-bodied ones, form clear limited type animals originating from annelids. Mollusks include mainly aquatic, less often terrestrial animals, characterized by the following characteristics.

The phylum Mollusca consists of large quantity very diverse forms - more than 100,000 species. These are soft-bodied, unsegmented animals with a true body cavity (coelom). They can range in size from a few millimeters to more than 20 m (as in the case of the giant squid Architheutis, the largest of the invertebrates). Among mollusks, several of the most interesting and valuable objects for neurobiological research have been found. As shown in Fig. 4.3 they evolved over 700 or 800 million years. Mollusks are divided into 7 classes.

1. Mollusks are bilaterally symmetrical animals, however, in some mollusks, due to a peculiar displacement of organs, the body becomes asymmetrical.

2. The body of mollusks is unsegmented; only a number of lower representatives show some signs of metamerism.

3. Mollusks are secondary cavity animals with a non-metameric residual coelom, represented in most forms by the pericardial sac (pericardium) and the gonad cavity. All spaces between organs are filled with connective tissue.

4. The body of mollusks, as a rule, consists of three sections - head, torso and legs. Very often the body grows onto the dorsal side in the form of an internal sac. The leg is a muscular unpaired outgrowth of the abdominal wall of the body, used for movement.

5. The base of the body is surrounded by a large fold of skin - the mantle. Between the mantle and the body there is a mantle cavity in which the gills, some sensory organs lie, and the openings of the hindgut, kidneys and reproductive apparatus open. All these formations, together with the kidneys and heart (located in close proximity to the mantle cavity) are called the mantle organ complex.

6. On the dorsal side of the body, as a rule, there is a protective shell secreted by the mantle, usually solid, less often bicuspid, or consisting of several plates.

7. Most mollusks are characterized by the presence in the throat of a special apparatus for grinding food - a grater (radula).

8. The circulatory system is characterized by the presence of a heart, consisting of a ventricle and atria; it is not closed, that is, part of its path the blood passes through a system of lacunae and sinuses that are not formed into vessels.

The respiratory organs are usually represented by primary gills - ctenidia. The latter, however, disappear in a number of forms or are replaced by respiratory organs of a different origin.

For excretion, the kidneys are used - modified coelomoducts, communicating at their internal ends with the pericardial sac.

9. The nervous system in primitive forms consists of a peripharyngeal ring and four longitudinal trunks; at higher forms on the trunks, as a result of the concentration of nerve cells, several pairs of ganglia are formed. This type of nervous system is called scattered - nodal.

10. Shellfish development is very similar to this polychaete worms; in the majority, fragmentation is of a spiral type, deterministic. In the lower representatives, a trochophore emerges from the egg, in most of the rest - a modified trochophore larva - a swallowtail (veliger).

The phylum of mollusks includes 7 classes. The most important of them:

• gastropods (Gastropoda) - slow crawling snails
• bivalves (Bivalvia) - relatively sedentary mollusks
• cephalopods (Cephalopoda) - mobile mollusks

With about 130,000 species, molluscs are second only to arthropods in number of species and represent the second largest phylum of the animal kingdom. Mollusks are predominantly aquatic inhabitants; only a small number of species live on land.

Mollusks have a variety of practical significance. Among them there are useful ones, such as pearl mussel and mother-of-pearl, which are mined to obtain natural pearls and mother-of-pearl. Oysters and some other species are harvested and even farmed for food. Some species are pests of agricultural crops. From a medical point of view, mollusks are of interest as intermediate hosts helminths.

Naked slugs cause damage to gardens, vegetable gardens, and vineyards, grape snail. Shellfish play a large role in the biological purification of water, constantly filtering it to obtain food. Bivalve mollusks are bred on marine plantations for food purposes.

19 species of representatives of this type of animal are included in the Red Book of the USSR.