Freshwater and sea mollusks: external, internal structure of the sink, its biology

Freshwater and sea mollusks are one of the sections of biology. You will find a description of the structure of their sink in the article.

Content

The shell of mollusks cannot but not notice, it immediately catches the eye. During excursions, we constantly meet these shells, sometimes by masses, off the coast of ponds. They attract our attention with a variety of their forms, on which the division of soft -bodied into the main systematic units is built - into species.

Read in another article on the topic: “Viruses - biology: brief presentation, structure scheme, reproduction, types, history of the discovery of viruses”.

These living creatures are unique. The structure of the sink alone is dismantled in schools in the lessons of both zoology and biology. Children are often interested in whether mollusk can be expelled from his “house” so that the animal chosen or does a new one, as can be observed from the larvae of curses. Therefore, we immediately note that Mollusk is closely and directly related to the sink, sometimes almost the same as a person with his skin, nails and hair. Read more about the structure of shells of different freshwater mollusks described below. Read further.

Freshwater mollusks of Russia: views

Freshwater mollusks are increasingly populating aquariums in our homes. In the waters of rivers, lakes and other reservoirs in Russia, you can see the belly and double mollusks.

The large belly -legged can be attributed pondoviks, Luzhanka, Coils and Bitinium.Of the two -leaf mollusks are most common beathered and one, the size of which on average reaches 5-15 cm, as well as their smaller species: peas and balls and drissen. Here is a list of types of mollusks, which are considered the most popular:

Coils
Lujanki
Bitini
Ponds
Peas
Sharkov
Dreissen
Colds
Bezkudok, etc.

Each of these species is beautiful in its own way. Below you will find a description of the structure of the sink. Read further.

How the sinks of freshwater mollusks are formed: Biology

In very rare cases, biologists managed to observe how, for example, the ponds left their shells and crawled without them, but it always ended in the death of the animal, and, obviously, the result of some kind of peculiar painful process.

Consider the developing caviar of the prince under the magnifying glass:

Already at this stage of the development of the animal, we clearly distinguish between the growing sink.
It differs from the darker body of the embryo with its transparency and a yellowish tint.
This fact definitely tells us that the sink is not an external formation from the body outsiders, as the larvae of curses, but the animal organ, functionally associated with it.

Having broken the sink, we thereby doom the death of the mollusk itself, the tender body of which will be devoid of reliable support and protection.

The external structure of the shells of freshwater and sea mollusks

The substance of the sink, as a kind of hardened the secret of the body, is distinguished by a certain part of the skin, which grows so that surrounds the body from all sides and is called mantia or epanchi.

In its composition and structure, the sink contains three different layers:

Inner - mother -of -pearl
Medium - porcelain or lime
External - conchioline or horn

The main substance from which it is built is lime, usually a lime spat, or close to it is aragonite. To clearly see the structure, we will conduct experience:

We throw the sink of an ordinary puddle in 30% -40% hydrochloric acid solution. We will immediately see how the gas bubbles will begin to stand out mass, which in the study turns out to be carbon dioxide.
At the same time, your attention will be attracted by the fact that these bubbles are completely out of the inside of the sink, but not from its outer surface.
After a quarter of an hour, the bubbles will stop standing out, and in our jar there will remain a dark soft film that retains the shape of the sink.
This film covers the sink outside, giving it a characteristic color. She prevented the acid from acting on the outer surface. Therefore, for this experience, it is better to take a lounge shell, since the latter in this film is quite strong and continuous.
This outer layer is called the cornea and consists of a special organic matter - conchiolin. In its chemical composition, it is close to chitin - the material from which the shells of many arthropods, for example, insects are built.

Conchiolin is a very persistent substance in relation to various kinds of chemical influences and therefore performs a very important role, protecting the substance of the sink from the dissolving influence of water. If we pre -weigh the entire sink, and then the dried conchioline shell remaining after treatment with acid, then in this way we determine the percentage of organic and inorganic substances in the sink.

According to the experiments of the scientist from America, M. Grovther, the sink of an ordinary pond (Limnaea Stagnalis) at 96.96% of inorganic substance has 3.04% of organic compounds. In a bivalve -toe -tooth (Anodonta) - 91.33 parts Inorganic salts occur 8.47 organic compounds.

It should be noted: Young mollusks have a relative amount of organic matter in the sink more than in old ones. Therefore, there is a process of calcifying the skeleton with age, something in a kind of what is available in vertebrates.

In turn, you can remove conchiolin from the sink, leaving only mineral salts. The easiest way for this is to burn the sink on the flame of the alcohol. Conchiolin burns, exposing the white lime mass. This experience also indicates the organic essence of this layer. In addition, you can boil the sink in a strong solution of caustic alkali (for example, in caustic to the side). This method is longer (boiling at least 30-40 minutes), but more neat, since burning makes the lime mass of the sink fragile.

Inner structure of the sink of freshwater and sea mollusks

Considering the shells, for example, the double -barrels from the inside, we notice that they are lined with a substance, which is already different in appearance from a white, not so dense mass that lies immediately under a conchioline film. Both of these layers dissolve well in acid, which indicates their lime composition, however, they still have to be divided, since they differ from each other as a number of physical properties:

Density
Strength
A kind of color, sometimes cast in the third layer with all the shades of the rainbow

This third layer is called mother -of -pearl and consists of the finest lime plates located in layers, parallel to the surface. A similar structure explains the aforementioned flower game, which is a purely physical light phenomenon: interference of light rays. The mother of pearl is almost completely absent in the freshwater belly -legged, but lines the shells of the plate -toothed double.

To familiarize yourself with the internal structure of the sink, you need to make with the help of a regular jigsaw for sawing - a longitudinal cut. For this purpose, it is best to take a puddle sink with the most durable walls. On such a cut, you can see the internal air -bearing chambers of the sink, which, reducing the specific weight of the mollusk, play a large role in its movement.

The volume of these air spaces can be well judged by some living mollusks with a transparent shell, for example, Planorbis Vortex.

Living mollusk with a transparent shell - Planorbis Vortex

In addition, on the sawn shell of the puddle, you can see a strong spindle -shaped column (columejla), which is formed by internal flexing walls of revolutions and serves to attach the mollusk to the sink.

Now let's look at the double -barrel. In order to navigate in the position of individual parts of the sink and in their names, we take a closed sink into the hand so that the spinal side where both sash are connected is directed up and the front end of the sink forward:

The structure of the sink of the bivalves

The right -hand sash will be then right, and the left -sided sulfur - the left half of the sink.
On each of them, near the upper edge, the so -called elite, which looks like a tubercle, rises - the oldest part of the sink.

The sash are interconnected by an elastic key ligament or heavy, which so tightly attracts the spinal edges of the wings to each other. As a result of this, their abdominal edges depart from each other, and the sink is half -twisted. In order for the sink to be in a closed state of a living mollusk, the work of two strong close muscles is needed, which, being attached to the sample from the inside, contracting, close the sink. As soon as they relax, like an elastic key ligament again reveals the wings. Thanks to this mechanism, the empty shell of the deceased mollusk is always half -rotten. Thus, the spinal litigation in its work is an antagonist of the closure muscles.

Double freshwater mollusks: how do the sinks restore the peel and toothless shells?

All of the above three layers described: conchiolin, porcelain and mother -of -pearl, can easily be seen on the transverse breaks river cervical sinks (unio). It is better to smooth out and polish the stolen edge on sandpaper.

The main painting of the sink, as well as the bristles on its surface, are usually laid in the upper stratum corneum. Therefore, when it is erased, the sink acquires an elongated lime appearance of a naked porcelain layer (this is clearly visible in the photo above). The coloring substance of sharply protruding hammocks (for example, in Luzhanka) is laid, on the contrary, in a porcelain layer.

If you once collected the sinks of freshwater mollusks, then perhaps you saw among them copies of the ponds, on the sink of which it is clear that it was once broken off at the hole. Then Mollusk restored the broken share, even with a great increase. A scarred scar or a scar clearly visible on the sink, shows the border of the old fracture, sometimes very wrong. It happens to find a mollusk, who had just begun to close up the damage caused to him, and then it is clearly visible how a very thin fragile shell of a newly formed shell is located along the edges of the break.

You can do some experiments yourself, slightly breaking the edges of still growing shells and throwing them into the aquarium. Literally after 2-3 months, under favorable living conditions, you can observe the above “repair”. The broken upper parts of the shells are also closed by Mollusk, but more slowly and not always successfully. Thus, we can say that the mantle throughout its length is capable of highlighting the substance of the sink.

The age of the sink of freshwater and sea mollusks: how to determine?

The shell of the mollusk, increasing in size, grows from the edges of the hole. The top changes little, mainly only thickening. In this way, it is the oldest part of the sink. Therefore, you can often find lively specimens of the toothless, (even single -winged belly -legged, in which the tops of the shells are erased, the upper cornea layer disappears and the porcelain mass is exposed. This is a common phenomenon on the old shells, which have long been influenced by external factors.

Since the growth, for example, the double -barrels occur mainly in the summer, almost completely stopping in the winter, and lasts a mollusk relatively evenly throughout the years of life.

On their sinks, the so -called so -called annual growth ringsseparated from each other with dark sharp lines (this is evident in the figure above).
Each such dark border corresponds to the winter period when the deposition of the sink substance almost stops.
Based on the calculation of the annual growth rings, we can accurately establish the age of each shell.

This is impossible to see on the shells of the belly -legged: there all the height is almost entirely carried out in the very first year of the life of Mollusk, and therefore the layering of the substance of the sink, are of a different nature than that of the double -barrels. And the so -called revolutions of the belly shell cannot be identified with annual growth rings in shells. Nevertheless, we can still have some judgment about the age of the belly -legged:

Until Mollusk, and therefore the sink is growing, the edge of the opening of the latter is very thin and fragile.
On such an easily breaking edge, we can always recognize a young animal.
When its growth stops, the edges of the opening of the sink become more durable, massive, dull - this characterize old specimens.

Do not confuse with this a born thinness of the sink in some types of mollusks. This is already a species feature, which is also taken into account when determining. The second indicator of the age of mollusk is the number of revolutions of its sink, which for each type is more or less constantly (for example, in the Provice of an ordinary 6-7 revolutions, in a horn coil of 5-6, etc.).

If we find a sink with an insufficient number of revolutions for this type, we can talk about a young, still growing individual. But for this, you need to have experience in biology and get used to the characteristic appearance of an adult mollusk of each species. As for the size of the sink, then as an indicator of age, this sign is very unreliable, since it is the result of a number of external influences. By the end of life in recent years, growth has been significantly slowed down.

The appearance of freshwater and sea mollusks: description, photo

As already mentioned, the shape of the shells of freshwater and sea mollusks in itself is one of the most interesting objects of study. Its appearance depends on a number of animal living conditions. This is also an indicator and the result of some genetic species composition. The most primitive sink in shape is a simple cap or cone, which fully corresponds to a mantle fold, tightening like a cloak of the whole body of the animal. Approximately this species has a small shell of the river and lake cup ( Ancylus) - in the photo below.

Subsequently, this simplest form was significantly complicated by various kinds of twisting, and now we have a number of all kinds of spirally twisted cones, which were described above. Now there are thousands of types of mollusks with an interesting appearance of shells.

The meaning of such twisting can be explained by the fact that if it were not for a similar “evolution”, the entire mass of the animal organs would have been too much protruding upward, which means that the sink itself should have been too high.
Such a device would make the sink unstable and unsafe, and the animal would be worth the extraordinary works to restrain a high sink with insides on the back.
Therefore, it is quite advisable that the high and long bag of the insides “curl” and took a more compact shape of the spiral, which causes greater stability to the entire animal.

The appearance of freshwater mollusks - gill belly -legged

A characteristic feature of gill belly -legged is a special lid that serves to close the holes of the sink, when drawing into the last animal itself.
At the same time, the leg folds in half (like a penny knife), and both halves lie on each other.
Watch several times after this process at Luzhanka.
The lid appears completely independently of the sink, and rests not on the mantle, but on the leg.

The appearance of freshwater mollusks - plate

The plate -knee mantle and the sink received a very peculiar development.

The mantle fold grew on each side in the form of an abdominal blade, so that the body is concluded between these blades, in contacting the belly and in some places growing up.
In its development, the sink is also obliged to this form of the mantle, which was originally whole.
The initial singleness is visible from the fact that at the back of the plate -and -shaped larvae, one flat shell plate is first laid on their backs, which only then begs like a book, along a median spinal line, and thus gives two wings.

In the adult state, both sashes in the middle of the spinal edge are connected to each other through the so -called key ligament or grade, so that in this place they pass into each other.

In which direction is the shell of mollusks twisted?

Mollusks sink usually twisted to the right

Let's now get acquainted with the design of the shellfish sink, which is simple. In order to understand the structure of any sink, it must be taken in the hands so that its hole “looks” at you, the top would be turned upward, and the base is downward. Then, if the hole is from you on the right side, they say that the sink is curled to the right, if on the left, then to the left. At the same time, you may confuse you curled in one plane, like a round spring, a sink-sink, in which you cannot disassemble either the top or the base. We will accept them curled to the right, since they descended from their ancestors curled to the right.

Mollusks sink can also be twisted on the left side

Interesting: As a rule, a sign of the certificate of the spiral in one direction or the other is very constant, but sometimes there are times when a sink that is curled to the left, belonging to some specific species, normally curls to the right. In this case, according to the lamper, "the heart of the biologist beats stronger with joy, due to a rare find."

The top and lower edge of the opening of the sink are connected to each other, the border between which is called the seam.
The lowest speeds of the sink do not contact each other. There is a recess called, called "Navel" or "Umbo".
The lowest of the revolutions (usually the largest), which forms the hole, is called the last turnover or siphonal outgrowth.
The combination of revolutions from the upper edge of the hole to the top of the sink is called a curl.
The hole distinguishes the outer edge or outer lip and the inner edge or inner lip, sometimes there is sometimes no latter.
The height of the sink is measured from the top to the lower edge of the hole, the diameter is the distance from the middle of the outer edge of the hole to the opposite part of the last turn.
The width of the hole is measured in the widest place, the height in the highest.

The height and width of the shells of mollusks

Castle in the shells of freshwater and sea mollusks

As mentioned above, if we consider the sink of freshwater and sea mollusks from the inside, then in most forms we find devices intended for a stronger short circuit of the sink and called in their aggregate - "Castle". The principle of this “device” resembles the “castle” of the joiners, with the help of which they are fastened in the corners of the boards.

On one of the wings, special protrusions are developing - "teeth"
On the other - corresponding recesses "Tooth holes"
When closing the wings - the teeth are tightly included in the dental fossa

Short rounded teeth are called the main ones, and longer, elongated along the spinal edge - subordinate. The castle’s teeth are especially acute in young specimens, then they thicken and lose their pronounced character under the influence of subsequent sediments of the sink substance.

Prints from muscles inside on the shell of mollusks

In front and back on the inside of the shells of shells, we see two strong pressures indicating the places of attachment of closing muscles. At a certain distance from the lower edge and parallel to it, a groove stretches, noting the place of attachment of the mantle, this is the so -called mantle line. The length of the sink indicates the largest distance from the front end of the sink to the rear, and the distance from the lower edge to the upper edge, the distance between the most convex parts of both wings.

The surface of the shells of mollusks

The surface of the shell of freshwater mollusks for the most part is smooth, but sometimes, for example, in types of genus Planorbis, Pisidium, Sphaerium, has microskulture, in the form of transverse or longitudinal stripes, spikes, ribs, etc. Best of all, these morphological details are distinguishable in early spring, until the sink has yet managed to form algae.

Respiratory organs of freshwater and sea mollusks

The sink that once appeared at the mollusks laid a certain imprint on their entire body. To the group of freshwater and sea mollusks from all other animals, the group of worms is closest. Most of these representatives do not have special organs for breathing, but breathe the entire surface of their skin.

A similar method of gas exchange to the proper extent can no longer be carried out in mollusks: this is hindered by a sink, like a dense armor that clothes the whole body. In this regard, we are observing their special respiratory organs, relatively difficult to arrange, which is very rare in neighboring, lower organized groups.

So, the main purpose of the sink is to protect the soft body of the mollusk, but it also performs the supporting function of the external skeleton. Let’s try to break the sink, for example, an ordinary pond, and in our palm a rather shapeless, almost semi -liquid, easily falling body body of the mollusk, which is devoid of functionality, will be in our palm. Meanwhile, the part of the animal that protrudes from the sink - the so -called leg has a good strength, elasticity and strength, which can be easily seen by touching it with a finger. The presence of the sink determines the difference in the development of muscles in the same organism, for example, worms and mollusks.