What do we know about them - about invertebrates of celiac animals, which are united in the type of mollusks? That they have a soft, boneless body, some have a calcareous shell (like snails), and some don't (like slugs). Some of them lead a peaceful lifestyle, eating grass, and some are dangerous predators and even parasites. Someone is very slow (for example, a snail creeps in an hour of only 12-20 centimeters), while a squid, using its "jet engine", speeds up to 70 km / h. There are species that are specially bred in order to eat (mussels, oysters) or for pearls, sepia, purple. And there are very poisonous mollusks that can cause death to ten people at once.
Among this extremely motley little family there are cannibals and even vampires. How does science subdivide this type? Mollusks are divided into three classes: bivalves, cephalopods and gastropods. But before talking about the differences between a grape snail, oyster and cuttlefish, let's isolate what unites these animals that are different in appearance, lifestyle and intelligence. They all have no internal skeleton. The shells of some animals serve as their “home", protection from overheating and predators. Their body consists of a trunk, legs and head. Most species have a “mantle” - a skin fold that covers the heart, liver, and kidney. Their blood is yellowish, but in some cephalopods it is reddish.
The Mollusk type was extremely common in the Jurassic, 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 available today. Moreover, 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" neopiline, which was caught in 1952 in the Pacific Ocean. Therefore, it is impossible to say with certainty which species has disappeared - information about its extinction may be exaggerated. It is believed that during the Jurassic the mollusks were gigantic, and this is confirmed by archaeological finds. The largest instance of the cephalopod found in the south of the USA is a three-meter squid that lived in the sea 325 million years ago. However, in 2003, two and a half meters long squid was caught off the coast of Antarctica.
The class of gastropods is unique in that its representatives mastered the land, and therefore it is the most numerous. Unlike cephalopods and bivalves, it has no giants. 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 body itself. Some of the gastropods have a shell, and some are “naked”. They move very slowly, contracting the muscle of the trunk-legs; eat mainly plant foods. Ground snails have a mouth at the bottom of the head, and sensitive tentacles at the top of which there are "eyes" placed on the tips. Internal organs are under the mantle.
What else is interesting for such a group of creatures as the type of Mollusks? Some of them (bivalves) live only in reservoirs. The largest animal of this class is tridacna: individuals weighing 300 kg are found. A characteristic feature of this species is the extremely strong muscles that hold the shell flaps together. The largest pearl extracted from the shell weighed 6 kg. It is interesting that the Black Sea ryan did not reach the Black Sea until the 30s of the 20th century, arriving with a ship from the Pacific Ocean. An aggressive gluttonous guest in less than 100 years has reduced the population of local mussels and oysters, putting them on the brink of extinction.
It is believed that neither gastropods nor bivalves have intelligence. However, such a mollusk as an octopus causes involuntary respect. This representative of cephalopods lends itself perfectly to training, recognizes the people who feed it, distinguishes a lot of shades of flowers, transfers information from one individual to another and has a brain that protects the “proto-skull” - a special cartilaginous membrane. With all this, a huge octopus can squeeze into a slot the thickness of a coin! These mollusks can build homes: from cans or dead shells, they assemble the structure even with an opening door!