Shellfish are one of the most ancient invertebrates.Characterized by the presence of the secondary cavity of the body and is difficult arranged internal organs.Many of them have calcareous shells, which is pretty well protected by their body attacks many enemies.
Habitat closes and the land, and water.Is an important link in the food chain, they eat many animals.In addition, many of them in the early stages of development are parasites of fish, often serve as reservoir hosts for many parasitic worms.In this regard, I would like to remind fans of tourism, which in any case should not drink water directly from open reservoirs.Contained in the liquid larvae opisthorchis will be happy to get into your digestive tract, from close to the liver!
It is not often recall this, but a lot of this type of species are predatory lifestyle.In doing so, they help the development of salivary glands.By the way, what is a salivary gland in clam?Under this umbrella term refers to a fairly extensive range of specific organs, which are located in the throat and mouth.They were intended for the secretion of various substances, whose characteristics can be very different from our understanding of the word "spit."
Typically, the clams have one or two pairs of glands, which in some species reach a very impressive size.Most predatory species a secret that they emit, contains from 2.18 to 4.25% of chemically pure sulfuric acid.It helps both to fend off predators and prey on their relatives (sulfuric acid dissolves them great calcareous shells).That's what the salivary gland in mollusks.
other natural value
Many species of slugs and snail, causing extensive damage to agriculture around the world.At the same time it is shellfish play a crucial role in the global water treatment, as they used to supply filtered out of it organic.In many countries, large clams are bred in marine farms, as they are a valuable food product, which contains a lot of protein.These representatives of shellfish (mussels and oysters) are used even in dietary.
In the former Soviet Union were considered rare and endangered just 19 representatives of this ancient type.Despite the diversity of molluscs, they must be treated with care, as they are crucial for the proper functioning of many natural habitats.
Generally, shellfish are often different and the important practical value for human beings.For example, pearl massively diluted in many coastal countries, as this kind of is a supplier of natural pearls.Some mollusks are of great value for medicine, chemical and process industries.
Want to know interesting facts about shellfish?In ancient times and the Middle Ages, sometimes inconspicuous cephalopods were the basis of the welfare of entire nations, as are extracted valuable purple, which were stained with royal robes and vestments know!
type of shellfish
total it has more than 130 000 species (yes, unbelievable diversity of mollusks).Clams on the total number surpassed only by arthropods, are the second most common living organisms on the planet.Most of them live in the water, and only a relatively small number of species have chosen their place of residence land.
General characteristics
Almost all animals that are part of this type, characterized by several specific features.Here are adopted today a general characteristic of shellfish:
- First, a three-layer.Their system bodies formed from the ectoderm, endoderm and mesoderm.
- bilateral symmetry of the type caused significant displacement of much of their bodies.
- body of non-segmented, in most cases, relatively secure solid limestone sink.
- There skinfold (mantle), which envelops all of their body.
- for movement is well-defined muscular outgrowth (leg).
- coelomic cavity very poorly expressed.
- There are almost all the same organ system (in a simplified version, of course), as well as in higher animals.
Thus, the overall response of shellfish indicates that in front of us - quite developed, but still primitive animals.No wonder that many scientists believe molluscs main ancestors of a large number of living organisms on our planet.For clarity, we present a table in which more detail painted the most common characteristics of the two classes.
| treat symptoms | Classes shellfish | |
| Bivalve | Snails | |
| type of symmetry | bilateral. | symmetry is missing, some bodies are completely reduced. |
| presence or absence of head | completely atrophied, as well as all organ systems, which historically belonged to her. | There is, as the whole set of organs (mouth, eyes). |
| Respiratory | gills. | gills or lungs (truncatula, for example). |
| type shell | Vertically. | -piece can be twisted in different directions (mollusks, ampulyarii), or in a spiral (coil lake). |
| sexual dimorphism, reproductive system | dioecious, males are often smaller. | Hermaphrodites, sometimes dioecious.Dimorphism is weak. |
| type of food | Passive (water filtration).Generally, these mollusks naturally contributes to the excellent water purification have been filtered therefrom ton of organic impurities. | active, there are predatory species (Cones (lat. Conidae)). |
| Habitat | sea and fresh water. | all types of reservoirs.There are also terrestrial mollusks (snail). |
Detailed specification
have gastropods body is still symmetrical, although bivalve species not observed.Dividing the body into segments preserved only in a very primitive species.The secondary body cavity presented a bag surrounding the heart muscle and sexual organs.All the space is completely filled between the parenchyma.
The body of most species of mollusks can be divided into the following departments:
- head.
- torso.
- muscular foot, by means of which the movement.
All bivalve species the head is completely reduced.Under the foot is meant massive muscular arm that develops from the base of the abdominal wall.Near the base of the body forms a large skin fold mantle.Between it and the body there is a fairly large cavity, which contains the following organs: the gills, and the conclusions of the sexual and excretory systems.That mantle secretes substances that reacts with water to form a solid shell.
sink can be a totally solid and consist of two flaps or more plates.The composition of this envelope includes a lot of carbon dioxide (of course, in the bound state - CaCO3) and conchiolin, special organic substance which is synthesized by the body of the mollusk.However, many species of mollusks shell is completely or partially reduced.In slugs from her was only microscopic size plate.
Feature digestive system
Gastropods
At the front end of the head has a mouth.The main body of it is a powerful, muscular language, which is covered with a particularly strong chitinous float (radula).With the help of snails scrape the algae bloom or other organic matter from all available surfaces.In predatory species (of which we discuss below) language was reborn in flexible and rigid proboscis, which is intended for opening the shells of other mollusks.
have Cones (of which also will be discussed separately) individual segments radular extend beyond the mouth and form a kind of harpoon.With their help, these representatives of the shellfish literally hurl their venom into the victim.Some predatory gastropods language has become a special "drill", which they just drilled in the shell of its prey hole injection of poison.
Bivalve
In their case, everything is much easier.They are simply fixed to the bottom (or hang firmly attached to the substrate), through the body profiltrovyvaya hundreds of liters of water with dissolved organics.The filtered particles are brought directly into a bulky stomach.
Respiratory
Most species breathe with gills.There is a "front" and "rear" types.At first gills are situated in front of the body and the tip is directed forwards.Accordingly, in the second case the tip looks back.Some nudibranchs have lost their gills in the literal sense of the word.These large clams breathe directly through the skin.
For this they have developed a special adaptive body skin type.In terrestrial species and secondary water mollusks (their ancestors went back into the water) of the mantle is wrapped to form a kind of light, the walls of which is densely penetrated by blood vessels.To breathe such snails rise to the surface of the water and gaining air supply with a special spiracles.Heart, located not far from the respiratory system, just "design", consists of one atrium and ventricle.
main classes that make up the type
How divided type of shellfish?Classes of mollusks (a total of eight pieces), "crowned" the three most numerous:
- Snails (Gastropoda).This includes thousands of species of snails of all sizes, the main feature of which is the low speed of movement and well-developed muscular foot.
- Bivalves (Bivalvia).The shell of two valves.As a rule, all members of the class of species are sedentary, inactive.Move can be like using a muscular foot and through the jet thrust, throwing water under pressure.
- Cephalopod (Cephalopoda).Mobile clams, scallops or denied altogether, or it is in its infancy.
Who else is part of the type of shellfish?Classes clams are quite diverse: in addition to all the above, there are still tusk shell, Tubular and Yamkohvostye, Borozdchatobryuhie and monoplacophora.All of them are now living and thriving.
What type of fossil contains shellfish?Classes of mollusks, which have already become extinct:
- Rostroconchia.
- tentaculites.
incidentally, the same monoplacophora until 1952 were considered completely extinct, but at the time the ship "Galatea" a research expedition on board the catch of several new organisms that have been assigned to a new species Neopilina galatheae.As you can see, the name of this species of mollusks were given on behalf of the research vessel, which they found.However, in scientific practice it is not uncommon: types more often designated in honor of the explorer who discovered them.
So it is possible that all of the following years and new research mission will be able to enrich the type of shellfish: clams classes, which are now considered extinct, it can be stored somewhere in the bottomless depths of the oceans.
Interesting facts
As strange as it may sound, one of the most dangerous and incredible predators on the planet ... apparently considered harmless gastropods.For example, snail Cones (lat. Conidae), a poison which is so unusual that it is used in the manufacture of modern pharmacists certain species of rare drugs.Incidentally, the name of the shellfish family is fully justified.Their shape and actually most like a truncated cone.
They can be persistent hunters exclusively ruthlessly cracking down on floodplain prey.Of course, as the latter often serve colonial, sedentary species, as for other snails simply can not keep up.Itself a victim of its size may exceed ten times the hunter.Want to know more interesting facts about shellfish?Yes please!
On the methods of hunting snails
most often insidious clam uses his most powerful body, strong, muscular foot.It can be attached to the production with a force equivalent to the application of a force of 20 kg!Predatory snail that is enough.For example, to "catch" the oyster opens in less than an hour with a force only ten kilograms!In short, the life of molluscs is more dangerous than is generally thought ...
Other gastropod and prefer to do nothing to push, gently prosverlivaya shell production with a special proboscis.That's just the process is simple and quick not to call at all desire.Thus, when the thickness of the shell only 0.1 mm drilling can take up to 13 hours!Yeah, this method of "hunting" is suitable only for snails ...
dissolution!
to dissolve foreign sink and most of its owner, the clam uses sulfuric acid (you already know what a salivary gland in mollusks).Since the destruction is much easier and faster.Once the hole is done, the predator begins to slowly eat away their prey of "packaging", using for this purpose its proboscis.To some extent, this body can safely be considered an analogue of our hands, because it is directly involved in the capture and hold prey.Furthermore, the arm can extend so often that exceeds the length of the body of the hunter.
That's how snails can get their prey even from deep cracks and large shells.Once again, that it is from the proboscis into the victim's body strong poison is injected, which is based on chemically pure sulfuric acid (released from the "innocent" of the salivary glands).In short, now you know exactly what the salivary gland in molluscs and why they need it.
