Type and classes of mollusks. The general characteristic of mollusks. What is the salivary gland in shellfish?

Molluscs belong to one of the most ancient invertebrates. They differ in the presence of a secondary body cavity and rather complicated internal organs. Many of them have a calcareous shell, which quite well protects their body from the encroachments of numerous enemies.

The habitat covers both land and water. They are the most important link in the food chain, they are fed by many animals. In addition, many of them in the initial stages of development are parasites of fish, often serve as reservoir hosts for many parasitic worms. In this regard, I would like to remind the fans of tourism that in no case can you drink water directly from open water bodies. The opisthorch larvae contained in the liquid will be only glad to get into your digestive tract, where you can reach the liver!

Not so often remember this, but many species of this type lead a predatory lifestyle. In this they are helped by the developed salivary gland. By the way, what is the salivary gland in mollusks? This generalizing concept implies a rather extensive spectrum of specific organs located in the pharynx and oral cavity. They are intended for the secretion of various substances, the characteristics of which can be very different from our understanding of the word "saliva".

As a rule, mollusks have one or two pairs of such glands, which in some species reach quite impressive sizes. In most carnivorous species, the secret that they secrete contains from 2.18 to 4.25% chemically pure sulfuric acid. It helps both to fight off predators and to hunt for their relatives (sulfuric acid perfectly dissolves their calcareous shells). This is the salivary gland in mollusks.

Other natural meanings

Many of the slug types, as well as the grape snail, cause huge damage to agriculture around the world. At the same time, it is the shellfish that plays a crucial role in the worldwide purification of water, since they use organic matter filtered out of it for feeding. In many countries large bivalve molluscs are bred on marine farms, as they are a valuable food product that contains a lot of protein. These representatives of mollusks (mussels and oysters) are used even in dietary nutrition.

In the former USSR, 19 representatives of this ancient type were considered rare and disappearing. Despite the variety of mollusks, they should be treated with care, as they are extremely important for the proper functioning of many natural habitats.

In general, mollusks often differ in the most important practical value for humans. For example, pearls are widely bred in many coastal countries, as this species is a supplier of natural pearls. Some mollusks are of great value for medicine, chemical and processing industries.

Want to know interesting facts about shellfish? In the Ancient period and the Middle Ages, the inconspicuous cephalopods were at times the basis of the welfare of entire states, since they produced the most valuable purple, which was decorated with royal robes and noble attire!

Type of shellfish

In total, it has more than 130 000 species (yes, the variety of mollusks is incredible). Mollusks in total numbers are inferior only to arthropods, they are the second most abundant living organisms on the planet. Most of them live in water, and only a relatively small number of species have chosen to live their land.

general characteristics

Virtually all animals that are part of this type, differ at once with several specific features. Here is the general characteristic of mollusks:

• First of all, three layers. Their system of organs is formed from ectoderm, endoderm and mesoderm.
• Symmetry of the bilateral type, caused by a significant displacement of most of their organs.
• The body is not segmented, in most cases protected by a relatively strong calcareous shell.
• There is a skin fold (mantle) that envelops the entire body.
• For movement is a well expressed muscular growth (leg).
• The whole cavity is very poorly expressed.
• There are practically all the same systems of organs (in a simplified version, of course), as in higher animals.

Thus, the general characteristic of mollusks indicates that before us are sufficiently developed, but still primitive animals. It's no wonder that many scientists consider shellfish to be the main ancestor of a large number of living organisms on our planet. For clarity, we give a table in which the characteristics of the two most common classes are described in more detail.

Detailed characteristic

In gastropod mollusks the body is still symmetrical, although this is not observed in bivalve species. The division of the body into segments has been preserved only in very primitive species. The secondary body cavity is represented by a bag surrounding the heart muscle and the genital organs. The entire space between the organs is completely filled with the parenchyma.

Torso in most species of mollusks can be divided into the following departments:

• Head.
• Torso.
• Muscular leg, through which movement is carried out.

In all bivalve species, the head is completely reduced. A foot is a massive muscular process that develops from the base of the abdominal wall. At the very base of the body, the skin forms a large fold, a mantle. Between her and the body there is a rather large cavity in which the following organs are located: the gills, and also the conclusions of the sexual and excretory systems. It is the mantle that secrets those substances that, when reacted with water, form a solid shell.

The shell can be either completely solid, or consist of two leaves or several plates. The composition of this shell includes a lot of carbon dioxide (of course, in the bound state - CaCO ₃ ), as well as conchiolin, a special organic substance, which is synthesized by the body of a mollusc. However, in many species of shellfish the shell is completely or partially reduced. Slugs from her remained only microscopic dimensions of the plate.

Characteristics of the digestive system

Gastropod molluscs

At the front end of the head there is a mouth. The main body in it is a powerful muscular tongue, which is covered with a particularly strong chitinous grater (radula). With its help, snails scrape off the incrustation of algae or other organic matter from all available surfaces. In predatory species (we will talk about them below), the tongue has degenerated into a flexible and rigid proboscis, which is intended for opening shells of other mollusks.

The cones (they will also be discussed separately) separate segments of the radula extend beyond the oral cavity and form a kind of harpoon. With their help, these representatives of mollusks literally throw their poison into a victim. In some predatory gastropods, the tongue has turned into a special "drill", by which they literally drill holes in the sink of their prey to inject poison.

Bivalve

In their case, everything is much simpler. They simply lay motionlessly on the bottom (or hang, firmly attached to the substrate), filtering through their body hundreds of liters of water with dissolved organic matter in it. The filtered particles enter directly into the bulky stomach.

Respiratory system

Most of the species breathe with gills. There are "front" and "rear" views. The first gills are located in the anterior part of the body and their apex is directed forward. Accordingly, in the second case, the top looks back. Some nudibranchs have lost gills in the direct sense of the word. These large mollusks breathe right through the skin.

To do this, they developed a special cutaneous organ of the adaptive type. In land species and secondary aquatic mollusks (their ancestors returned to water again), part of the mantle is wrapped, forming a kind of lung, the walls of which are densely permeated with blood vessels. To breathe, such snails rise to the surface of the water and gain a supply of air with a special spiracle. The heart, located near the respiratory organs, the simplest "construction", consists of one atrium and a ventricle.

The main classes that make up the type

How is the type of shellfish divided? The classes of mollusks (eight in all) are "crowned" with the three most numerous:

• Gastropoda. This includes thousands of species of snails of all sizes, the main distinguishing feature of which is the low speed of movement and well-developed muscular leg.
• Bivalvia. Sink from two leaves. As a rule, all species entering the class are sedentary, inactive. Moving can both with the help of a muscular leg, and through jet traction, throwing out water under pressure.
• Cephalopoda. Movable mollusks, shells or deprived at all, or it is in its infancy.

Who else is in the clam? Clam classes are quite diverse: in addition to all the above, there are also Lopatopods, Scabies and Yawns, Boraceae and Monoplacophores. All of them relate to the living and the living.

What kind of fossils does the type of mollusks contain? Clam classes that have already become extinct:

• Rostroconchia.
• Tentaculites.

By the way, the same Monoplakophores were considered completely extinct until 1952, but at that time the ship "Galatea" with the research expedition on board caught several new organisms that were assigned to a new species of Neopilina galatheae. As you can see, the name of the mollusks of this species was given by the name of the research vessel that discovered them. However, in scientific practice it is not uncommon: species are more often indicated in honor of the researcher who discovered them.

So it is possible that all subsequent years and new research missions will be able to enrich the type of mollusks: the classes of mollusks that are now considered extinct may well be preserved somewhere in the bottomless depths of the world's oceans.

Interesting Facts

However strange it may sound, one of the most dangerous and incredible predators on our planet is ... seemingly innocuous gastropods. For example, snails Cones (Latin Conidae), the poison of which is so unusual that it is used by modern pharmacists in the manufacture of certain types of rare medicines. By the way, the name of the mollusks of this family is completely justified. Their shape is indeed more like a truncated cone.

They can be persistent hunters, extremely ruthlessly dealing with floodplain prey. Of course, in the role of the latter are more often colonial, inactive species of animals, because other snails simply can not keep up. The victim itself by its size can exceed the hunter in dozens of times. Want to know more interesting facts about shellfish? Yes please!

About methods of snail hunting

Most often, the insidious mollusc uses its most powerful organ, a strong muscular leg. It can be attached to mining with a force equivalent to applying a force of 20 kg! A predatory snail of this is enough. For example, the "caught" oyster is revealed in less than an hour with an effort of only ten kilograms! In a word, the life of mollusks is much more dangerous than it is customary to think about ...

Other species of gastropods also prefer not to press anything, neatly drilling a shell of prey with a special proboscis. But this process is simple and quick not to call at all desire. So, with a shell thickness of only 0.1 mm, drilling can take up to 13 hours! Yes, this way of "hunting" is suitable only for snails ...

Dissolution!

To dissolve another's shell and its owner, the mollusc uses sulfuric acid (you already know what the salivary gland is for mollusks). So the destruction goes much easier and faster. After the hole is done, the predator begins to slowly eat out its prey from the "packaging", using for this his proboscis. To some extent, this body can be safely considered an analogue of our hand, as it takes a direct part in capturing and retaining the prey. In addition, this manipulator can often be stretched so that it exceeds the length of the hunter's body.

This is how snails can get their prey even from deep cracks and large shells. Once again we recall that it is from the proboscis that a strong poison is injected into the body of the victim, the basis of which is chemically pure sulfuric acid (isolated from the "harmless" salivary glands). In a word, from now on you know exactly what the salivary gland is for mollusks and what they need it for.