What do mollusks have. Type and classes of mollusks. General characteristics of mollusks. What is the salivary gland in mollusks? Structural Features of Mollusks
Classes of mollusks and their features
All representatives of this class have a soft, non-articulated body, a shell or its remains, and a special fold of the skin - the mantle, which forms the mantle cavity.
Their mantle releases substances from which the shell is formed (horny substances, lime and mother of pearl). Some mollusks have a head, muscular leg and torso. Many of them have small eyes.
Octopus (lat. Octopus vulgaris) refers to cephalopods
Shellfish differ not only in size, but also in their anatomical structure and behavior. For example, about 80% of the species of these animals belong to the class of gastropods, about 19% make up the class of bivalves, and only about 1% to the rest of the class of mollusks.
Scallop (lat. Pectinidae) belong to the family of marine bivalves
Clam classes: gastropod
Gastropods (snails) is the largest class in the mollusk family (about 90 thousand species). This group includes grape snails, slugs, coils, ponds. Coils and ponds live in small fresh water bodies, and slugs in moist places on land (usually in gardens and fields), grape snails only in vineyards.
Almost all snails eat plants, but sometimes they eat small insects. Among them there are predators, for example, rapana (they live in the seas - eat mussels and oysters).
Sea gastropods mollusks (lat.Gastropoda)
The structure of gastropods
The gastropods of the mollusk class have a single shell, which looks like a small curl. And in some of the mollusks (for example, slugs), the shell is reduced or completely hidden under the skin itself. Like all representatives of this species, they have a leg, trunk and head. On their head they have a mouth, eyes and two or one pair of tentacles. The muscular leg of the mollusks occupies almost the entire abdominal part of the body.
In gastropods, the mantle looks like a pocket that forms a “lung” with breathing holes. Oxygen from the atmosphere’s air fills the “lung” and penetrates directly through the mantle wall into the blood vessels branched by it, and carbon dioxide from the blood vessel exits.
All gastropod mollusks scrape food with the help of the so-called grater - the tongue, which is covered with numerous cloves (horny). They have salivary glands - from the ducts flow directly into the anterior intestines, there is a digestive gland that combines the functions of the liver and pancreas.
Clam classes: cephalopods
In addition, cuttlefish, squid, squid, octopus (about 675 modern species) belong to the order of cephalopod mollusks. These mollusks are inhabited mainly by warm, salty seas and feed on fish crabs and other animals. Cuttlefish and squid actively pursue their own prey, and octopuses watch over it.
Nautilus (lat.Nautilus pompilius) - marine cephalopod, which appeared 500 million years ago, is considered the only among modern cephalopods with an external chamber shell
The structure of cephalopods
In addition, they can quickly change the color of their body, which in cephalopod mollusks consists of a head and a body. Most animals have a crown around their mouths consisting of 8 arms (for cuttlefish and squid) a pair of tentacles with large suckers. Tentacles and arms formed from particles of legs. But the second part of the legs forms a funnel, which is connected with the mantle cavity itself.
The shells of cephalopods are internal, often reduced or completely absent. It is important to note that their mantle cavity functions similarly to a jet engine: through mantle slots, water is drawn directly into the mantle cavities, and then it is ejected with forces through the funnel itself. Cephalopods are crushed by thick and powerful horny jaws, and others are grater. They have two pairs of internal salivary glands.
Squid (lat. Teuthida) - another of the representatives of cephalopods
The origin of the mollusks.
Many scientists are of the opinion that all mollusks came from their ancestors - worm-like marine organisms, or rather, from annelids. In evidence, they cite the similarity of the larvae of many gastropod marine mollusks, as well as the larvae of polychaete marine worms. In addition, part of the primitive mollusks have a fairly large similarity directly with the annelids themselves.
However, some scientists believe that the mollusks were descended from a genus of giant cephalopod mollusks living in the Ordovician period, 470-440 million years ago (Cameroceras lat. Cameroceras)petrified shells of which have been found in North America, South America, and Spain.
Cameroceras (lat. Cameroceras) belongs to the genus of giant cephalopod orthocons
And in more detail with the most interesting representatives of the mollusk class, new articles on the pages of the online magazine "The Underwater World and All Its Secrets" will be introduced to you by new and these videos:
The world of aquatic invertebrates is rich and diverse, and these articles will tell about them:
The type of mollusks, numbering about 130,000 species, is second only to arthropods in terms of the number of species and is the second largest type of animal world. Shellfish are predominantly aquatic; only a small number of species live on land.
Shellfish have a diverse practical meaning. Among them there are useful ones, such as pearl pearl and pearl barley, which are mined in order to obtain natural pearls and nacre. Oysters and some other species are mined and even bred for use in food. Some species are pests of crops. From a medical point of view, mollusks are of interest as intermediate hosts for helminths.
General characteristic of type
Animals belonging to the type of mollusks are characterized by:
- three-layer, i.e. organ formation from ecto-, ento- and mesoderm
- bilateral symmetry, often distorted due to organ displacement
- non-segmented body, usually covered with a shell, whole, bivalve or consisting of several plates
- skin fold - a mantle that fits the whole body
- muscle outgrowth - the leg that serves to move
- poorly pronounced coelomic cavity
- the presence of basic systems: apparatus of movement, digestive, respiratory, excretory, circulatory system, nervous and reproductive
The body of mollusks has bilateral symmetry, in the gastropod (for example, the pond), it is asymmetric. 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 cavity of the gonads. The space between the organs is filled with connective tissue (parenchyma).
The body of the mollusk consists of three sections - the head, trunk and legs. In bivalve mollusks, the head is reduced. The leg - the muscular outgrowth of the abdominal wall of the body - serves to move.
At the base of the body, a large skin fold is developed - the mantle. Between the mantle and the body there is a mantle cavity in which the gills, sensory organs are located, openings of the posterior intestine, excretory and reproductive system open here. The mantle secrets a shell that protects the body from the outside. The sink may be continuous, bivalve or consist of several plates. The shell contains calcium carbonate (CaCO 3) and the organic substance concholin. In many mollusks, the shell is more or less reduced (for example, in part of cephalopods, in naked slugs, etc.).
The circulatory system is open. Respiratory organs are represented by gills or a light, formed part of the mantle (for example, in pond, grape and garden snails, naked slugs). Excretory organs - the kidneys - with the inner ends connected to the pericardial sac.
The nervous system consists of several pairs of nerve nodes interconnected by longitudinal trunks.
The mollusk type includes 7 classes. The most important of them:
- gastropods (Gastropoda) - slowly crawling snails
- bivalves (Bivalvia) - relatively settled mollusks
- cephalopods (Cephalopoda) - moving mollusks
| Table 1. Characteristic features of bivalves and gastropods | ||
| Sign | Class | |
| Bivalve | Gastropods | |
| Symmetry type | Bilateral | Asymmetric with reduction of some right organs |
| Head | Reduced with its related organs | Developed |
| Respiratory system | Gills | Gills or lung |
| Sink | Bivalve | Spiral twisted or cap-shaped |
| Reproductive system | Diclinous | Hermaphroditic or dioecious |
| Nutrition | Passive | Active |
| Habitat | Sea or freshwater | Marine, freshwater or land |
Class of gastropods (Gastropoda)
This class includes mollusks having a shell (snails). Its height ranges from 0.5 mm to 70 cm. Most often, the gastropod shell looks like a cap or spiral, only in representatives of the same family a shell of 2 wings connected by an elastic ligament develops. The structure and shape of the shell are of great importance in the taxonomy of mollusks [show] .
- Placospiral sink - a strongly twisted sink, the revolutions of which are located in the same plane
- Turbo-spiral sink - sink revolutions lie in different planes
- Right-Wound Sink - The spiral of the sink spins clockwise
- Left-handed sink - the spiral twists counterclockwise
- Covert spiral (involute) shell - the last turn of the shell is very wide and completely covers all previous ones
- Open spiral (evolute) shell - all revolutions of the shell are visible
Sometimes a sink is provided with a lid located on the dorsal side in the back of the leg (e.g. at the puddles). When the legs are pulled into the sink, the cap covers the mouth tightly.
In some species that have switched to a floating mode of life (e.g., pterygopods and klenopods), there is no shell. Shell reduction is also characteristic of some terrestrial gastropod mollusks living in soil and forest litter (e.g., slugs).
The body of the gastropod mollusk consists of a well-separated head, leg, and trunk — an internal sac; the latter is placed inside the sink. On the head are a mouth, two tentacles and at their base are two eyes.
Digestive system. At the front end of the head is the mouth. It has developed a powerful language, covered with a solid chitinous grater, or radula. With its help, mollusks scrape algae from the ground or aquatic plants. In predatory species, a long proboscis develops in the front of the trunk, capable of twisting through an opening on the lower surface of the head. In some gastropods (e.g. cones), individual teeth of the radula may protrude from the mouth opening and are in 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 their shells, releasing saliva containing sulfuric acid.
Through the esophagus, food enters the saccular stomach, into which the ducts of the liver flow. Then the food enters the intestine, which bends like a loop and ends on the right side of the body with the anus - the anus.
Nerve nodes are collected in the periopharyngeal nerve ring, from which nerves depart to all organs. On the tentacles are tactile receptors and organs of chemical sensation (taste and smell). There are organs of balance and eyes.
In most gastropods, the trunk protrudes above the leg in the form of a large, spirally twisted bag. Outside it is covered with a mantle and is closely adjacent to the inner surface of the shell.
The respiratory organs of mollusks are represented by gills located in the front of the body and directed with the apex forward (antero-gill mollusks) or located in the right rear part of the body and directed with the apex back (posterior). In some gastropods (e.g., nudibranchs), real gills were reduced. As respiratory organs, they develop the so-called. adaptive skin gills. In addition, in land and secondary aquatic gastropod mollusks, a part of the mantle forms a peculiar lung, numerous blood vessels develop in its walls, and gas exchange occurs here. The pond, for example, breathes atmospheric oxygen, so it often rises to the surface of the water and opens a circular breathing hole to the right at the base of the shell. Next to the lung is the heart, which consists of the atrium and ventricle. The circulatory system is open, the blood is colorless. Excretory organs are represented by one kidney.
Among gastropod mollusks, there are both dioecious species and hermaphrodites, whose sex gland produces both spermine and eggs. Fertilization is always cross, development, as a rule, with metamorphosis. All land, freshwater and some marine gastropods have a direct development. Eggs are laid in long mucous filaments attached to moving objects.
To the class of gastropods
- An ordinary pond, often found on aquatic plants in ponds, lakes and rivers. Its shell is continuous, 4-7 cm long, spirally twisted, with 4-5 curls, a sharp apex and a large opening - the mouth. A leg and a head can protrude through the mouth.
The intermediate owners of trematodes also belong to gastropods.
- The intermediate host of the cat fluke - bitinia (Bithynia leachi) - is widespread in the freshwater bodies of our country. It lives in the overgrown vegetation of the coastal zone of rivers, in lakes and ponds. The shell is dark brown, has 5 convex revolutions. The height of the sink is 6-12 mm.
- An intermediate host of the hepatic fluke, the small pond (Limnea truncatula), is widespread in Russia. The sink is small, no more than 10 mm high, forms 6-7 revolutions. It lives in ponds, swamps, ditches and puddles, where it is often found in huge numbers. In some areas, more than 1 million ponds per hectare of bogs. When the swamps dry, the ponds burrow into the ground, experiencing a dry time in the ground.
- Intermediate hosts of the lanceolate fluke are terrestrial mollusks of 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, helicellas often accumulate in clusters on plants, thus escaping from drying out. Helicella has a low conical shell with 4-6 curls; the shell is light, with dark spiral stripes and a wide rounded mouth. Zebrina has a highly conical shell with 8-11 curls; the shell is light, with brown stripes extending from the top to the base; the mouth is incorrectly oval.
Class Bivalvia (Bivalvia)
This class includes mollusks with a shell consisting of two symmetrical halves, or cusps. These are sedentary, sometimes completely motionless animals that live at the bottom of the seas and freshwater bodies. Often they burrow into the ground. The head is reduced. In freshwater reservoirs, toothless or pearl barley are widespread. Of the marine forms, oysters are of greatest importance. Very large species are found in the tropical seas. The shell of a giant tridacna weighs up to 250 kg.
Barrier, or Toothless lives on the silty and sandy bottom of rivers, lakes and ponds. This inactive animal feeds passively. Toothless foods are particles of detritus suspended in water (the smallest remains of plants and animals), bacteria, unicellular algae, flagellates, and ciliates. The mollusk filters them out of the water passing through the mantle cavity.
The toothless body, up to 20 cm long, is externally covered with a bivalve shell. Distinguish between the widened and rounded front end of the shell, and the narrowed pointed back end. On the dorsal side of the sash are connected by a strong elastic ligament, which supports them in a half-open state. The shell closes under the action of two contact muscles - the front and back, - each of which is attached to both wings.
Three layers are distinguished in the shell - the horny, or concholinic, which gives it an externally brownish-green color, the middle thick porcelain-like layer (consists of pads of lime carbonate; perpendicular to the surface - sinks) and the inner pearl layer (there are thin layers between the finest calcareous leaves strata of conhiolin). A mother-of-pearl layer is laid on each of the two folds with a yellowish-pink fold of the mantle. The mantle epithelium secrets a shell; in some species of freshwater and marine pearls, it forms pearls.
The body is located in the dorsal part of the shell, a muscular outgrowth - the leg - leaves from it. In the mantle cavity, on either side of the body there is a pair of lamellar gills.
In the back part, both shell flaps and mantle folds do not fit tightly against one another, there are two openings between them - siphons. The lower, introductory, siphon serves to introduce water into the mantle cavity. A continuous directed flow of water is due to the movement of numerous cilia, which cover the surface of the trunk, 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 is excreted along with excrement.
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 move food particles to the mouth with their movement. Due to the reduction of the head in the pearl barley and other bivalve mollusks, the pharynx and organs associated with it (salivary glands, jaws, etc.) are reduced.
The digestive system of the pearl barley consists of a short esophagus, a saccular stomach, a liver, a long, loop-like curved middle intestine, and a short hind gut. An opening of a saccular outgrowth opens in the stomach, inside of which there is a transparent crystalline stalk. With its help, food is ground, 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 open; colorless blood flows not only through the vessels, but also between the organs. Gas exchange occurs in the gill filaments, from where 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 begin 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 sensory organs are poorly developed in connection with the reduction of the head and a sedentary lifestyle.
Cephalopods class (Cephalopoda)
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, to which most squids belong, have an elongated, torpedo-shaped body. For benthic species, among which octopuses predominate, a saccular body is characteristic. In cuttlefish living in the near-bottom layer of water, the body is flattened in the spin-direction. Narrow, spherical or jellyfish-like planktonic species of cephalopods are distinguished by their small size and gelatinous body.
Most modern cephalopods have no outer shell. It turns into an element of the internal skeleton. Only the Nautilus retains an outer, spirally twisted shell, divided into internal chambers. In cuttlefish, the shell, as a rule, has the appearance of a large porous calcareous plate. Spirula retains a spiral-wrapped shell hidden under the skin. In squids, only a thin horny plate, which extends along the dorsal side of the body, is preserved from the shell. In octopuses, the shell is almost completely reduced and only small crystals of lime carbonate remain from it. In female argonauts (one of the species of octopuses), a special brood chamber develops, in shape very similar to the outer shell. However, this is only an apparent similarity, since it is secreted by the tentacle epithelium and is intended only to protect developing eggs.
One of the hallmarks of cephalopods is the presence of an internal cartilaginous skeleton. Cartilage, similar in structure to vertebral cartilage, surrounds the head cluster of ganglia, forming a cartilaginous capsule. The processes that reinforce the eye openings and the balance organs depart from it. In addition, supporting cartilages develop in cufflinks, the base of the tentacles and fins.
The body of the cephalopod mollusk consists of a head with complex eyes, a crown of tentacles, or arms, a funnel, and a trunk. Large complex eyes are located on the sides of the head and in their complexity are not inferior to the eyes of vertebrates. The eyes have a crystalline lens, a cornea and an iris. Cephalopods have developed not only the ability to see in stronger or weaker lighting, but also accommodation. True, it is achieved not by changing the curvature of the lens, as in humans, but by approaching or moving away 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 for catching prey; in male octopuses, one of the tentacles is used to transfer sexual products. During the breeding season, this tentacle changes, and during the mating period it breaks away and, due to its ability to move, penetrates 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 abdominal side of the body, opening at one end into the mantle cavity, and the other into the external environment. The mantle cavity in cephalopods is located on the ventral side of the body. At the place of transition of the torso to the head, it communicates with the external environment through the transverse abdominal opening. For its closure in most cephalopods, paired lunate fossae form on the ventral side of the body. Opposite them on the inner side of the mantle are two solid, cartilage-reinforced tubercles, the so-called cufflinks. As a result of muscle contraction, the cufflinks enter the semilunar depressions, tightly fastening the mantle to the body. When the abdominal opening is open, water freely penetrates the mantle cavity, washing the gills lying in it. After this, the mantle cavity is closed and its muscles are reduced. 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 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 horn jaws to kill. They are located in the muscular throat and resemble the beak of a parrot. It also contains a radula - a chitinous tape with 7-11 rows of cloves. 1 or 2 pairs of salivary glands open in the throat. Their secret contains hydrolytic enzymes that break down polysaccharides and proteins. Often, the allocation of the second pair of salivary glands is poisonous. The poison also helps 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 hind gut. It gives off a dark secret (ink), which can stir up a large amount of water. Ink serves as a smoke screen, disorienting the enemy, and sometimes paralyzing his sense of smell. Cephalopods use it, fleeing the pursuit of 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. Ganglia of the nervous system form the general nervous mass - a multifunctional brain that 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 learning ability. For the perfection of the brain, cephalopods are called "primates of the sea."
Unique skin photoreceptors of cephalopods respond to the slightest changes in illumination. Individual 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, are armed with one or two modified hands - hectocotyls, with the help of which spermatophores are carried in the period of copulation with “packets” of seminal fluid. 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 membrane of the eggs. After fertilization, the females attach clusters of eggs to the bottom objects. Some species take care of offspring and protect developing eggs. The female protecting the offspring can starve for more than 2 months. In octopuses, cuttlefish and nautiluses, a minicopy of parents hatch from each egg, only in squid development occurs with metamorphosis. Young growth grows quickly and often reaches puberty by the year.
The value of mollusks
Freshwater pearl shells with a mother-of-pearl layer thickness of about 2.5 mm are suitable for the manufacture of mother-of-pearl buttons and other jewelry. Some bivalves (mussels, oysters, scallops), a grape snail from gastropods (in some European countries it is bred in snail farms) are consumed, and cephalopods are especially valuable in calories and protein composition (more than 600 thousand of them are harvested annually in the world . t).
The river zebra mussel in enormous quantities is found 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 is surrounded by stones, piles and various hydraulic structures: watercourses, pipes for technical and drinking water supply, protective grilles, 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 water to pass through; therefore, continuous cleaning of zebra mussel is necessary; apply mechanical, chemical, electrical and biological control methods. Some bivalve mollusks make passages in the bottoms of ships, wooden parts of port facilities (shipworm).
Perlovica and some other bivalve mollusks 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 contaminated 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 reservoir self-cleaning system. Of particular importance in the system of biological self-cleaning of water bodies are mollusks, which have special mechanisms of resistance to pollution of water bodies with toxic substances and mineral salts, as well as adapted to living in water with a reduced amount of oxygen. The molecular mechanism of such adaptation is based on carotenoids contained in the nerve cells of mollusks. Pearl barley and other mollusk filters need protection. They can be bred in special containers and used to clean artificial ponds from pollution, waste disposal and additional food.
Shellfish fishing is especially important in Japan, the USA, Korea, China, Indonesia, France, Italy, and England. In 1962, the extraction of mussels, oysters, scallops and other bivalve mollusks amounted to 1.7 million tons; to date, the 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 in the experimental farm in the northwestern part of the Black Sea (productivity is 1000 cc mussels per year), research on breeding mussels has been carried out in the basins of other seas washing the shores of our country. Shellfish meat is easily digested, it contains many vitamins, carotenoids, trace elements (iodine, iron, zinc, copper, cobalt); it is used for food by the population, as well as for the feeding of domestic animals. Clam-filtrators can also be used in a biomonitoring system to monitor the chemical composition of water in water bodies.
Cephalopods, common in all seas except desalinated, despite being predators, often serve as food for many fish and marine mammals (seals, sperm whales, etc.). Some cephalopods are edible and are subject to fishing. In China, Japan, and Korea, the use of these animals as food goes back centuries; in Mediterranean countries it also has a very long history. According to the testimonies of Aristotle and Plutarch, octopuses and cuttlefish were common food in ancient Greece. In addition, they were used in medicine, perfumery and in the manufacture of first-class paints. At present, congenital programs of complex behavior are being studied in laboratory conditions on cephalopods.
Type Mollusks, or Soft-bodied, includes more than 100 thousand species of 7 or 8 (according to various classifications) of the living class. However, most species belong to the classes of gastropods and bivalves. Representatives of the soft-bodied: snails, slugs, barley, oysters, squids, octopuses, etc.
Different types of mollusks belonging to different classes differ quite strongly in structure, and often in the life cycle.
Body sizes from less than a millimeter to more than 10 m.
The external structure of mollusks
The body of the mollusk has bilateral (bilateral) symmetry or asymmetric due to distortion of bilateral symmetry in the process of individual development.
The body is not divided into segments. However, the most simply arranged mollusks have some signs of segmentation. Therefore, the soft-bodied either could have common ancestors with annelids, or their ancestors themselves were annelids.
The body of many mollusks consists of a head, trunk and leg. In bivalves, the head is absent, the leg is reduced. In cephalopods and a number of others, the leg turned into a swimming organ.
The body forms a mantle, which is a skin fold covering the body. A mantle cavity is formed between the body and the mantle, where the openings of the excretory organs, sometimes the genitals, and the anus open. Here are the gills (or lung) and some sensory organs.
In many mollusks, a solid shell having a mineral nature covers the body from the dorsal side. It is formed from substances that are secreted by the mantle. This is mainly crystalline calcium carbonate (CaCO 3) with an admixture of organic matter. Often on top of the shell is covered with horn-like organic matter, and on the inside has a calcareous layer called pearl.
The sink may be solid, bivalve or consisting of several plates. It is usually well developed in slowly moving and motionless mollusks. Others may be small or completely absent. So, for example, there are no squid shells, octopuses, as well as slugs.
On the head of the mollusks there is a mouth opening, tentacles and eyes.
The leg is an unpaired muscular outgrowth of the abdominal side of the body. Used to crawl. May carry balance organs ( statocysts).
The internal structure of mollusks
Like annelid worms, mollusks belong to the primary, the secondary, and the three-layer.
Despite the fact that mollusks belong to the secondary cavity, the secondary body cavity (as a whole) is well developed only in their embryos. In adults, the whole remains only in the form of a pericardial sac and cavity of the genital gland, and the spaces between the organs are filled with connective tissue (parenchyma).
Digestive system
A pharynx goes behind the mouth of the mollusk, in which many species have pleased (grater). The radula consists of a ribbon and cloves located on it, with which vegetable food is scraped off or animal (grains, crustaceans, etc.) are grasped.
In some predatory mollusks, the salivary glands, whose secret contains poison, open in the oral cavity.
In bivalve mollusks, which feed on microorganisms and small organic particles, the esophagus immediately follows the mouth, i.e. they do not have a pharynx with a grater.
Respiratory system
Aquatic mollusks have paired gills ( ctenidia), which are skin outgrowths in the mantle cavity. Terrestrial have a lung, which is a fold (pocket) of the mantle, filled with air. Its walls are penetrated by blood vessels. Despite the presence of respiratory organs, mollusks also have skin respiration.
Circulatory system
Unlike annelids, mollusks have an open circulatory system. Although in the most complex soft bodies, it is almost closed. For some, the oxygen-transporting pigment contains manganese or copper, rather than iron. Therefore, the blood may be blue.
There is a heart, which in most species consists of one ventricle and two atria.
The aorta leaves the heart, then the arteries go, which pour blood into the spaces between the organs. Then venous blood is already collected again in other vessels and goes into the gills or lung. From there, through the vessels in the heart.
Excretory system
Mollusks have 1 to 12 kidneys similar to methanephridia. Inside, they open into the cavity of the pericardial sac, with the other end into the mantle cavity. In the kidneys, uric acid accumulates.
Nervous system and sensory organs
The nervous system of mollusks includes several pairs of ganglia connected by nerve trunks. From the trunks nerves depart.
In different representatives of the type, the degree of development of the nervous system is different. The simpler ones have a ladder type, the rest have a scattered-nodal type.
There are organs of touch, chemical feelings, balance. In mobile forms, especially in fast-swimming cephalopods, the organs of vision are developed.
Shellfish breeding
Among the mollusks there are both dioecious species and hermaphrodites (less commonly). Fertilization is external or internal. The genital glands open as a whole, and the reproductive products are excreted into the mantle cavity through the kidneys.
A planktonic larva (sailfish) or small mollusk develops from an egg.
The value of mollusks
Benthic bivalve mollusks filter water, which purifies it not only from organic, but also from mineral particles.
Shellfish serve as food for other animals, including birds, mammals and humans. People breed, for example, oysters.
In the shells of pearls, pearls are formed, which people use as jewelry.
Scientists determine the age of sedimentary rocks from the shells of fossil mollusks.
Some marine bivalves destroy wood, which can cause damage to ships and hydraulic structures.
Ground slugs and snails can harm orchards and vineyards.
Mollusks
Mollusks, representatives of over 80,000 species of invertebrate animals such as Mollusca. These include snails known to all, bivalves and squid, as well as many lesser-known species. Initially, the inhabitants of the sea, mollusks are now found in the oceans, in fresh water and on land. Classes of mollusks include: primitive BRUCIPOUS mollusks, single-winged (slugs and snails), Bivalve mollusks, shovel-legged mollusks and HEADED (squids, etc.). The body of the mollusk consists of three parts: head, leg and torso. There is also a skin fold attached to the body called mantleproducing a calcareous shell (shell), characteristic of most mollusks. The head is well developed only in snails and cephalopods, which have eyes, tentacles, and a well-shaped mouth. The body contains the internal organs of blood circulation (blood vessels and heart), respiration (gills), secretion (kidney) and reproduction (gonads). Shellfish are usually dioecious, but there are many types of hermaphrodites. Cephalopods, bivalves and gastropods are important fossils - evidence of the geological past. see alsoHERMAPHRODITES.
Clams. Remarkable experts in the study of the new habitat, snails, previously lived in the sea, but gradually about 22,000 species adapted to life on land, having lost their gills and developing lungs, breathing air. Most species of land snails, such as the Helix pomatia grape snail pictured here, live on the ground, are dimly colored, there are several woody species that tend to be brightly colored. Other species have returned to life in the water and must periodically surface to breathe.
Scientific and technical encyclopedic dictionary.
See what is "Mollusks" in other dictionaries:
Soft-bodied (Mollusca), a type of invertebrate animal. Arisen presumably in Precambrian; from the lower Cambrian already known several. classes M. Probably came from small-segment worm-like ancestors (annelids) or directly from flat ... ... Biological Encyclopedic Dictionary
Mollusks - Molluscs, or soft-bodied (Mollusca), a well-closed type of invertebrate animal. The body is soft, undivided, typically carries a conch. The integuments form a fold mantle covering the body or fusing along the edges with its surface. ... ... Big Medical Encyclopedia
- (new lat. mollusca, from lat. mollis soft). Soft animals, slugs. Dictionary of foreign words included in the Russian language. Chudinov AN, 1910. MALLUSS novolatinsk. mollusca, dated mollis, soft. Soft animals. The explanation ... ... Dictionary of foreign words of the Russian language
- (from lat. molluscus soft) (soft-bodied) type of invertebrate animals. The body of most mollusks is covered with a shell. On the ventral side is a muscular outgrowth of the leg (organ of movement). 2 subtypes: side-shell and shell-type; St. 130 thousand species. They live in ... ... Big Encyclopedic Dictionary
Modern Encyclopedia
Mollusks - Mollusks, a type of invertebrate animal. The body of most is covered with a shell. The head has a mouth, tentacles and often eyes. The muscular outgrowth (leg) on \u200b\u200bthe ventral side is used for crawling or swimming. About 130 thousand species, in the seas (most), ... ... Illustrated Encyclopedic Dictionary
- (Mollusca) type of animals with a whole non-segmented body. Most representatives have a calcareous shell, whole or consisting of two, rarely several separate parts. The organ of movement is a muscular unpaired ... ... Geological Encyclopedia
mollusks - the body of most m is covered with a shell. ▼ side-bones. armored: tunic of tonicella. saline gastric: echinomenia. caudofoveates. conch. monoplacophores: neopilin. gastropods, snails, gastropods: prothoracic: kauri. littorins. abalone trumpeters ... Ideographic dictionary of the Russian language
mollusks - A type of soft-bodied, non-segmented invertebrate animals that usually secrete a substance for building a calcareous shell: snails, saucers, bivalves, tunics, squids. ... ... Technical Translator Reference
- (Mollusca) (from lat. Molluscus soft), soft-bodied, type of invertebrate animals. 7 classes: gastropods, Monoplacophores, Shell-mollusks, Belly-bellied mollusks, Bivalve mollusks, Shovel-legged mollusks and ... Great Soviet Encyclopedia
Books
- J.-L. Cuvier. The kingdom of animals. Mollusks, R. Aldonina. This publication introduces the reader to the section `Mollusks` from the four-volume work of the French naturalist and naturalist Georges-Leopold Cuvier` The animal kingdom, distributed according to ...
Mollusks- bilaterally symmetric or second-asymmetric three-layer animals. They live in marine and fresh water bodies, on land.
In the body of most species of mollusks, three sections can be distinguished: head, trunk and leg. On the head are the mouth opening, sensory organs. The strongly thickened abdominal 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 into a flat sole.
The body is surrounded by a skin fold - the mantle. Between the mantle and the body, a mantle cavity is formed, into which openings of the digestive, excretory and reproductive systems open. In the mantle cavity there are also respiratory organs and organs of chemical sensation (osfradia). All of the above is called the mantle complex of organs.
The muscles in mollusks are well developed and consist of muscle bundles. They are especially developed in the leg of the animal.
As a whole, it is reduced to the pericardial sac and the cavity in which the gonads are located. The space between the other organs is filled with parenchyma.
The digestive system is divided into three sections: anterior, middle and posterior. The anterior and posterior divisions are of ectodermal origin, the middle one is endodermal. In the throat of many species there is a specific organ for grinding food - radula, or grater. The ducts of the salivary glands open into the pharynx, and the ducts of the liver into the middle intestine.
Respiratory organs are represented by gills or lungs. Lungs are present not only in terrestrial species, but also in forms that have secondarily passed to an aquatic way of life. Gills and lungs are modified sections of the mantle. In species living in water, 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 in the space between the organs. Mollusks have a heart made up of two or more chambers. The heart is in the pericardial sac (pericardium).
Excretory organs - kidneys, which are modified metanephridia. The kidney begins with a funnel in the pericardial sac and opens with a secretory hole in the mantle cavity.
The nervous system in most mollusks is represented by several pairs of nerve nodes that are located in different parts of the body. The nervous system of this type is called scattered-nodal. In addition to reflex activity, the nervous system performs the functions of regulating growth and reproduction by secreting various neurohormones. The mollusks have organs of chemical sensation (osfradia), balance, numerous tactile receptors are scattered in the skin. Many species have eyes.
The predominant number of species of mollusks is diclinous animals, but bisexual species are also found. The development of all terrestrial species, the majority of freshwater and part of marine inhabitants is direct. If the development proceeds with metamorphosis, then either a larva of the trochophore type or a larva - veliger (sailfish) emerges from the egg.
The type of mollusks is 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 mollusks and annelids is confirmed by the similarity of embryogenesis (spiral fragmentation, metamerism of the primordia of some organs, teloblastic laying of the mesoderm) and the presence of a trochophore larva similar to trochaophore in the lower mollusks. It is assumed that the primary mollusks were bilaterally symmetrical animals with a low body, covered with a slightly convex shell, with a muscular flat leg and an almost non-isolated head. Two lines of evolutionary development depart from the primary mollusks. The first line leads to the formation of squamous mollusks, this group is not considered in this manual. The second evolutionary line leads to the appearance of conch mollusks. Among the shell mollusks, the most primitive are monoplacophores. It is believed that bivalves, gastropods, and cephalopods came from the ancient monoplacophores.
Description of classes, subclasses and orders of the Mollusk type:
- Class Gastropoda
Class Cephalopods
- Subclass Coleoidae
