Mollusks are one of the most ancient invertebrate animals. They are distinguished by the presence of a secondary cavity of the body and rather complicated internal organs. Many of them have a calcareous shell, which protects their body quite well from the attacks of numerous enemies.
The habitat covers both land and water. They are the most important link in the food chain, many animals feed on them. In addition, many of them at the initial stages of development are fish parasites and often serve as reservoir hosts for many parasitic worms. In this regard, I would like to remind tourism lovers that in no case should you drink water directly from open reservoirs. The opisthorch larvae contained in the liquid will only be glad to enter your digestive tract, from where it is a stone's throw to the liver!
Not so often they recall this, but many species of this type lead a predatory lifestyle. The developed salivary gland helps them in this. By the way, what is the salivary gland in mollusks? This generalizing concept means a fairly wide range 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 very impressive sizes. In most predatory species, the secret they secrete contains from 2.18 to 4.25% chemically pure sulfuric acid. It helps both fight off predators and hunt their relatives (sulfuric acid dissolves their calcareous shells perfectly). This is what the saliva gland of mollusks is.
Other natural significance
Many types of slugs, as well as grape snails, cause enormous damage to agriculture around the world. At the same time, it is mollusks that play a crucial role in the global purification of water, since they use organics filtered from it to feed them. In many countries, large bivalve mollusks are bred on marine farms, as they are a valuable food product that contains a lot of protein. These mollusks (mussels and oysters) are used even in diet food.
In the former USSR, 19 representatives of this ancient type were immediately considered rare and endangered. Despite the variety of mollusks, they should be treated with care, since they are extremely important for the proper functioning of many natural biotopes.
In general, mollusks often differ in the most important practical value for humans. For example, pearl mussel is widely bred in many coastal countries, since 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 Antiquity and the Middle Ages, inconspicuous cephalopods were sometimes the basis of the welfare of entire states, since they extracted the most valuable purple, which painted the royal mantles and robes of the nobility!
Mollusk type
In total, it has more than 130,000 species (yes, the variety of mollusks is incredible). Mollusks are second only to arthropods in terms of total number and are the second most common living organisms on the planet. Most of them live in water, and only a relatively small number of species have chosen land as their place of residence.
general characteristics
Almost all animals that are part of this type are distinguished by several specific features. Here is the general characteristic of mollusks accepted to date:
- Firstly, three-layer. Their organ system is formed from ectoderm, endoderm and mesoderm.
- Symmetry of 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 their entire body.
- For movement is a well-defined muscular outgrowth (leg).
- The coelomic cavity is very poorly expressed.
- There are almost all the same organ systems (in a simplified version, of course), as in higher animals.
Thus, the general characteristic of mollusks indicates that we have before us quite developed, but still primitive animals. It is not surprising that many scientists consider mollusks to be the main ancestors of a large number of living organisms on our planet. For clarity, we present a table in which the characteristics of the two most common classes are described in more detail.
Characteristic features of gastropods and bivalves Feature under consideration | Clam classes |
Bivalve | Gastropods |
Symmetry type | Bilateral. | Symmetry is absent, some organs are completely reduced. |
The presence or absence of the head | Fully atrophied, like all organ systems that historically belonged to it. | There is, like the whole set of organs (oral cavity, eyes). |
Respiratory system | Gills. | Gills or lungs (pond, for example). |
Type of sink | Bivalve. | One-piece, can be twisted in different directions (ponds, ampoule) or in a spiral (lake coil). |
Sexual dimorphism, reproductive system | Dioecious, males are often smaller. | Hermaphrodites, sometimes dioecious. Dimorphism is weak. |
Type of food | Passive (water filtration). In general, these mollusks in nature contribute to excellent water purification, since tons of organic impurities are filtered out of it. | Active, predatory species are found (Cones (lat. Conidae)). |
Habitat | Sea and fresh water. | All types of ponds. Ground mollusks are also found (Grape Snail). |
Detailed feature
In gastropods, the body is still symmetrical, although this is not observed in bivalve species. The division of the body into segments was preserved only in very primitive species. The secondary body cavity is represented by a bag surrounding the heart muscle and genitals. The entire space between the organs is completely filled with parenchyma.
The body of most species of mollusks can be divided into the following departments:
- Head.
- Torso.
- The muscular leg through which movement is performed.
In all bivalve species, the head is completely reduced. By foot is meant 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 it and the body there is a rather large cavity in which the following organs are located: the gills, as well as the conclusions of the reproductive and excretory systems. It is the mantle that secrets those substances that, having reacted with water, form a durable shell.
The sink can be either completely continuous, or consist of two wings or several plates. This shell contains a lot of carbon dioxide (of course, in a bound state - CaCO
3 ), as well as conhiolin, a special organic substance that is synthesized by the body of a mollusk. However, in many species of mollusks the shell is fully or partially reduced. In slugs, only a microscopic-sized plate remained from it.
Digestive system
Gastropods
There is a mouth at the front end of the head. The main organ in it is a powerful muscular tongue, which is covered with a particularly strong chitinous grater (radula). With its help, snails scrape off a coating of algae or other organics from all accessible surfaces. In predatory species (we will talk about them below), the tongue degenerated into a flexible and hard proboscis, which is intended to open the shells of other mollusks.
In the Cones (they will also be discussed separately), individual segments of the radula extend beyond the mouth and form a kind of harpoon. With their help, these representatives of mollusks literally throw their poison into a sacrifice. Some predatory gastropods have turned their tongue into a special “drill” with which they literally drill holes in the sink of their prey for injecting poison.
Bivalve
In their case, everything is much simpler. They simply lie motionless at the bottom (or hang tightly attached to a substrate), filtering hundreds of liters of water with dissolved organic matter through their bodies. Filtered particles enter directly into the voluminous stomach.
Respiratory system
Most species breathe with gills. There are "front" and "rear" views. In the first, the gills are located in the front of the body and their tip is directed forward. Accordingly, in the second case, the top looks back. Some nudibranchs have lost their gills in the direct sense of the word. These large mollusks breathe directly through the skin.
To do this, they developed a special adaptive-type skin organ. In land species and secondary aquatic mollusks (their ancestors returned to the water again), part of the mantle is wrapped, forming a kind of lung, the walls of which are densely penetrated by blood vessels. To breathe, such snails rise to the surface of the water and gain air with the help of a special spiracle. The heart, located near the
respiratory system, the simplest "design", consists of one atrium and ventricle.
The main classes that make up the type
How is the type of mollusk divided? The mollusk classes (eight in total) are “crowned” by the three most numerous:
- Gastropods (Gastropoda). This includes thousands of species of snails of all sizes, the main distinguishing feature of which is the low speed of movement and a well-developed muscular leg.
- Bivalvia (Bivalvia). Sink of two wings. As a rule, all species included in the class are sedentary, inactive. They can move both with the help of a muscular leg, and by means of jet traction, throwing out water under pressure.
- Cephalopods (Cephalopoda). The moving mollusks, shells are either completely deprived, or it is in its infancy.
Who else is a mollusk type? Classes of mollusks are quite diverse: in addition to all of the above, there are also Shovels, Carapace and Pit Tailed, Furrow-bellied and Monoplacophores. All of them relate to the living and living.
What minerals does the type of mollusk contain? Classes of mollusks that are already extinct:
- Rostroconchia.
- Tentaculitis.
By the way, the same Monoplacophores were considered completely extinct until 1952, but at that time the Galatea vessel, with a research expedition aboard, caught several new organisms that were assigned to the new species 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 this is not uncommon: species are often designated 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 mollusk: the mollusk classes, which are now considered extinct, may well remain somewhere in the bottomless depths of the oceans.
Interesting Facts
No matter how strange it may sound, but one of the most dangerous and incredible predators on our planet are considered ... seemingly harmless gastropod mollusks. For example, snails Cones (lat. Conidae), the poison of which is so unusual that modern pharmacists use it in the manufacture of some types of rare medicines. By the way, the name of the mollusks of this family is fully justified. Their shape is, in fact, most similar to a truncated cone.
They can be persistent hunters, exceptionally ruthlessly cracking down on floodplain prey. Of course, the latter often plays the role of colonial, sedentary species of animals, since it is simply impossible to keep up with other snails. The victim itself can be tens of times larger than the hunter. Want to know more interesting facts about shellfish? Yes please!
About snail hunting methods
Most often, the insidious mollusk uses its most powerful organ, a strong muscular leg. It can attach to prey with a force equivalent to a force of 20 kg! This is enough for a predatory snail. For example, a “caught” oyster opens in less than an hour with an effort of only a dozen kilograms! In a word, the life of mollusks is much more dangerous than it is customary to think about ...
Other types of gastropods even prefer not to crush anything by carefully drilling a prey shell using a special proboscis. Here are just a process that is not simple and fast to name with all desire. So, with a shell thickness of only 0.1 mm, drilling can take up to 13 hours! Yeah, this way of "hunting" is suitable exclusively for snails ...
Dissolution!
To dissolve another shell and its owner, the mollusk uses sulfuric acid (you already know what the salivary gland of mollusks is). So destruction is much simpler and faster. After the hole has been made, the predator begins slowly to eat its prey from the “package”, using its proboscis for this. To some extent, this body can be safely considered an analogue of our hand, since it takes a direct part in the capture and retention of prey. In addition, this manipulator can often be stretched so that exceeds the length of the body of the hunter.
This is how snails can get their prey even from deep crevices and large shells. Once again, we recall that it is from the proboscis that strong poison is injected into the victim’s body, the basis of which is chemically pure sulfuric acid (secreted from the “harmless” salivary glands). In a word, from now on you know exactly what the salivary gland of mollusks is and why they need it.