This mysterious and diverse type - Mollusks

What do we know about them - about the invertebrates of the secondary-living animals, which are united in the Mollusca type? That they have a soft, boneless body, some have a calcareous shell (like a snail), and some do not (like slugs). Some of them lead a peaceful lifestyle, feeding on grass, and some of them are dangerous predators and even parasites. Someone is very slow (for example, a snail crawls in an hour only 12-20 centimeters), whereas a squid, using its "jet engine", develops a speed of up to 70 km / h. There are species that are specially bred for eating (mussels, oysters) or for the sake of pearls, sepia, purpura. And there are very poisonous mollusks capable of causing death to ten people at once.

Among this extremely diverse family there are cannibals, and even vampires. How does science subdivide this type? Molluscs are divided into three classes: bivalves, cephalopods and gastropods. But before talking about the differences between a grape snail, an oyster and cuttlefish, let's isolate what unites these different in appearance, way of life and the level of intelligence of animals. They all do not have an internal skeleton. Shells of some animals serve as their "home", a protection against overheating and predators. Their body consists of a trunk, a leg and a head. Most species have a "mantle" - a skin fold that covers the heart, liver, kidney. Their blood is yellowish, but for some cephalopods it is reddish.

The type of mollusks was extremely common in the Jurassic period, when most of the Earth's surface was covered with water. More than 130 thousand species of fossil creatures are known - almost as many as the species nowadays. And among them there are those who were considered extinct, and they are caught alive and unharmed from the depths of the waters. This happened with the "fossil" neopilinoy, which was caught in 1952 in the Pacific Ocean. Therefore, it is impossible to say with certainty which species disappeared - information about its extinction may be exaggerated. It is believed that during the Jura period, the mollusks were gigantic, and this is confirmed by archaeological finds. The largest, found in the south of the United States, a specimen of the cephalopod is a three-meter squid that lived in the sea 325 million years ago. However, in 2003 near the coast of Antarctica was caught a squid in two and a half meters long.

The class of gastropods is unique in that its representatives have mastered the land, and therefore it is the most numerous. Unlike cephalopods and bivalves, there are no giants in it. Some tropical species can reach a maximum of 60 cm, but most are small animals 8-12 cm long. It is called so because of the "leg", which in fact is not a limb, but the trunk itself. Some of the gastropods have a shell, and some are "naked." They move very slowly, cutting the muscle of the trunk-legs; Feed mainly on plant foods. The ground snails have a mouth at the bottom of the head, and at the top are sensitive tentacles with "eyes" on their tips. The internal organs are under the mantle.

What else is interesting for such a group of creatures as the Mollusca type? Some of them (bivalves) live only in water bodies. The largest animal of this class is tridakna: there are individuals weighing 300 kg. A characteristic feature of this species are extremely strong muscles that fasten the sink flaps. The largest pearl, extracted from the shell, weighed 6 kg. It is interesting that the Black Sea rapan came to the Black Sea only in the 30s of the 20th century, when it arrived with a ship from the Pacific Ocean. Aggressive gluttonous guest for incomplete 100 years reduced the population of local mussels and oysters, putting them on the verge of extinction.

It is believed that neither the gastropods nor the bivalves have intelligence. However, such a marine mollusk, like an octopus, causes involuntary respect. This representative of the cephalopods is excellent for training, recognizes the people who feed it, distinguishes a lot of shades of colors, transmits information from one individual to another and has a brain that protects the "protocerek" - a special cartilaginous shell. With all this, a huge octopus can squeeze into a slot thick with a coin! These shellfish can build dwellings: from cans or dead shells, they collect the construction even with the door opening!