Bivalve mollusks (classlamellar-gill) - as a rule, inactive or immobile animals. They dwell on the bottom of the reservoirs. Known species of mollusks today are represented in an amount of more than fifteen thousand. Most of them inhabit the seas, the smaller - fresh water.
Bivalve molluscs havebilaterally symmetric body. It consists of a leg and a trunk, the head is reduced (reduced). Typically, the leg is wedge shaped. In mobile shellfish, it is able to protrude from the shell, burrow into the ground at the bottom of the reservoir, pulling the whole animal behind it. In specimens attached to the bottom or lying on it, the leg is usually reduced to a lesser or greater extent.
Покров двустворчатых моллюсков богат разными glands. They excrete mucus and other substances. So, for example, in the rock-carvers on the cover of the legs are glands, which secrete acid, capable of destroying lime. This allows the animal to freely penetrate into the calcareous rocks. Mussels and Dreissena have glands that secrete the bisus. This substance in water hardens in the form of thin threads, by means of which bivalve mollusks are firmly fixed on the substrate.
The mantle cavity is limited by two folds,which hang on the sides. Inside are the gills. The rectum, the genital tract and the renal ducts also open into the mantle cavity. Water penetrates through the gill siphon, it is removed through the siphon cloacal, which is formed by the posterior mantle margin.
The shell of animals is formed by two leaves,which are connected by an elastic ligament on the top or by a lock - teeth that are located along the upper edge of the flaps. In most representatives of the class, the valves are of the same size. However, in some species lying on the bottom, they are different in size and shape.
The opening of the doors is carried out by the actionthe ligament connecting, which is endowed with elastic properties. Rapprochement is provoked by the reduction of the barriers. These powerful muscles bind both valves. The increase in the shell is due to the secretion of glands located in the underlying mantle. In winter, bivalve mollusks practically do not grow. As a result, annual leaves are formed on the leaves. By their number, you can determine the age of the animal.
The nervous system is formed of three pairs of nerve nodes. One is located in the back of the body, the second - in the leg, and the third - above the pharynx. Commissars - nervous cords - connect all the nodes.
The circulatory system of mollusks consists of vesselsand hearts. In the heart there is only one ventricle, several atria. Their number corresponds to the number of gills due to the fact that each of them is represented by the expansion of the vessels that depart from the respiratory organs. The heart is located on the dorsal side of the body.
У двустворчатых моллюсков достаточно слабо developed sense organs. Sensory cells are scattered in different parts of the body. On the gills, there are organs of chemical senses - ophradia. Statocysts are located in the leg. These are the organs of equilibrium. Some representatives of mollusks are distinguished by the presence of numerous ocelli along the mantle edges.
The mouth is located above the foot base in the frontparts of the trunk. There are two blades on the sides. They are covered by a ciliated epithelium - its cilia podgonyayut food particles to the mouth. Esophagus short. It is connected to a small stomach into which the hepatic ducts open. From the stomach goes the gut. It forms several loops, rises to the dorsal side, passes through the ventricle and pericardium bag back and ends in a cloacal siphon.
Mollusks are widely used in human economic activities. Many, for example, scallops, oysters, mussels, are used for food.
