Gastropods are torsion, that is, turning the internal sac by 180°. In addition, most gastropods are characterized by the presence of a turbospiral shell.
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One can note freshwater snails, which are very popular among aquarists. These snails are a decoration of the aquarium due to their exotic appearance.
Torsion
As a result of torsion, the visceral sac is rotated 180° counterclockwise. As a result, the shell curl turns out to be directed backward, and the edge of its growth and the mantle cavity are directed forward. Thus, the concha becomes endogastric. It is believed that torsion arose during the transition from a pelagic to a benthic lifestyle, since when existing in benthos, an exogastric (the curl is directed forward) pre-torsion shell is very inconvenient.
Torsion can be observed during the embryonic development of primitive gastropods such as Patella (Archigastropoda). In this case, the larva unfolds its internal sac due to muscle effort. This process is called physiological torsion. However, most modern species of gastropods, torsion is exclusively “evolutionary” and in embryonic development The internal sac is inserted already turned.
It should be noted that for the group Opistobranchia detorsion is characteristic, that is, a rotation of the internal sac by 90° clockwise.
Turbo-spiral shell and asymmetry of the internal structure
The emergence of a turbo-spiral shell is associated with the fact that this shape provides its greatest strength with equal volume. It is believed that turbospirality is a key factor in the formation of asymmetry in the internal structure of gastropods. Thus, gastropods with saucer-shaped shells (family Fissurillidae as part of Archigastropoda) the internal structure is symmetrical, except that the right kidney is larger than the left, and there is only one gonad - the right one (the latter is typical for all gastropods). When a turbo spiral occurs, the center of gravity of the shell shifts, and in order to restore its position, the shell must be shifted to the left. Such a displacement naturally causes a reduction in the right half of the internal organs, since, firstly, greater pressure is exerted on them, and, secondly, the flow of water through the mantle cavity becomes asymmetrical. So in families Haliotidae And Pleurotomariidae (Prosobranchia) the right ctenidium undergoes reduction. In families Trochidae And Turbinidae (Prosobranchia) the right ctenidium is completely absent, and the right atrium is greatly reduced and does not bear a functional load. And finally, at Caenogastropoda(rest Prosobranchia) the right ctenidium, osphradium, hypobranchial gland and atrium are completely absent. The left kidney becomes the main organ of excretion, and the right one is part of the reproductive system as the renal gonoduct (distal part of the reproductive ducts).
Nervous system
The nervous system of gastropods is of the scattered-nodular type. In most advanced representatives of this class, the nerve elements are concentrated at the anterior end of the body.
As a result of torsion in gastropods, the location of the visceral nerve trunks changes, and they form a cross - a visceral loop. As a result of this process, the initially right intestinal ganglion is located above the esophagus, and the left one is located below the esophagus. This phenomenon is called chiastoneuria.
However, in groups Opistobranchia And Pulmonata there is a return to the original structure of the nervous system: Opistobranchia due to detorsion, and Pulmonata due to the displacement of the ganglia forward.
There are 5 pairs of ganglia: cerebral (head), pedal (foot), pleural (mantle), parietal (respiratory), visceral (internal organs). Sense organs: eyes, osphradia, organs of touch, edges of the mantle.
Excretory system
Most representatives Prosobranchia - Caenogastropoda, and also for everyone Opistobranchia And Pulmonata The excretory system is represented by one left kidney. However, in primitive Prosobranchia there are two kidneys, and the right one is larger than the left one.
Respiratory system
Initially, the respiratory organs are ctenidia. Their presence is typical for everyone Prosobranchia. Ctenidia are also present in Opistobranchia, however, in this group the respiratory function can be taken over by secondary gills (order Nudibranchia). U Pulmonata ctenidium is completely reduced in the process of adaptation to life on land. Instead, a dense network of blood vessels develops in the mantle cavity. The mantle cavity itself is filled with air and has one hole connecting it with the external environment - a pneumost. Those representatives Pulmonata, which transition to life in water for the second time, breathing occurs either during periodic ascents to the surface, or (in a small number of species) the mantle cavity is filled with water, that is, it works like gills, or secondary gills appear. In addition, some Pulmonata a semblance of a tracheal system arises, that is, from the lung to all organs there are channels through which air is transported.
Reproduction
Snails usually lay their eggs in special egg capsules. These capsules have a hard outer shell. So that the larvae can get out of the capsule, there is a special cap on the capsule - by the time the offspring are ready to leave the capsule, the cap falls off or dissolves. Snails usually lay eggs in large groups - clutches. If the capsules look like small glasses on legs, then they are arranged in rows in the masonry. If the capsules are oval, then the masonry looks like a lump. It often happens that there are no eggs in the capsules located at the edges of the clutch - a predator that attacks such a clutch will gnaw through several empty capsules and leave without causing any harm to the eggs.
Pelagic larvae - veliger - hatch from the clutches of many sea snails. The veliger moves with the help of large blades or outgrowths covered with cilia. These cilia constantly vibrate, creating a flow of water, which allows the larvae to swim and also collect small particles of food. Veligers of some species can live in the water column for weeks. The veliger's sail gradually decreases, and the snail itself becomes more and more similar to an adult snail. Finally she falls to the bottom and begins to crawl.
For many snails, only a few snails (non-pelagic) mature in one clutch. The remaining eggs are needed only as food for young snails. The more trophic eggs, the larger the snails emerging from the clutches will be. There are also viviparous snails.
Classification
More than 400 modern families and about 200 extinct ones are known. In old systems, 4 subclasses of gastropods were distinguished:
- Opisthobranchia(opisthobranchs) - pteropods
- Gymnomorpha(shellless)
- Prosobranchia(prosobranchs) - limpets, livebearers, helmet snails, abalones
- Pulmonata(pulmonary) - grape snail, coils, pond snails, slugs, ambers
According to the new system (Bouchet & Rocroi, 2005), taking into account the structure of DNA modern classification lost its subclasses and units (they were replaced with treasures), and it is now like this:
- Clade Patellogastropoda
- Clade Vetigastropoda
- Clade Cocculiniformia
- Clade Neritimorpha (= Neritopsina)
- Clade Cyrtoneritimorpha
- Clade Cycloneritimorpha
- Clade Caenogastropoda
- Architaenioglossa
- Sorbeoconcha clade (example: Bithynia)
- Clade Hypsogastropoda
- Clade Littorinimorpha
- Clade Neogastropoda
- Clade Ptenoglossa
- Clade Heterobranchia
- "Lower Heterobranchia" (= Allogastropoda)
- Opisthobranchia - includes the clades Cephalaspidea, Thecosomata, Gymnosomata, Aplysiomorpha (= Anaspidea), Sacoglossa, Umbraculida, Nudipleura and the groups Acochlidiacea and Cylindrobullida.
- Pulmonata
- Eupulmonata
sea slug Elysia chlorotica assimilates algae chloroplasts Vaucheria litorea, which are capable of photosynthesizing in slug cells for several months.
The small pond snail is the main carrier of the liver fluke; because of this agriculture suffers colossal losses.
Gastropods: body structure
The body of representatives of this type is divided into the head, torso, and muscular leg. Most species have one or two pairs of tentacles on their heads. As for the body, it is asymmetrical and is completely located in the shell. The shell of gastropods is solid and forms a spiral with a different number of curls. The leg is used for movement and fixation - there is a special sole on the underside. In case of danger, the head and leg are pulled inside the shell.
The animal's mantle cavity is located in the lower whorls of the shell. The ducts of the excretory system, as well as the anus, open here. In some representatives of the class, the ducts of the reproductive system also exit here. It is interesting that part of the mantle has turned into gills, and in land dwellers the lungs are contained here.
The integument of the body is represented by epithelium, which is rich in specific glands that produce mucus.
Gastropods: internal structure
The digestive system begins with the oral cavity, the opening of which is located on the underside of the head. In the pharynx there is a specific formation called the radula, which is a chitinous grater with the help of which food is crushed and enters the esophagus. Next comes the stomach (hepatic ducts open here) and the intestines, which ends with a hole.
Well formed and nervous system, which is classified as diffuse-nodular type. Most animals have five pairs of main ganglia. It is worth noting that they have well-developed eyes and tactile receptors.
Gastropods can be either dioecious (marine species) or hermaphrodite (terrestrial and freshwater representatives). Fertilization, as a rule, occurs in the body of the female, after which she lays eggs. It is interesting that at one time the gastropod lays several eggs at once, forming a clutch.
The eggs are covered in a capsule with a small cap at the top. Sometimes unfertilized, empty capsules are attached to the outer sides of the clutch with the help of mucus. This technique is protective in nature - the predator gnaws through several empty eggs and then leaves. As for development, it can be either direct or with the formation of larvae.
Gastropods: the most popular species
The most well-known representative class is the so-called It is worth noting that large groups of these animals can harm the grape harvest. At the same time, specially grown snails are considered an expensive delicacy in some countries.
And some housewives are familiar with the slug - a small mollusk with a reduced shell that feeds on plants. This is a real pest of vegetable crops, so chemicals are used to destroy populations.
One can note freshwater snails, which are very popular among aquarists. These snails are not only capable of cleaning the walls of the aquarium from overgrown algae, but are also its decoration, having a beautiful exotic appearance.
Torsion
As a result of torsion, the visceral sac is rotated 180° counterclockwise. As a result, the shell curl turns out to be directed backward, and the edge of its growth and the mantle cavity are directed forward. Thus, the concha becomes endogastric. It is believed that torsion arose during the transition from a pelagic to a benthic lifestyle, since when existing in benthos, an exogastric (the curl is directed forward) pre-torsion shell is very inconvenient.
Torsion can be observed during the embryonic development of primitive gastropods such as Patella (Archigastropoda). In this case, the larva unfolds its internal sac due to muscle effort. This process is called physiological torsion. However, in most modern species of gastropods, torsion is exclusively “evolutionary” and in embryonic development the visceral sac is formed already rotated.
It should be noted that the Opistobranchia group is characterized by detorsion, that is, a rotation of the visceral sac by 90° clockwise.
Turbo-spiral shell and asymmetry of the internal structure
Anatomical structure of gastropods
The emergence of a turbo-spiral shell is associated with the fact that this shape provides its greatest strength with equal volume. It is believed that turbospirality is a key factor in the formation of asymmetry in the internal structure of gastropods. Thus, in gastropods with a saucer-shaped shell (family Fissurillidae within Archigastropoda), the internal structure is symmetrical, with the exception that the right kidney is larger than the left, and there is only one gonad - the right one (the latter is typical for all gastropods). When a turbo spiral occurs, the center of gravity of the shell shifts, and in order to restore its position, the shell must be shifted to the left. Such a displacement naturally causes a reduction in the right half of the internal organs, since, firstly, greater pressure is exerted on them, and, secondly, the flow of water through the mantle cavity becomes asymmetrical. Thus, in the families Haliotidae and Pleurotomariidae (Prosobranchia), the right ctenidium undergoes reduction. In the families Trochidae and Turbinidae (Prosobranchia), the right ctenidium is completely absent, and the right atrium is greatly reduced and does not bear a functional load. Finally, Caenogastropoda (the rest of the Prosobranchia) completely lack the right ctenidium, osphradium, hypobranchial gland and atrium. The left kidney becomes the main organ of excretion, and the right one is part of the reproductive system as the renal gonoduct (distal part of the reproductive ducts).
Nervous system
The nervous system of gastropods is of a scattered-nodular type. In most advanced representatives of this class, the nerve elements are concentrated at the anterior end of the body.
As a result of torsion in gastropods, the location of the visceral nerve trunks changes and they form an overlap - a visceral loop. As a result of this process, the initially right intestinal ganglion is located above the esophagus, and the left one is located below the esophagus. This phenomenon is called chiastoneuria. However, in the groups Opistobranchia and Pulmonata there is a return to the original plan of the structure of the nervous system: in Opistobranchia due to detorsion, and in Pulmonata due to a forward displacement of the ganglia. There are 5 pairs of ganglia: cerebral (head), pedal (foot), pleural (mantle), parietal (respiratory), visceral (internal organs). Sense organs: eyes, osphradia, organs of touch, edges of the mantle.
Excretory system
In most representatives of Prosobranchia - Caenogastropoda, as well as in all Opistobranchia and Pulmonata, the excretory system is represented by one left kidney. However, primitive Prosobranchia have two kidneys, the right one being larger than the left one.
Respiratory system
Initially, the respiratory organs are ctenidia. Their presence is characteristic of all Prosobranchia. Ctenidia are also present in Opistobranchia, but in this group the respiratory function can be taken over by secondary gills (order Nudibranchia). In Pulmonata, the ctenidium is completely reduced in the process of adaptation to life on land. Instead, a dense network of blood vessels develops in the mantle cavity. The mantle cavity itself is filled with air and has one hole connecting it with the external environment - a pneumostomus. In those representatives of Pulmonata that transition to life in water for the second time, breathing occurs either during periodic ascents to the surface, or (in a small number of species) the mantle cavity is filled with water, that is, it works like gills, or secondary gills appear. In addition, some Pulmonata develop a semblance of a tracheal system, that is, from the lung to all organs there are channels through which air is transported.
Classification
More than 400 modern families and about 200 extinct ones are known. In old systems, 4 subclasses of gastropods were distinguished:
- Opisthobranchia (opisthobranchia) - Pteropods
- Gymnomorpha (shellless) -
- Prosobranchia (prosobranchia) - Limiters, Livebearers, Helmet snails, Abalones
- Pulmonata (pulmonary) - Grape snail, Coils, Ponds, Slugs, Ambers
According to the new system (Bouchet & Rocroi, 2005), taking into account the structure of DNA, the modern classification has lost subclasses and orders (they are replaced by clades) and it is now as follows:
- Clade Patellogastropoda
- Clade Vetigastropoda
- Clade Cocculiniformia
- Clade Neritimorpha (= Neritopsina)
- Clade Cyrtoneritimorpha
- Clade Cycloneritimorpha
- Clade Caenogastropoda
- Architaenioglossa
- Clade Sorbeoconcha
- Clade Hypsogastropoda
- Clade Littorinimorpha
- Clade Neogastropoda
- Clade Ptenoglossa
- Clade Heterobranchia
- "Lower Heterobranchia" (= Allogastropoda)
- Opisthobranchia - includes the clades Cephalaspidea, Thecosomata, Gymnosomata, Aplysiomorpha (= Anaspidea), Sacoglossa, Umbraculida, Nudipleura and the groups Acochlidiacea and Cylindrobullida.
- Pulmonata
- Basommatophora
- Eupulmonata
The class Gastropods belongs to the phylum Mollusca and is the most numerous in this phylum. There are about 100 thousand species of gastropods. They live in the seas, fresh water bodies, and on land. Typical representatives are the large pond snail and the horn reel.
They feed on plants and organic debris. On the underside of their body they have a grater with which they scrape off the tissues of the stem and leaves.
They have a developed sole along the entire ventral side of their body, which contracts in waves and thus allows the snail to crawl.
Most gastropods have a curled shell consisting of a horn-like substance and lime. This shell serves as protection for the mollusk. In slugs, the shell is reduced and consists of remains under the skin.
The body of a gastropod can be distinguished by a head, torso and leg. The head has tentacles and eyes.
There is a fold of skin on the body of the mollusk - the mantle. The mantle secretes a special substance that causes the shell to increase in size. This is necessary as the mollusk grows.
Most aquatic gastropods have one or two gills in the mantle cavity. In coil snails, pond snails, and grape snails, the mantle cavity acts as a lung. The mantle cavity is filled with air, oxygen from which enters the blood vessels through the wall of the mantle. Carbon dioxide leaves the blood vessels.
Gastropods have a grater, which is a tongue-like outgrowth of the pharynx. The grater is covered with horny teeth. The salivary glands empty into the pharynx. There is a liver, the ducts of which open into the stomach. The intestine has a long middle and posterior sections.
The structure of the pulmonary cochlea: 1 - shells; 2 - digestive gland; 3 - light; 4 - anus; 5 - pneumostomy; 6 - eye; 7 - tentacle; 8 - brain; 9 - radula; 10 - mouth; 11 - goiter; 12 - salivary gland; 13 - gonopore; 14 - penis; 15 - vagina; 16 - mucous gland; 17 - oviduct; 18 - bag of love arrows; 19 - leg; 20 - stomach; 21 - kidney; 22 - mantle; 23 - heart; 24 - vas deferensThe circulatory system is not closed. There is a heart consisting of an atrium and a ventricle. From the heart, blood flows through the blood vessels through the organs and pours into the spaces between the organs, and from there it again enters the blood vessels and returns to the heart.
Gastropods have one or two kidneys. They receive substances that are unnecessary for the body from the blood.
Gastropods are characterized by a scattered-nodular nervous system, which consists of several pairs of nerve ganglia connected by nerves. From the nodes, nerves extend to all organs.
Among gastropods there are both dioecious animals and hermaphrodites (ponds, coils, slugs). They lay eggs, from which small snails emerge that look like large ones. However, marine gastropods have a larval stage that is not like an adult, called a swallowtail.
Which live not only in reservoirs, but also on land. That is why gastropods are the most numerous class in terms of the number of species. Sizes of gastropods are relatively small - the maximum large tropical species can reach 60 cm.
A typical representative of the Gastropod class, which almost all of us have met, is Great pond snail. This gastropod lives in stagnant bodies of water, ponds, lakes, and creeks. The body of the Greater pond snail can be divided into the head, torso and leg, which occupies the entire surface of the abdomen, which gives the name to the class.
Pond snail body twisted in a spiral sink and covered mantle. Pond snail moves by stretching and contracting the leg muscles. The pond snail has a mouth at the bottom of its head, and on the sides there are tentacles, at the base of which there are eyes.
The pond snail's diet consists of plant foods. In the pharynx there is a muscular tongue, which has many teeth on the lower side. The pond snail uses these cloves to scrape the soft tissues of plants that it eats. After swallowing, food passes through the pharynx and esophagus into the stomach. Digestion of food in the pond snail's body occurs in three stages: first in stomach, then in liver and the process ends in intestines. Through anal hole undigested food remains are expelled.
Respiratory system The pond snail consists of a lung - this is the cavity of the mantle, where air enters through the breathing hole. During the process of breathing, the pond snail rises to the surface to draw air into this cavity. The surface of the lung is intertwined with many blood vessels, due to which the blood is saturated with oxygen and carbon dioxide is released.
Circulatory system of the pond snail consists of two-chamber heart(atrium and ventricle), vessels And capillaries. The circulatory system of the pond snail is not closed, since through the capillaries the blood goes directly into the cavity with internal organs. From there, saturated with carbon dioxide, it is collected in a vessel leading to the lung, where carbon dioxide is released and oxygen enters. From there, enriched (arterial) blood enters the atrium, passes into the ventricle and is released into the arteries, capillaries and further along the same circle.
Excretory system of the pond snail presented kidney, through which blood passes and is cleared of toxic metabolic products. These substances are then excreted through the excretory opening, which is located next to the anus.
Fertilization in pond snails happens by cross type, despite the fact that pond snails- hermaphrodites. They lay eggs on the surface of aquatic plants. Young animals immediately develop from the eggs; the type of development of the pond snail is direct.
Gastropods also include slugs, which get their name from the lack of a shell, which forces them to secrete huge amounts to avoid drying out. Slugs live in damp places, but on land. They feed on mushrooms, plants, for example, cabbage and other crops, which causes the harvest to spoil.
The most notable are marine gastropods, many species of which have beautiful shells used as souvenirs and also for buttons (mother-of-pearl layer). Some peoples of Asia and Africa even make money and jewelry from the shells of a small cowrie mollusk.