Variety of shellfish message. Diversity, meaning and common features of mollusks - Knowledge Hypermarket. Characteristics of the digestive system
>>Diversity, importance and common features of molluscs
§ 20. Diversity, importance and common features of mollusks
The body is clearly divided into a head with eyes, tentacles and a mouth, a leg and a torso. The turtle is whole, spirally twisted 4-7 cm long.
Conditions of existence - fresh water, coastal territory.
Movement - on the ventral side due to the contraction of the leg muscles.
Nutrition - feeds on plants, small animals and animal corpses, from which layers of tissues are zishkryabyut using a radula. The intestine is bent in a loop and ends with an anus in the anterior part of the mantle.
The transport of substances is carried out by an unlocked circulatory system. Colorless blood flows through the vessels and between the cells of the parenchyma. The movement of blood is provided by the contraction of a two-chambered heart (20-30 contractions per minute).
Excretion of substances - one kidney, the duct of which opens into the mantle cavity.
Responding to stimuli - the nervous system has a well-defined main ganglion. There are eyes, an organ of balance (in the foot), organs of touch, and organs of chemical sense by which the quality of water is recognized. In case of danger, it quickly hides in the shell.
Reproduction - hermaphrodites. Fertilization is internal cross. Eggs are covered with mucus, laid on the surface of underwater plants. The larval stage continues in the egg, the young mollusk comes out in a fully formed state.
Class Bivalves (toothless).
The body is bilaterally symmetrical, distributed over the trunk and leg. The shell consists of two valves, interconnected by an elastic ligament and closing muscles, due to their contraction, the valves close quickly. In the dorsal part there are inlet and outlet siphons. Through the first, water enters the mantle cavity, through the second - it is brought out.
Conditions of existence - lives at the bottom of fresh water bodies, plunging into sand and silt.
Movement - with the help of the foot goes deep into the sand or silt and moves at a speed of 20-30 cm / hour.
Nutrition - feeds on small organisms and their remains enter the mantle cavity with water through the inlet siphon. In the mantle cavity there is a mouth with two blades, through which water with food particles enters the intestine. The intestine forms several loops and ends with an anus in the mantle cavity.
Respiration - breathes oxygen dissolved in water, which it absorbs through the gills located in the mantle cavity.
The transport of substances is an open circulatory system with a three-chambered heart.
Excretion of substances - two kidneys.
Responding to stimuli - the nervous system consists of three pairs of nerve ganglia located near the mouth, the posterior adductor muscle and in the leg. This arrangement of the nerve ganglia ensures the instantaneous closure of the valves of the toothless in case of danger. At the edges of the mantle there are tactile receptors and chemoreceptors, and at the foot - balance organs.
Class Cephalopoda.
The body is divided into head, torso and tentacles (forked leg). The body is covered with a mantle, under which, on the dorsal side, the remains of a shell are contained (only the nautilus has an external turtle). Tentacles surround the mouth. Body sizes can reach 2.5 m (giant squid).
The conditions of existence are exclusively marine animals. They swim with the help of tentacles and in a jet way - they draw water into the mantle cavity and sharply push it out through the funnel. Some squids can reach speeds of up to 40 km/h.
Food is predators. They catch prey with tentacles, kill it with a chitinous beak and eat it.
Breathing is the same as in bivalves.
Transport of substances - the circulatory system is well developed capillaries, practically closed.
Excretion of substances - kidneys.
Reaction to stimuli - differ in complex forms of behavior, capable of learning and easy formation of conditioned reflexes. The nerve ganglia merge into the brain, surrounded by a cartilaginous sheath. They have large, complex eyes. They have a subtle sense of smell. A defensive form of behavior in some is the release of an ink cloud from a special cavity when an enemy approaches.
Reproduction - dioecious animals. Fertilization is internal. Direct development - the larva has all the organs of an adult, but sometimes differs in body shape. Characteristic care of their offspring.
The type of molluscs is divided into 7 classes: shellless, monoplacophores, armored, spadefoot, bivalves, gastropods and cephalopods.
Shellless (Aplacophora) mollusks have a worm-like body up to 30 cm long, completely covered by a mantle, there is no shell. On the ventral side, they have a groove with a roller - a rudiment of the leg. There are no nephridia. This group of mollusks are hermaphrodites.
One of two subclasses - furrowed bellies mollusks - live in the seas at a depth of 15 m to 4 km. They burrow into mud or live on corals. 250–300 species.
Monoplacophora (Monoplacophora) - marine, mainly fossil forms. The head and leg can be pulled into the shell by muscles. They breathe with 5-6 pairs of feathery gills. The heart consists of 2 ventricles and 4 atria. The nervous system includes four longitudinal nerve trunks connected by a peripharyngeal ring.
The heyday of the monoplacophorans was from the Cambrian to the Devonian. So far, 1 genus with 8 species has been preserved.
The class of shellfish (Polyplacophora) includes about 1000 species of marine bottom animals found in all seas, mainly in shallow water. Shellfish live on rocks and rocks and feed on algae and detritus. Some of them are used by humans for food.
An elongated body 0.5–30 cm long is divided into a head, a trunk, and a leg, with which the shellfish stick to the substrate. The dorsal side of the body is covered with a shell consisting of eight scutes. Respiratory organs - gills, the heart consists of two atria and one ventricle. Of the sense organs, there are eyes located on the dorsal surface of the body, and organs of touch. Most shellfish are dioecious with external fertilization; develop with metamorphosis.
The body of shovel-footed (Scaphopoda) mollusks is enclosed in a tusk-like shell. Body length 0.4–25 cm. There are holes at the ends of the shell; through the front of them, spadefoot can push out the head and leg. Above the base of the head are trapping tentacles that serve to touch and capture food (mainly foraminifera). These mollusks are dioecious; fertilization is external. A floating larva emerges from the egg.
About 600 species lead a burrowing lifestyle in the seas at various depths (up to 6 km).
The shell of bivalves (Bivalvia) mollusks consists of two valves covering the body of the mollusk from the sides. From the back, the valves are interconnected by an elastic jumper - ligament, and from the inside - by muscles. The thickened dorsal edge of the valves bears protrusions that form a lock. The shell has dimensions from a few millimeters to tens of centimeters. The giant tridacna grows up to 1.5 m in length, and the mass of this animal can exceed 200 kg. Tridacna can live up to a hundred years.
Bivalve mollusks have no head - this is the result of adaptation to a sedentary lifestyle. For the same reason, the sense organs are poorly developed: there are organs of touch, balance (statocysts), chemoreceptors (osphradia on the gills). Some have eyes. On the ventral side of the body there is a leg that serves to attach to the substrate. Respiratory organs - bipinnate gills (in primitive forms) or gill plates. The heart consists of a ventricle and two atria.
The class has been known since the Cambrian. About 150 families and 20,000 species. Bivalves that live in marine and fresh waters feed on plankton and detritus by filtering water through siphons at the back of the shell. Some drill through hard rock and wood (using sharp shell teeth or by dissolving the rock with acid). shipworm damages the bottoms of ships and piers, turning long passages into them. Some bivalves (oysters, mussels, scallops) are eaten.
The shells of gastropods (Gastropoda) mollusks are twisted into a spiral and are distinguished by a wide variety of shapes. In some mollusks, the shell is immersed inside the body or is absent altogether. The head has a pair of tentacles with eyes. In the course of evolution, gastropods have lost their bilateral symmetry. In many species, the symmetrical organs located on the right side of the body were reduced. Some species have a kind of lung - a cavity filled with air or water with oxygen dissolved in it. There are both hermaphrodites and dioecious forms.
Various species of the class live on land (from alpine highlands and tundra to tropical forests and deserts) and in water. Land snails, which live for several years, endure the winter in slime-filled burrows while hibernating. Water forms crawl along the bottom; some are part of the plankton, moving with the help of a leg modified into a fin or keel. A typical freshwater representative is a pond snail. The shells of the porcelain cowrie snail were used as coins in many countries, and red and purple dyes were mined from murex. Slugs are agricultural pests. grape snail consumed by humans. About 40,000 (according to some sources, more than a hundred thousand) species are divided into three subclasses: anterior branchial, posterior branchial and pulmonary. Extinct gastropods are known from the Cambrian or even the Proterozoic; 15,000 species.
Class cephalopods (Cephalopoda) - the most highly organized group of molluscs. The head is clearly defined. Part of the leg has evolved into 8 or 10 tentacles ("arms") surrounding the mouth. At the end of the tentacles, with which the animal grabs its prey, there are suckers, often equipped with horn hooks. In the mouth are powerful horny jaws resembling the beak of a parrot. With its help, cephalopods tear food, and the teeth of the radula grind it into a pulp. The fact is that the brain of these mollusks surrounds the esophagus from all sides, not allowing them to swallow large pieces of food.
The remains of the shell are sometimes preserved under the skin in the form of a horny plate; the outer shell was mainly in extinct forms. The only modern cephalopods that still retain an outer spiral shell are nautiluses. The circulatory system is well developed; blood has a blue color due to hemocyanin, which is part of red blood cells. Cephalopods breathe with gills, some are capable of a long stay on land (several hours or even days) thanks to the water stored in the mantle cavity.
At the entrance to the mantle cavity there is a funnel (siphon), which is the second part of the modified leg. Due to the reactive force arising from the water ejected from it backwards, the animal moves forward with the rear end of the body. Muscle contractions occur at a very high frequency, which ensures uniformity of movement. This is achieved, in particular, by the high conductivity of the nerves - in some squids, their thickness reaches 18 mm. Squids have been recorded moving at 55 km/h. Cephalopods can also swim using their tentacles to help themselves. Some squids, pushing water out of the siphon near the sea surface, can rise several meters into the air.
The organs of vision are perfect. Human-like eyes have a lens and a retina; in giant squids, their size exceeds 40 cm. There are also miniature thermolocators on the fins. The sensitive organs of smell (or taste) are concentrated on the inner surface of the tentacles and on the suckers. Developed organs correspond to a large brain.
For passive protection from enemies, autotomy is used (cephalopods “throw away” the tentacles by which they are grabbed by the enemy) and ink curtains, possibly poisonous, are sprayed to the side. In addition, special cells scattered over the skin -
Cuttlefish and squid. Top row, left to right: cuttlefish, firefly squid, giant squid, Tasmanian euprimna. Bottom row, left to right: common cuttlefish squid, flying squid, Atlantic histoeuthys, common loligo Cephalopods live in the seas (up to a depth of 5 km), preferring warm water bodies. Some forms live among the coastal rocks, others - at great depths. Some swim in the water column, others crawl along the bottom. Almost all are predators that feed on fish, crustaceans, and other mollusks; prey is caught with tentacles, killing it with the secret of poisonous glands. Many cephalopods (squid, cuttlefish, octopus) are eaten by humans. The class is divided into two subclasses: four-gill (extinct ammonites and the only surviving nautilus genus now) and two-gill (cuttlefish, squid, octopus and extinct belemnites). About 600 modern species.
The grape snail has value as a food product and is eaten boiled in a number of countries. She finds special demand in Spain, France and Italy. With mass reproduction, it can be harmful to fruit farms. grape snail
Naked slug Slugs do not have a shell, and the body is covered with thick mucus that prevents it from drying out. They prefer damp places and are nocturnal. Field slugs eat winter plants, cabbage and other vegetables, causing harm to agriculture. The slugs themselves are a source of food for toads and moles.
The Cone family includes mollusks with very beautiful cone-shaped shells. They belong to the group of predators. They live in shallow waters, among corals and algae. They feed on worms, molluscs, and fish. They hunt with the secret of poisonous glands. Cones Cones are very dangerous for humans, their attacks are often fatal.
Scallop Scallops are constantly moving from place to place. They move with the help of water, which is first taken into the shell, and then thrown out with force. The scallop has become a symbol of travelers. In the Middle Ages, pilgrims wore scallop shells on their hats to show that they had come a long way by sea.
They reach 1.5 meters in diameter and weigh more than 300 kg. In the old days, among pearl divers, the name “killer mollusk” stuck to the tridacian. If the diver's leg or hand accidentally gets between the valves, the mollusk will quickly close the shell and the person may die. Tridacna Tridacna is a real giant among bivalves.
The valves of the shells located in the anterior part of the body are dotted with rows of sharp and hard teeth. Moving the valves, the mollusk works them like a grater, rubbing the passage in the wood. The body of this bivalve mollusk is strongly elongated (up to almost two meters), but the shell is quite small (shorter than the body at a time). Shipworm - teredo
Octopuses Giants weighing up to 25 kg with tentacles 3 m long and crumbs weighing only 1 g Have 8 tentacles with suction cups. The most "smart" among invertebrates. At the moment of danger, ink is thrown out, putting up a "smoke screen". They can throw back tentacles, distracting the enemy and hiding safely. They also have real poison, which is secreted by the salivary glands.
Cuttlefish The cuttlefish lives near the seabed and feeds on shrimp, detecting them with a jet of water that "blows off" the surface of the sand. In a similar way, the cuttlefish hides, stirring up the sand, which, settling, hides the animal. The mollusk is perfectly camouflaged, changing the color of the body depending on the color of the substrate.
Nautilus One of the oldest groups of animals - four-gill cephalopods. Nautiluses for their amazingly beautiful mother-of-pearl shell are called pearl boats. They swim with the shell forward, pushing themselves with jets of water released from the mantle cavity. Currently survived only in the western part of the tropical Pacific.
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