Zoological excursions to fresh water. Gastropods: pond snails, meadows, bitinia, coils Based on the text of the paragraph, prove that mollusks are at a higher level of evolutionary development than worms
In Russia and Europe there are different types of pond snails. Among them, the largest is the common pond snail, the shell of which can reach 7 centimeters. All species breathe through the lungs, therefore, from time to time they are forced to swim to the surface. You can often observe how the pond snail, the photo of which is presented in this article, smoothly and slowly slides along the lower part of the surface film of water, collecting oxygen from the air.
If mollusks “suspended” in this way are somehow disturbed, they immediately release an air bubble from the breathing hole and fall like a stone to the bottom. The long-eared pond snail is the closest relative of the common pond snail. Its shell reaches 2.5 centimeters, which depends on the abundance of food and the temperature in its reservoir.
The common pond snail and other species of its family (besides those listed above, in our reservoirs you can find ovoid, small and marsh) are very variable. The shapes, sizes, thickness of the shell, and the color of the body and legs of the snails vary. Along with those that have a strong shell, there are species with a very fragile, thin shell that breaks even with the lightest pressure. There may also be different forms of curl and mouth. The color of the body and legs varies from sandy-yellow to blue-black.
Structure
The body of the mollusk is enclosed in a spirally twisted shell, which has an mouth and a sharp apex. The shell of the common pond snail is covered with a layer of lime, a horn-like greenish-brown substance. It is a reliable protection for his soft body.
In the body of a snail, 3 main parts can be distinguished: the leg, the head and the torso - although there are no sharp boundaries between them. Only the front part of the body, leg and head can protrude from the shell through the mouth. The leg is very muscular. It occupies the abdominal part. Such snails are called gastropods. At the same time, sliding along objects with the sole of the foot or hanging from the lower film of water, the mollusk smoothly moves forward.
The body copies the shape of the shell, fitting very closely to it. It is covered in the front part by a mantle (a special fold). The space between it and the body is called the mantle cavity. The body in front passes into the head, which has a mouth on the underside and two sensitive tentacles on the sides. When a pond snail is lightly touched, it immediately pulls its leg and head into the shell. Near the bases of the tentacles there is one eye.
Circulation
The common pond pond has a rather interesting structure. So, he has a heart that pushes blood into the vessels. In this case, large vessels are divided into small ones. And from them blood flows into the spaces between the organs. Such a system is called “unclosed”. The interesting thing is that blood washes each of the organs. Then it is collected again into the vessels that lead to the lung, after which it goes directly to the heart. In such a system, it is much more difficult to ensure the movement of blood than in a closed one, since it slows down between organs.
Breath
Despite the fact that the snail lives in water, it breathes atmospheric air. To do this, the common pond snail, the structure of which is described in this article, floats to the surface of the reservoir and opens a round breathing hole at the edge of the shell. It leads to the lung - a special pocket of the mantle. The walls of the lung are densely braided. In this place, carbon dioxide is released and the blood is enriched with oxygen.
Nervous system
This mollusk has a circumpharyngeal concentration. From them, nerves extend to all organs.
Nutrition
The snail's mouth leads to the pharynx. There is a muscular tongue covered with teeth ─ the so-called grater. The common pond snail, a photo of which can be seen in this article, uses it to scrape off plaque from all kinds of microorganisms that form on various underwater objects, and also rubs various parts of plants. Food from the pharynx goes to the stomach and then to the intestines. The liver also facilitates its digestion. The intestine opens through the anus into the mantle cavity.
Movements
If a caught pond snail is placed in a jar, it immediately begins to actively crawl along its walls. At the same time, a wide leg extends from the shell opening, which is used for crawling, as well as a head with two long tentacles. By sticking the sole of its foot to various objects, the snail slides forward. In this case, gliding is achieved by wave-like, smooth muscle contractions, which can be easily observed through the glass of the vessel. It is interesting that the common pond snail can wander along the lower surface of the water, as we have already discussed above. In doing so, it leaves a thin ribbon of mucus. It stretches across the entire surface of the water. It is believed that snails moving in this way use fluids, hanging from below to an elastic film that forms on the surface due to this tension.
Such crawling can be easily observed on a calm surface of a reservoir, while going on an excursion or relaxing in nature.
If a pond snail, crawling in this way, plunges into the water again under a little pressure, you will see how it rises to the surface again, like a cork. This phenomenon is easily explained: there is air inside the respiratory cavity. It supports the snail, just as the Prudovik can compress its respiratory cavity at will. In this case, the mollusk becomes heavier, and therefore sinks to the very bottom. But when the cavity expands, it floats to the surface in a vertical line without any push.
Try immersing a pond snail floating on the surface of a pond and disturb its soft body with a touch of tweezers or a stick. The leg will immediately be pulled back into the shell and air bubbles will be released through the breathing hole. Next, the mollusk will fall to the bottom and will not be able to independently rise to the surface in any other way than by climbing onto plants, due to the loss of the air float.
Reproduction
The pond snail is a hermaphrodite, although it undergoes cross-fertilization. The snail lays eggs, which are enclosed in slimy transparent cords attached to algae. From the eggs emerge small pond snails with a very thin shell.
If you decide to keep a common pond snail, then you need to understand that a prerequisite for keeping it is a water temperature of about 22 ° C and its moderate hardness.
Zoological excursions to fresh water
A fresh body of water is easy to find in almost any area, including within the city. A river, stream, lake, pond, swamp and even a simple ditch can serve as a place for an excursion.
Unlike forests and open spaces, the species composition of the inhabitants of water bodies is quite constant and changes slightly during the warm season. The inhabitants of reservoirs are diverse and represent a wide variety of ecological groups, but at the same time there are not so many of them and it will not be difficult to prepare a brief description of them and talk about them.
Conducting an excursion requires preliminary preparation. It makes sense to map out in advance the route and method of approaching or approaching a reservoir (or reservoirs, if you plan to visit several of them), inspect the banks and identify the most convenient (and vice versa, the most inconvenient and dangerous) places for catching and taking samples. Immediately before work, it is necessary to give a brief briefing on the rules of conduct near a reservoir. For example, it is necessary to strictly prohibit any games in the immediate vicinity of water - a child falling into the water, even if it is a small ditch, in cool weather can disrupt the entire excursion. An equally important task is to make the children not passive listeners, but active researchers of the reservoir. To do this, you need to inform them in advance about basic information about the biology of certain aquatic animals, interest them in a story about their interesting features and draw up a work plan, explaining where, how and who to look for.
You need to take with you nets with long handles (2 m) made of durable fine-mesh fabric - preferably one for each group of 5-7 people, shallow plastic buckets and light photo cuvettes for analyzing the catch, tweezers, small nets and jars, and for displaying the most small organisms - pipettes, test tubes and magnifying glasses.
When catching, the net should be held against the current or as if describing a figure eight with it.
It is advisable that the hoop of the net be strong, and that a strip of dense, strong fabric is sewn onto it - this will allow you to catch bottom organisms. To do this, a hoop is passed along the bottom, and before removing the net from the water, it is rinsed without turning it over, washing out small particles of soil.
During the excursion, everyone should be given the opportunity to be a “hunter”, but you should not get carried away only with catching and sorting out “prey” - at least some of the caught animals should be told in more detail. The animal about which the story is told must be transplanted into a jar or test tube and passed around in a circle so that everyone can see it. The story should be accompanied by problematic questions, inviting children to take a closer look at the animals, think out loud and discuss their answers.
How can aquatic animals be interesting? Here are some topics to discuss.
Breathing methods in aquatic animals
Breathing through the entire surface of the body. Sponges, bryozoans, flatworms, and leeches do not have special respiratory organs. The absorption of oxygen and the release of carbon dioxide occurs simply through the skin.
Gill breathing. Gills - special outgrowths with a large surface area, entangled in a dense branched network of blood vessels, are found in various crustaceans, bivalves and some gastropods (lawn) mollusks, amphibian larvae, and, of course, fish.
Breathing using tracheal gills and trachea. The trachea is a system of thin air-filled tubes that permeate the body, through the walls of which gas exchange occurs.
Typically, the trachea opens on the surface of the body with special openings - spiracles. But in the tracheal gills these tubes are closed and do not directly communicate with the environment (water) - gas exchange is carried out by diffusion through thin covers. The tracheae here are similar to the blood vessels in ordinary gills.
Tracheal breathing is characteristic of insects.
The larvae of stoneflies, mayflies, caddis flies, and flies have noticeable tracheal gills in the form of outgrowths on the sides, and in the larvae of small (uniptera) dragonflies, at the end of the body.
The locations of the spiracles in aquatic insects that do not have tracheal gills are very diverse. The lungs are organs of air respiration, which are a cavity inside the body, the walls of which are intertwined with blood capillaries. Among aquatic animals, pulmonary gastropods (ponds, coils) and adult amphibians breathe through their lungs.
Types of movement in aquatic animals
Swimming using wave-like movements of the body. Leeches, some other worms.
Swimming due to the bends of the back of the body with an extended organ located at the end - a fin. Fish, newts, larvae of some mosquitoes and a number of other insects, crayfish.
Swimming using limbs. Planktonic crustaceans, swimming bugs (smooth bugs), larvae of swimming beetles and adult water beetles, larvae of some other insects from different orders. In arthropods that swim with the help of their limbs, the rowing legs are oar-shaped or equipped with hairs. Frogs swim using their hind limbs; their hind legs have swimming membranes.
Jet propulsion (water shot). The larvae of heteroptera dragonflies take water into the hindgut and, abruptly throwing it out, move quite quickly in the water column.
Movement using cilia on the surface of the body. Planaria and ciliates (but it is difficult to demonstrate microscopic ciliates on a tour).
Crawling along the bottom or through aquatic vegetation using limbs.
A whole range of insects and their larvae, water donkeys, crayfish. The limbs of such animals differ from swimming ones in the absence of extensions and the presence of claws. Crawling using muscle contractions
. Shellfish Movement on the surface film
. Water striders glide easily and quickly along the surface of the water, gastropods crawl along the film below, and, having attached themselves to it, smooth bugs hang motionless, lying in wait for prey. Filtration
. Straining out nutrient particles and small organisms from water is characteristic of bivalves, sponges, and daphnia crustaceans (daphnia strain out food particles using their abdominal legs). Feeding on dead organic matter (detritus)
. It is characteristic of many aquatic animals - the larvae of a number of mosquitoes and flies, the water burro, crayfish, partly gastropods and tadpoles.. It is characteristic of water-loving beetles (but the larvae of water-loving beetles are predators), combing bugs, the larvae of a number of caddis flies, and partly gastropods. The caterpillars of the water lily moth butterfly, which develop in the water, feed on aquatic plants. The larvae of tailless amphibians (tadpoles) eat small fouling algae.
A – broad swimmer and its larva; B – female water donkey with a brood chamber
Predation. Most inhabitants of water bodies are predators. Actively catching their prey are cyclops crustaceans, leeches, larvae of dragonflies, water striders, some caddis flies, swimming beetles and their larvae, spinning beetles, smoothies, water striders, and water scorpions. Predators are the larvae of tailed amphibians (newts).
Features of reproduction
Sexual and asexual reproduction. Sexual reproduction in the vast majority of aquatic animals is carried out by spawning reproductive products into the water, where fertilization and egg development occur.
This can be most clearly demonstrated in the spring using the example of amphibian eggs (frogs and toads). Asexual reproduction (budding) is characteristic of primitive animals - sponges, bryozoans, hydras. Caring for offspring (spawn or juveniles)
. Carrying eggs on the female's body, which ensures much greater survival of the offspring than with simple spawning into water, can be observed in the snail leech, cyclops, water donkey, and crayfish. Some mollusks - balls and meadows - are characterized by ovoviviparity. Development of insects with complete and incomplete metamorphosis.
Development with complete transformation is characteristic of beetles (beetle larvae can be demonstrated), dipterans (mosquito larvae and floating pupae can be shown) and caddisflies.
It is interesting to demonstrate caddisfly larvae in canopy houses, and if you are lucky, interesting mobile floating pupae of these insects. Incomplete transformation, in which the larva looks like an adult insect from the very beginning, is observed in bedbugs. Dragonflies, mayflies, stoneflies, and flies also develop with incomplete metamorphosis, but their larvae differ markedly from the adult forms. This is a very interesting example that allows us to show adaptations to different habitats in the same species: larvae of dragonflies and mayflies live in water, and adult insects leave the reservoir and fly. On the stems of plants rising from the water, you can see the dry skins of dragonfly larvae, crawling out of the water before the last moult, before the appearance of the adult winged form.
Using the example of dragonflies, mayflies and mosquitoes, we can also talk about the transfer of chemical elements and energy from the aquatic community to the terrestrial one. The larvae of these insects develop and grow using the nutrients of the reservoir.
Adult insects fly near water, where they are eaten by birds, animals and other land inhabitants, who thus receive energy originally stored not by terrestrial, but by aquatic plants.
Larvae of aquatic insects: A – stoneflies; B – silt fly; B – dragonflies (1 – homoptera, 2 – heteroptera)
What animals, which are characterized by one or another of the listed features, are most often encountered during an excursion to a pond, and what other interesting moments of their life can you tell about?
Type Flatworms
Planarians, representatives of ciliated worms, are found in our fresh water bodies. These are small animals that slowly crawl along the bottom with the help of numerous cilia, invisible to the naked eye. Planarians feed using a retractable muscular pharynx located on the ventral side of the body. From the pharynx, food enters the extensive, highly branched and blindly closed intestine. Planaria are hermaphrodites that lay cocoons with eggs on the lower parts of aquatic plants.
An interesting feature of planarians is the ability to quickly regenerate - restore lost body parts. Sometimes they even reproduce through self-mutilation. We most often meet white planaria with an almost transparent body through which the internal organs are visible, and brown planaria
with a rounded head end, dark in color.
Type Annelids
In fresh water bodies you can find a variety of leeches (Leech class). These are quite large worms with clearly defined segmentation, often with a pattern on the body.
Most leeches in the middle and northern parts of the Russian Plain are active predators, feeding on worms, insect larvae, and mollusks. They (not all) swim quickly in the water column, wriggling with their whole body, but they can also “walk” along the bottom, sucking either with the front or the rear suction cup. Leeches: A – large pseudocone; B – cochlear The most common leeches are small And
Due to its small size and weak jaws, the small false horse leech is much inferior to the large false horse leech in gluttony. False-cone leeches cannot attach themselves to humans, but in the southern regions of the European part of Russia a related species is found medical leech, which has long been used in medicine.
Snail leech has an oval-shaped body, greenish-yellow color. The size of this leech is up to 3 cm. Unlike the horse and false horse leech, the snail leech has a proboscis, with which it sucks out small inhabitants of water bodies, mainly snails (hence its name). This leech takes care of its offspring: eggs and young develop on the ventral surface of its body. The snail leech cannot swim, so it crawls along the bottom and plants.
Type Shellfish
Class Bivalve
Bivalves have a shell consisting of two valves connected on the dorsal side. The muscles that close the shell valves are so powerful that even a person must make an effort to open a large shell. The shell consists of three layers: the outer horny layer, the middle layer, the hardest layer - porcelain and mother-of-pearl, lining the shell from the inside.
Bivalve mollusks live at the bottom of reservoirs and are able to move with the help of a muscular leg, with which they attach to the substrate and pull up the rest of the body. The speed of movement is naturally low - up to 30 cm/h.
Bivalves breathe by drawing water through the inlet siphon (a tube formed by the edge of the mantle) into the gill cavity and releasing it through the outlet siphon. By type of nutrition, all bivalves are filter feeders, and their absorption of food occurs simultaneously with respiration.
Looks like toothless pearl barley has a more elongated and thick shell of a greenish color. It has industrial significance: mother-of-pearl buttons are made from its shells.
Peas The most common leeches are balls- smaller bivalve mollusks, the length of their shell does not exceed 1.5 cm. In peas, the top of the shell is shifted to the rear end, and in balls it is located in the center of the shell.
Molluscs: A – horny ballfish; B – river pea
Class Gastropods
Gastropods have a spirally twisted shell containing a body, a leg and a head equipped with a pair of tentacles and two eyes. The mollusk's mouth is also located on the head.
The mouth of the shell can be open (in pulmonate mollusks - pond snails, coils) or closed with an operculum (in meadows breathing through gills, bitinia).
Gastropods move with the help of wave-like contractions of the muscular leg, as if gliding along the substrate. They can also crawl from below on the water film.
Pulmonary mollusks breathe air, after which they rise to the surface from time to time.
All our freshwater snails feed mainly on plant food, which they obtain using a “grater” (an organ on the tongue), scraping off the covers of plants. They also eat dead remains. Gastropods: A – common pond snail; B – horn-shaped coil
Reels The most common leeches are They differ from pond snails by having a flattened shell, as if compressed from the sides. Coil egg clutches are round in shape.
Luzhanki (livebearers)
bithinia
They vaguely resemble pond snails, but the mouth of their shells is closed with a lid. They live at the bottom of muddy ponds. Luzhanka is a fairly large snail, its shell is 2–3 cm long and relatively round in shape. Bitinia are smaller and their shells are more similar to those of pond snails. Bitinia lays eggs on plants, and the meadow “gives birth” to small snails, carrying eggs in the body. Bitinia is an intermediate host of the cat fluke. Phylum Arthropod
Class Crustaceans
Water donkey
from the order of isopods (to which terrestrial woodlice belong) can always be found in reservoirs polluted by rotting plants. Despite the fact that this is a relatively large crustacean (females reach 15 mm in length, and males - 20 mm), it is quite difficult to detect due to its good camouflage. A water burro, caught by a predator, can voluntarily discard its limbs, which then regenerate. from the group of leaf-footed crustaceans. These crustaceans can reach quite impressive sizes - 2–5 cm. Their body is covered with a rounded shield, from under which two tail appendages protrude. Shields are ephemeral animals. Emerging from eggs in the spring, they grow quickly, multiply, and when the puddle or pond dries out, they die. The eggs remain on the dry bottom until next spring, but remain viable in dry form for a much longer time - 6–9 years.
To be continued
Prudoviki
The pond snail (Limnaea) belongs to the molluscs (Mollusca), the class Gastropoda, the order Pulmonata, and the pond snail family (Limnaeidae).
The largest of our pond snails is the common pond snail Limnaea staghalis L. (shell height up to 55 mm, width up to 27 mm) with a highly elongated conical shell.
Eared pond snail L. auricularia L. (shell height 26 mm, width 21 mm), with a short whorl and an even more swollen last whorl, and the shell is similar to a human ear.
The marsh pond snail L. palustris Mull, (shell height 32 mm, width 10 mm) is similar to the common one, but the shell has the shape of a very sharp cone with a small hole and is dark brown in color.
Egg-shaped pond snail L. ovata. (shell height 18 mm, width 12 mm) has a short whorl and a final whorl with a wide ovoid opening.
Ponds. Everyone eats. led
1 - common pond snail; 2 - ear pond snail; 3 - marsh pond snail; 4 - pointed physis (Physa acuta); 5 - ovoid pond snail (L. ovata); 6 - key physis (Physa fontinalis).
Pond snails are very common in freshwater bodies of water. The common pond snail is especially common everywhere. To collect pond snails, there is no need to even use a net, since they stay close to the surface, and sometimes even on the very surface of the water, and can easily be removed from underwater objects by hand. Very often you have to catch empty pond snail shells.
Movements. If you put a caught pond snail in a jar of water, it slowly begins to crawl along the walls of the vessel. At the same time, a wide leg with a flat sole, used for crawling, and a head with two long triangular tentacles, at the base of which sit a pair of eyes, emerge from the hole in the shell. Sticking its sole to underwater objects, the snail smoothly glides forward. Sliding is achieved by wave-like contractions of the muscles of the sole, which is easy to observe through the glass of the vessel. It is very remarkable that snails can wander on the surface of the water, hanging from it using their soles. In this case, the animal leaves a ribbon of mucus that stretches along the surface of the water and can be detected if you pass a stick behind the crawling snail or dust the surface of the water with lycopodium (moss seed). It is assumed that snails moving in this way use the surface tension of the liquid, hanging from below to the elastic film that exists on the surface of the water due to tension.
The described crawling is easy to observe on an excursion when the surface of the reservoir is calm.
If a snail crawling in this way is slightly pushed so that it sinks into the water, you will see that the animal floats to the surface again, like a cork. This phenomenon is explained by the fact that there is air inside the respiratory cavity of the animal, which supports the snail like a swim bladder. The snail can voluntarily compress its respiratory cavity: then the mollusk becomes specifically heavier and sinks to the bottom. On the contrary, when the cavity expands, the cochlea floats to the surface in a vertical line without any push.
Try to immerse a pond snail floating on the surface of the water into the water and disturb its soft body by touching it with some kind of point (tweezers, stick, etc.). The leg will immediately retract into the shell and air bubbles will escape through the breathing hole. After this, the mollusk will sink to the bottom and will no longer be able to rise to the surface except by crawling onto aquatic plants, due to the loss of its air float.
Breath. The pond snail belongs to the pulmonary molluscs and breathes atmospheric air. The way in which he takes in air is not difficult to observe on an excursion. Rising to the surface of the water, the pond snail opens its breathing hole, which is located on the side of the body, near the edge of the shell. In a calm state, this hole is closed by the muscular edge of the mantle. The air is drawn into the vast pulmonary integrity, the walls of which are formed by the mantle, penetrated by a rich network of blood vessels. The exchange of gases occurs through the thin wall of the mantle, with the mantle cavity playing the role of a lung.
A pond snail can stay under water without refreshing the air in its lung cavity for a very long time. This is explained by the fact that the air enclosed in the pulmonary cavity is used very completely during breathing, and the oxygen in the air is gradually replaced by carbon dioxide. In addition, snails probably breathe through their skin, extracting oxygen dissolved in water.
Pond snails feed on plant foods: leaves and stems of aquatic plants on which they live. The plant tissue is scraped off using a special organ covering the tongue, aptly called a “grater”; the mouth opening of the mollusk can be clearly seen through the glass of the vessel as it crawls along the wall of the jar.
Large pond snails (Limnaea stagnalis) are extremely voracious and cause considerable damage to plants in the aquarium, which is why only small species should be planted in aquariums. Sometimes pond snails devour, in addition to parts of plants, small animals (hydra, protozoa), eat fish eggs, meat and even the corpses of fallen fish and dead snails.
Pond snails reproduce by eggs, which are laid on aquatic plants or other objects (Fig. 189). The eggs are connected by a common mucous membrane, and the entire clutch has the appearance of a transparent gelatinous sausage. Each animal lays several such clutches (up to 20) during the summer.
Clays of mollusks. Eating. led (Orig.)
1 - pond snail; 2 - coils; 3 - bithinia.
After 20 days, tiny snails emerge from the eggs and grow quite quickly, feeding on plant foods. The development of eggs is very convenient to observe in an aquarium.
Interestingly, pond snails can reproduce far from reaching their maximum growth. For example, the common pond snail becomes sexually mature at the end of the first year of its life, when it grows to only half its normal size. All pond snails are hermaphrodites. In some species, even self-fertilization has been observed (eared pond snail).
When water bodies inhabited by pond snails dry out, not all mollusks die. When the pond dries out, the large pond snail (L. stagnalis) secretes a dense film that closes the opening of the shell. Some of the most adapted forms of mollusks can survive being out of water for quite a long time. Thus, the common pond snail lives without water for up to two weeks. L. peregra Mull. - more than a month. L. truncatula Mull is particularly resilient in this regard. (Fig. 190), which sticks to the substrate with mucus, and L. palustris Mull. In one experiment, 4 specimens of the latter species remained alive after a month in a desiccator over calcium chloride.
Pond snail (Limnaea truncatula). Eating. led
When water bodies freeze, mollusks do not die, freezing into the ice, and come to life when they thaw. According to S. A. Zernov’s calculations, more than a hundred organisms are known in the Moscow vicinity that have this ability, including 5 species of mollusks.
Most pond snails are animals that are either indifferent to humans or even useful to him, such as the large pond snail and the oval pond snail, which sometimes serve as food for fish. But among pond snails there is one extremely harmful small species - L. truncatula (Fig. 190), which contributes to the development of helminthic disease in sheep and cattle, known as fascioliasis, or dropsy of the liver (popular name).
The species we are interested in is distributed throughout the country. The mollusk lays eggs (9-25 pieces) several times a year, reaches maturity in 6-7 months and lives for about two years. It lives along the banks of rivers and lakes, in swamps, in small puddles and ditches, even in road ruts filled with water (dwarf forms).
Sometimes this pond pond reproduces in large numbers: in one case, 323 specimens were found at 4 m. In another case, an average of 125 pieces were found per liter, which gives 1.25 million per 1 hectare. Thus, thousands of sheep per hectare can become infected with the fluke.
Naturally, the question arises about the fight against this harmful mollusk.
One of the remedies is not to graze animals in wet meadows and to dry out flooded pastures. Among the chemical methods of control, it is recommended to destroy the transmitter using lime milk (1000-1400 kg of lime per 1 ha), table salt, caustic potassium, copper sulfate. However, the fight against Limnaea truncatula is very difficult: chemical methods are expensive, and drying pastures does not always give positive results due to the noted ability of the mollusk to resist drying out.
Lawns and bitinia
Meadows (Viviparus viviparus L., Paludina vivipara) belong to the class of gastropods (Gastropoda), to the order Prosobranchia, to the family of meadows (Viviparidae).
Luzhanka is a large snail with a spirally curled shell, which looks like a blunt cone of yellowish-brown color (shell height 40 mm, width 30 mm). Three dark brown stripes run along the whorls of the shell. The opening of the shell can be tightly closed with a horny cap.
Lawns and bitinia. Eats Vel. 1 - true lawn (Viviparus viviparus), 2 - striped lawn (V. fasciata), 3 - tentaculata (Ltithynia tentaculata).
There are two closely related species, very similar to each other: the true meadow, characteristic of stagnant bodies of water (V. viviparus), and the striped meadow (V. contectus Millet), found in running waters. The latter species is somewhat smaller than the first and has a more blunt apex of the shell and the shell opening is pointed upward. In addition, there are also various local varieties, which we will not dwell on.
A representative of the close family Hydrobndae is somewhat similar to the lawn - Bithynia tentaculata L., a small snail (shell height 10 mm, width 6 mm) with a conical shell equipped with a calcareous cap.
Both lawns and bitinia are common inhabitants of our reservoirs and are often found in abundance. Luzhanki usually live in reservoirs with a muddy bottom, sometimes completely dotting it. When fishing for meadows, you should run the net along the very bottom of the reservoir where the animals are kept. Due to the presence of the operculum, it is difficult to confuse the meadowsweet and bitinia with other similar mollusks, and even the most inexperienced excursionist will distinguish them at first sight.
Movements. Meadows caught from the water lie motionless with their lids closed. But, being lowered into the water, the snails after a while open their lids and push their dark-colored body, dotted with small yellow specks, out of the shell. The head of the meadow is extended in front into a short proboscis, on which the mouth is located. On the head there is a pair of thin tentacles, at the outer base of which an eye is placed. The wide flat leg gives the snail the ability to slowly crawl over underwater objects.
Both the meadow and the bitinia always stay at the bottom of the reservoir and do not float to the surface, like pond snails and spools. When in danger, they close the shell with a lid, which serves as an excellent protective shield for these snails.
Breath. Unlike the pond snail and the reel snail, the lawn snail and the bithinia belong to the gill snails, which extract oxygen from the water using a gill apparatus hidden under the shell. The lawnfish has a well-developed comb-like gill with numerous branchial projections that somewhat resemble the gills of fish. Thanks to water respiration, lawns and bitinias are very sensitive to water quality and, under unfavorable conditions, die much sooner than pond snails and coils.
Snails feed on various plant debris found at the bottom of reservoirs. Bitinia readily eats the green coating of algae on underwater objects.
The propagation of lawn grass is of considerable interest. Its Latin name Viviparus - viviparus - indicates that it gives birth to live young, carrying eggs and young in its body, which distinguishes it from other freshwater gastropods. Young meadows are not similar to adults not only in their size, but also in the shape of their shell. The latter appears to be faceted and covered with hard bristles, which subsequently fall off. Unlike pond snails and reels, lawns are dioecious. Having found a mature female, you can immediately open her shell during an excursion and, breaking open the turns of the spiral, show young, unborn meadows at various stages of development.
Unlike the lawn, bitinia reproduces by laying eggs on aquatic plants. Its clutch has a very unique shape and is easily distinguishable from the clutches of other mollusks: it consists of hexagonal eggs, which are located in a double row and covered with a gelatinous substance, forming an elongated mucous cord (Fig. 189).
Let us finally note the often observed overgrowth of meadows with algae, which cover their shells in the form of a green coating, completely hiding their characteristic pattern. Sometimes the algae grow so abundantly that they completely cover the shell with a sort of green fluff.
A similar overgrowth of algae is characteristic, although to a lesser extent, of pulmonate mollusks, for example, squirrels.
The durability that the meadows show when the reservoir they inhabit dries out is remarkable, which is greatly facilitated by the presence of a lid. Thus, it was observed that the striped lawnfish (Viviparus contecfts) survived out of water for up to 10 months, burrowing into the ground to a depth of 15 cm.
Reels
Coils (Planorbis) belong to the class Gastropoda, to the order Pulmonata, to the family of coils (Planorbidae).
The coil can be distinguished at first glance due to its extremely characteristic shell, curled in one plane in the form of a spiral cord.
The one that most attracts the attention of excursors is the horny coil (P. corneus L.), the largest among the rest (shell diameter 30 mm, height 12 mm), reddish-brown in color. This reel is found everywhere in both pond and lake waters.
Reels. Eating. led (Orig.)
1 - horny coil (Planorbis corneus); 2 - edge coil (P. marginatus); 3 - keel coil (P. carinatus), 4 - circular coil (P. vortex); 5 - curled coil (P. contortus); 6 - smooth coil (P. glaber)
Among the medium-sized coils, we note the marginal coil (P. marginatus L.) (shell diameter 15 mm, thickness 3.5 mm), the whorls of which are equipped with a thread-like keel running in the middle of the whorl; much more common than the previous one is the keel coil (P. carinatus L.), almost the same size, in which the thread-like keel is shifted to the lower side. The circular coil (P. vortex L.) has a somewhat smaller size (shell diameter 10 mm, thickness 1 mm) of a very flat shape, with closely wound whorls, with a keel without a thread-like appendage.
Next, we note an almost black curled coil (P. contortus L.) (shell diameter 4-5 mm, thickness 1.8 mm), the turns of which are wound very closely, so that their number reaches 7-8. Almost the same size, but with a small number of rapidly growing whorls, is the shell of P. complanatus L.
The movements of the coils resemble the movements of pond snails. When crawling, snails expose their dark, soft body far from the shell and move along underwater objects using their wide, flat legs. The head has a pair of thin tentacles, at the base of which are eyes. Coils, just like pond snails, can wander along the surface of reservoirs, suspended from a film of surface tension of the liquid.
The coils breathe atmospheric air, drawing it into the pulmonary cavity formed by the walls of the mantle. The breathing hole leading into the indicated cavity opens on the side of the body, near the edge of the shell. It opens when the coil rises to the surface of the water for a supply of air. When there is a lack of air, the coil uses a special leathery outgrowth, which is placed on the body near the pulmonary opening and plays the role of a primitive gill. In addition, the coil, in all likelihood, breathes directly through the skin.
Nutrition. Coils feed on plant matter by eating plant parts that are scraped off using a grater. These snails are especially willing to eat the green coating of small algae that forms on the walls of the aquarium. From the outside, through the glass, it is not difficult to observe how the animal uses its grater, raking up plaque like a spatula. It is very possible that the coils can also feed on animal food. At least in captivity, they willingly pounce on raw meat.
Reproduction. Coils reproduce using eggs that are laid on the leaves of aquatic plants and other underwater objects. The clutch of the horny coil is constantly encountered on excursions and is so characteristic that it can be distinguished without difficulty: it looks like a flat gelatinous oval plate of yellowish or light brown color and contains several dozen round pinkish transparent eggs. After two weeks or more (depending on the water temperature), the eggs hatch into tiny snails that grow quite quickly. The eggs of spools, like other snails, are readily eaten by fish and are consumed by them in large quantities. Like the pond snail, reels are hermaphrodites.
The behavior of the coils when the reservoirs in which they are found dry out is interesting. They burrow into moist mud, like the large horn spool (P. corneus). Sometimes this coil remains on the surface of the soil, sticking its mouth to the silt if there are residual moisture in it, or it releases a dense film insoluble in water, which closes the hole of the shell. In the latter case, the body of the mollusk gradually contracts, eventually occupying a third of the shell, and the weight of the soft parts drops by 40-50%. In this state, the mollusk can survive out of water for up to three months (marginal coil P. marginatus P. planorbis).
Bibliography
B.E.Raikov, M.N.Rimsky-Korsakov. Zoological excursions. 1
Current page: 3 (book has 18 pages total) [available reading passage: 12 pages]
1. Why did annelids get their name?
2. What is the difference between annelids and roundworms?
General characteristics. Among the various worms, annelids are the most progressive group. Its representatives are predominantly free-living worms. Their body can be distinguished by a head section, a torso and a tail section. The body consists of rings - segments, the number of which varies in different species (Fig. 24). Body length from 0.5 mm to 3 m.
Annelids have bilateral symmetry. The body consists of three layers of cells and is divided by partitions along and across the body (see Fig. 155). The internal cavity of the worm is divided by partitions into separate segments. There is liquid inside. Movement is provided by bundles of circular and longitudinal muscles, as well as special paired outgrowths of the body located on the sides of each segment - parapodia(like legs), which not all annelids have.
Rice. 24. Annelids
Annelids have sense organs: sight, touch, taste, smell, hearing, balance.
Most annelids closed circulatory system, that is, blood does not pour freely into the body cavity, but moves only through the vessels. There is no heart; its function is performed by the contracting walls of blood vessels.
The digestive system includes the mouth, pharynx, esophagus, mid and hind intestines, and anus. Breathing is carried out through the moist surface of the body or with the help of gills (see Fig. 156). The excretory system is located in each segment of the worms' body. The nervous system is characterized by a cluster of nerve cells above the pharynx - peripharyngeal ring(this is the primitive brain) and ventral nerve cord with nerve branches in each segment (Fig. 25).
There are annelids that are dioecious and hermaphrodites. Reproduction is possible asexually and sexually. During asexual reproduction, the worm's body breaks up into several parts, and then each of them completes the missing head and tail sections. Sexual reproduction occurs with the participation of two individuals, even in hermaphrodites. When they come into contact, they exchange germ cells. After fertilization, the eggs enter a special formation on the body - belt, which then, like a clutch, slides off the front end of the body and remains in the soil.
Rice. 25. Longitudinal section of the body of an annelid worm
Rice. 26. Polychaete worms - polychaetes
The type of annelids is divided into several classes, among which the most significant are three: Polychaetes, Oligochaetes and Leeches.
Class Polychaetes, or Polychaetes. These worms are typically marine animals; only certain types of polychaetes live in fresh water (Fig. 26). They got their name from the numerous bristles, located on parapodia.
Lifestyle. Most polychaete annelids lead a free lifestyle. However, they are known to live in the bodies of sponges, mollusks, starfish, and fish. They are found at different depths in warm and cold waters, reaching their greatest diversity in the coastal zone of tropical seas. Many annelid polychaete worms live on the seabed in large numbers; for example, in the Barents Sea, the population density of annelids reaches 90 thousand specimens per 1 m2.
Polychaetes live among algae, reefs, sand, soft mud, some of them build horny, sandy and calcareous tubes and live in them.
Parapodia with bristles allow them to move well in water, on the surface and in the thickness of the soil, inside tubes.
Among polychaete worms there are predators that feed on crustaceans, mollusks, coelenterates and worms. There are omnivores that filter water and eat plants.
Free-living polychaetes swim throughout their lives in the water column, transported by sea currents. Bottom ringers live on the bottom of the sea and feed on the organic remains of aquatic plants and animals.
The development of polychaetes occurs with alternation of life forms. Their larvae do not resemble adults. Each life form performs different functions: reproduction, dispersal, self-preservation. Some polychaetes are observed caring for offspring for example, they guard laid eggs. The less eggs the female lays, the more active the care for the offspring is. Among polychaetes there are viviparous ones.
Parapodia. Closed circulatory system. Polychaetes. Bristles. Circumpharyngeal ring. Ventral nerve cord. Caring for offspring.
Questions
1. What are the differences in the structure of roundworms and annelids?
2. Why did polychaetes get this name?
3. What is the significance of polychaetes in nature?
4. Why do some polychaetes settle in the body of sponges, on starfish and fish?
Tasks
1. Read the last paragraph of the paragraph and, using various sources of information, name polychaete worms that play an important role in nature.
2. Suggest a version of a table that allows you to systematize the knowledge obtained about worms of all types studied.
Do you know that…
The palolo worm is one of the species of polychaetes, reaching a length of 1 m. One of the representatives of polychaete worms of the genus Nereis was specially brought to the Sea of Azov to improve the food supply of fish.
1. What do all types of worms have in common?
2. What is the significance of annelids in nature and human life?
Class Oligochaetes, or Oligochaetes
Lifestyle. Oligochaete worms, or oligochaetes, live in soil, fresh and salt waters, and are found living in two environments at once - in water and on land. Among the oligochaetes there are small (0.5 mm long) and large (up to 40 cm long) worms. Instead of parapodia, most oligochaetes have setae on their bodies. There are worms completely devoid of setae or with them arranged in 4 tufts around a segment, like an earthworm (Fig. 27).
Aquatic oligochaetes can live at the bottom of a reservoir among rotting algae or heavily polluted sewage. They are unpretentious to the oxygen content and can tolerate its complete absence for several days. The front end of the body of such worms is immersed in silt, and the rear end makes oscillatory movements, providing the body with oxygen. They feed by swallowing silt with sand, passing it through the intestines and digesting the organic substances contained in the silt. Oligochaetes are very resistant to environmental pollution.
Rice. 27. Earthworm
Rice. 28. Seta on the body of the oligochaete
Among soil oligochaetes there are species found even in the harsh conditions of Novaya Zemlya and Greenland. There are species that live at an altitude of 3–4 km above sea level and on the walls of mountain glaciers.
Worms of many soil species feed on plant debris, dragging them from the surface into soil passages. Oligochaetes can overwinter at a depth of 5–6 m or more. In soil species, in case of unfavorable conditions, diapause: the worms crawl to depth, curl up into a ball and, secreting mucus, form a protective capsule. The number of oligochaete worms in the soil can reach 2–3 tons per 1 hectare. Soil oligochaetes also include the well-known earthworm.
Oligochaetes play an important role in soil formation, decomposing organic residues. They improve the structure of the soil, the permeability of air and moisture into it, purify water from harmful impurities, and are an excellent food for other animals.
Laboratory work No. 3
External structure of an earthworm
Equipment:
Petri dish, wet filter paper, magnifying glass.
Progress:
1. Examine the body of an earthworm (Fig. 27).
Determine the body shape, color, size, and segmentation of the body.
Find the front and back ends of the body, the girdle.
Determine which body segments the girdle is located on.
2. Find the convex (dorsal) and flat (abdominal) parts of the body. Gently run your finger along the ventral or side of the worm's body from the back to the front end (you will feel the touch of the bristles). Use a magnifying glass to examine the location of the bristles on the worm's body.
3. Pay attention to the skin of the worm. Determine whether it is dry or wet. What do you think is the significance of such skin and such bristles for the life of a worm in the soil?
Leech class
Rice. 29. Leeches
Leeches live mainly in fresh water bodies, but can also live in slightly salty waters, seas, as well as in soil and on the land surface. Some leeches live in the cold waters of Chukotka, Kamchatka and Lake Baikal. Leeches that live in cold waters can enter into anabiosis- a state of the body in which life processes (for example, metabolism) are so slow that all visible manifestations of life are absent.
Leeches have a better developed nervous system than other worms. The sense organs perceive light, chemical, mechanical and other stimuli. Breathing is carried out through the entire surface of the body. When attacking a victim, leeches secrete a special substance - hirudin, preventing blood clotting both in the wound on the victim’s body and in the stomach of the leech itself. However, not all leeches have a circulatory system. In a number of species, instead, a system of tubules develops, through which nutrients and gases are transported.
Leeches are hermaphrodites. After mating, they lay numerous cocoons containing varying numbers of eggs.
Many types of leeches are very sensitive to the purity of water and die if it is polluted.
Some types of leeches are used in medicine for hypertension and the threat of hemorrhage.
Oligochaetes. Diapause. Protective capsule. Leeches. Hirudin. Anabiosis.
Questions
1. What features of annelids allowed them to populate most of the planet?
2. What adaptations do annelids have to endure unfavorable conditions? How does this happen?
3. What allows scientists to classify polychaetes, oligochaetes and leeches as one type?
4. After rain, you can observe a massive emergence of earthworms to the surface of the earth. What is the reason for this phenomenon?
5. Why can we conclude from the number of oligochaetes in a reservoir about the degree of its pollution?
6. How do oligochaetes improve the structure of the soil, the permeability of air and moisture into it, and purify water from harmful impurities?
7. Why have doctors used leeches since ancient times for hypertension and the threat of hemorrhage?
Do you know that…
In many countries around the world, earthworms are bred for zoological reclamation - improvement of crop lands, as well as for processing sewer and wastewater sludge, and for creating composts.
The largest leeches can be up to 30 cm long, and the smallest - up to 5 mm.
One medicinal leech can suck out 50 g of blood in one session. This amount is enough for her for six months. At the same time, she can go without food for 2 years.
1. How did mollusks get their name?
2. What mollusks live in your area?
General characteristics. Mollusks are soft-bodied animals that inhabit salt and fresh waters, land and have mastered various types of nutrition (filter feeders, herbivores, carnivores). There are approximately 130 thousand species. The body length of mollusks ranges from 1 mm to 20 m.
Mollusks have a muscular body, on which a head is distinguished (not in bivalves), a torso and a leg. The head contains tentacles, a mouth opening, eyes, and balance organs. The body of most species of mollusks is completely or partially covered sink, there is no segmentation. The shell consists of three layers: horny (outer), porcelain and mother-of-pearl (inner). Many shells have bizarre shapes and vary in color and structure (Fig. 30). The shell substance is secreted by cells mantles, formed by the dorsal fold of skin.
Between the mantle and the body is located mantle cavity. It contains the respiratory and chemical sense organs. The anal, genital and excretory openings open into it.
The organs of mollusks are combined into systems: respiratory, circulatory, digestive, nervous, excretory, and reproductive (Fig. 31).
Respiration in mollusks living in water is carried out gills(see Fig. 156), and in terrestrial ones - bag-shaped light. Air enters them, and blood vessels branch in the walls of the lungs. Some aquatic mollusks carry out gas exchange through the surface of the mantle.
Rice. 30. Clam shells
Circulatory system open(except for cephalopods). It includes heart(an organ that ensures the movement of blood through the vessels and cavities of the body) and vessels. The heart is made up of ventricle and one or two atria. Blood vessels pour blood into the space between the cells of the organs. The blood then collects again in the vessels and enters the gills or lungs.
The digestive system is different depending on the type of food. Filter-feeding mollusks have inlet siphon, along it, water with microorganisms suspended in it enters the mantle cavity, where this suspension is filtered and, with the help of cilia, is directed into the mouth and pharynx; then enters the esophagus, stomach, intestines and through the anus enters the outlet siphon.
In the oral cavity of mollusks (except bivalves) there is a muscular tongue with chitinous teeth, forming the so-called grater In herbivores, the grater is used to scrape off plant food; in carnivores, it helps to retain prey. From the pharynx, food passes through the esophagus into the stomach and intestines. Undigested food remains are expelled through the anus. In the body of herbivorous and predatory mollusks it acts digestive gland. Open into the oral cavity salivary glands.
The nervous system varies in complexity and is most developed in cephalopods. The least developed mollusks (bivalves) have three pairs of nerve ganglia connected to each other (see Fig. 174); eyes (a collection of light-sensitive cells) are located in various places of the body (along the edge of the mantle, in the gills, on the tentacles, at the base of the siphons), there can be up to 100 of them.
Rice. 31. Internal structure of mollusks: 1 – nerve ganglia; 2 – nerves; 3 – light; 4 – heart; 5 – blood vessels; 6 – intestine; 7 – stomach; 8 – grater
Excretory organs of mollusks – kidneys(two or one).
Among mollusks there are many dioecious species, but there are also hermaphrodites. Mollusks reproduce by laying fertilized eggs. In some representatives of the type they are glued together into mucous lumps, in others they are free of mucus.
Laboratory work No. 4
Features of the structure and life of mollusks
Equipment:
Petri dishes with shells and live mollusks.
Progress:
1. Examine the mollusk shells offered to you. Divide them into groups of gastropods and bivalves.
2. In gastropods, note: the presence or absence of symmetry; a twisted shell to the right or left. Find out if there is a difference in the number of curls, color, size, outgrowths (tubercles, rays, spines, etc.). Pay attention to the types of shellfish found in your area (name them or find out their names).
3. For bivalves, note: the difference between the inner and outer layers of the shell valves; the presence of growth rings on the outer layer; shape, size and color of shells.
4. Consider the live shellfish offered to you. Determine which classes they belong to.
Pay attention to the nature of the movement of mollusks on glass and paper. What trace remains on them? Watch the clam crawling on the glass while looking at the glass from the other side.
5. Examine the sole of the gastropod while holding it in your hands. Compare its area with the area of the sole of a freely crawling mollusk. Pay attention to the operation of the grater and the mark it leaves as the mollusk moves across the glass of an aquarium overgrown with algae. Look for shellfish moving on the surface film of water. Think about how they do it. Note whether the grater works.
6. Study the reactions of mollusks to various stimuli.
Touch the body of the mollusk with a dissecting needle, a strip of paper soaked in alcohol, and thread. What is the animal's reaction to stimuli? Take the clam in your hand. What is his reaction to your action?
7. Place two shellfish of the same type in vessels with water of different temperatures. Observe their behavior. What conclusion can you draw about the behavior of these animals?
Shellfish Sink. Mantle. Mantle cavity. Lung. Gills. Heart. Grater. Glands: digestive, salivary. Eyes. Kidneys.
Questions
1. What is the function of the shell in mollusks?
2. What does the unequal development of sensory organs in different mollusks indicate?
3. What are the structural features of mollusks compared to annelids?
4. How and why does a decrease in ambient temperature affect the life activity of gastropods?
Tasks
Using different sources of information, explain the origin of the name of the mollusk “Cuttlefish”.
1. What characteristics of mollusks served as the basis for identifying three main classes in the phylum Mollusks?
2. What is the significance of mollusks in nature and human life?
Class Gastropods(Fig. 32). This is the most numerous class of mollusks. Its representatives are found in the depths of the ocean, in the coastal zone of oceans and seas, in fresh waters, in mountains and caves, and rocky deserts. The shell of these mollusks is solid, often asymmetrical (twisted to the right or left, cone-shaped). Representatives of gastropods - slugs do not have shells. The body is divided into head, trunk and leg.
Gastropods have well-developed organs of touch and chemical sense. These mollusks are very sensitive to environmental temperature: the speed of processes in their body depends on it.
Rice. 32. Gastropods
Class Bivalves. Exclusively aquatic animals. However, some species can remain without water for a long time (Fig. 33). Body sizes and weights vary. The largest tridacna mollusk reaches up to 1.5 m in length and weighs up to 250 kg. In all bivalve shells, the shell consists of two valves, tightly connected to each other by an elastic ligament. Both halves of the shell are firmly secured by the protrusions and recesses of such a “lock”.
Rice. 33. Bivalves
A characteristic feature of the structure of bivalves is the absence of a head. Bivalves can lead both mobile and sessile lifestyles, attaching to the rocky bottom or the shells of other mollusks.
Some shellfish, for example oysters And mussels, form very large clusters, the so-called banks. This feature makes it easy to obtain them in large quantities and breed the most valuable ones.
Mollusks are quite sensitive to water salinity and other environmental factors.
The importance of bivalves is great: they are excellent water filters, food for many animals and humans, producers mother of pearl and natural pearls(spherical formation in the shells of some mollusks, mainly pearl oysters, produced as a protective reaction to irritation of the mantle by some foreign body, for example a grain of sand). Shells and pearls are used to make jewelry, buttons and other items, and are also highly collectible. Some mollusks, such as the shipworm, so named for its body shape, harm wooden structures in water.
Class Cephalopods. The most developed mollusks (Fig. 34). The body of most cephalopods lacks an external shell. The leg is transformed into tentacles (usually 8–10), i.e. legs surrounding the head (hence the name of the class).
Cephalopods are large mollusks up to 20 m long. They actively move in the water column and near the bottom. They can reach swimming speeds: squids - up to 40 km/h, octopuses - up to 15 km/h. Swimming is aided by the pulsating ejection of water from the mantle cavity - jet propulsion. When escaping from persecution, squids can fly several tens of meters above the water. Cephalopods have well-developed sensory organs. The structure of the eyes is the same as that of higher, more developed animals.
The nervous system of cephalopods is the most developed compared to all invertebrates.
Some mollusks, such as cuttlefish and octopuses, in case of danger are capable of changing color or throwing out an ink substance produced in a special organ - ink sac. Before using this remedy, the mollusk first darkens, then releases ink in a film shaped like its body. The pursuer grabs the ink “bomb” - the film breaks and the ink stains a large volume of water. In addition, they paralyze the enemy’s sense of smell and save the life of the mollusk: after releasing ink, it quickly turns pale and swims away almost invisible. The mollusk can make similar emissions 5–6 times in a row, then rests for half an hour and is ready to repeat them again.
Rice. 34. Cephalopods
Almost all cephalopods are predators. They have special adaptations for catching prey and saving them from enemies. Located on the tentacles suckers.
Among the enemies of cephalopods are fish, birds, mammals, including humans. The cephalopods themselves feed on mollusks, crustaceans, and fish. Among squids and octopuses, cannibalism is quite widespread, that is, eating individuals of their own species.
All cephalopods reproduce once in their lives, after which they die.
There are many legends and fictions about cephalopods, but there are not so many reliable facts of their attacks on people.
Gastropods. Bivalve. Cephalopods. Jet propulsion. Nacre. Ink bag. Pearl.
Questions
1. Explain how mollusks are adapted to their environment.
2. What are the similarities and differences between gastropods and bivalves?
3. How do mollusks protect themselves from enemies?
4. What allows cephalopods to move quickly in water and escape from enemies?
5. How do people use mollusk shells?
Tasks
1. Suggest options for combating gastropods - pests of garden and vegetable crops, based on the structural features and vital functions of these animals.
2. Having united in groups, prepare projects for creating cages for growing commercial bivalve mollusks in sea water, and grape snails on land.
Do you know that…
The most commercially important bivalves are edible oysters, mussels, and scallops; from gastropods - grape snail; of the cephalopods - squids.
Extinct gastropod ammonites had shells up to 2 m in diameter.
The blood of mollusks can be not only red, but also blue. It depends on which element is included in the blood cells - iron or copper.
A bright red dye, purple, was extracted from the gastropod mollusks murex: 1 g of purple was obtained from 10 thousand mollusks.
Those of you who have an aquarium at home are familiar with the large pond mollusk. Having studied this animal, you will understand why the class to which it belongs is called Gastropods, you will learn how gastropods move, what is the structure of their organ systems and how they function.
The large pond snail (Fig. 27.1) lives in rivers, lakes and ponds. Its cone-shaped shell, 4-5 cm high, is twisted in a spiral of 4-5 turns and ends with a sharp tip. The entire body of the mollusk is placed in the shell. But the pond snail can only push its head, leg and front part of the body through the hole in it (mouth); the rest of it is covered with a mantle, firmly connected to the shell.
On the head of the pond snail, two tentacles are visible, the organs of touch and smell, and next to them there is a pair of eyes. The mouth is located at the bottom of the head. It can be seen when the mollusk crawls along the glass of the aquarium, scraping off a thin coating of algae.
The large muscular leg (Fig. 27.2) of the pond snail resembles an iron. By contracting its muscles in waves, the pond snail slowly moves along the surface of stones and aquatic plants. A large pond snail can also move along the underside of the surface film of standing water in the “upside down” position. The mucus secreted by the epithelium of the leg facilitates the movement of the mollusk. You can see how the pond snail crawls using a magnifying glass at the moment when the mollusk moves along the glass wall of the aquarium.
Diversity of gastropods, their role in nature. This is the most numerous class of mollusks; they have mastered both the aquatic environment and land.
Most of them have a spiral-shaped shell. The grape snail is common in Ukraine (Fig. 27.6), and the river viviparus (Fig. 27.6), which breathes through gills, lives in water bodies. The rapana mollusk living in the Black Sea is a migrant. It came here in the 40s from the Sea of Japan on the bottoms of ships. Having not encountered any natural enemies in the Black Sea Sea, rapana “took root” and replaced the Black Sea oyster and many other mollusks. Material from the site
Among gastropods there are species that do not have a shell (Fig. 27.7). Slugs willingly settle in cabbage and strawberry beds, causing harm to these plants. In the Carpathians live slugs up to 8 cm long with dark purple, dark green and reddish colors. Many gastropods live in the tropics on land and in the sea (Fig. 27.8).
On this page there is material on the following topics:
The role of gastropods in nature and significance for humans
Mollusks gastropods large pond snail habitat and lifestyle
Report on pond snail natural history
The role of the pond snail in human life
Gastropods and their significance in nature
Questions about this material:
