Bivalve mollusks (class lamellar-gill) - as a rule, inactive or immobile animals. They live at the bottom of reservoirs. Known species of mollusks today are represented in the amount of more than fifteen thousand. Most of them inhabit the seas, the smaller - fresh water.
Bivalves have a two-sided symmetrical body. It consists of a leg and a body, the head is reduced (reduced). Typically, the foot is wedge-shaped. In moving mollusks, it is able to protrude from the shell, bury itself in the ground at the bottom of the reservoir, pulling the whole animal behind it. In specimens attached to or lying on the bottom, the leg is usually reduced to a lesser or greater extent.
The cover of bivalve mollusks is rich in various glands. They secrete mucus and other substances. So, for example, in stonecutters on the veil of the legs are glands that secrete acid that can destroy lime. This allows the animal to freely penetrate the calcareous rocks. Mussels and zebra mussels have glands that secrete byssus. This substance in water hardens in the form of thin filaments, with the help of which bivalves are firmly fixed on the substrate.
The mantle cavity is bounded by two folds that hang down on the sides. Gills are located inside. The rectum, genital tract and renal ducts also open into the mantle cavity. Water penetrates through the gill siphon, and is removed through the cloacal siphon, which is formed by the rear mantle edge.
The animal shell is formed by two cusps, which are connected by an elastic ligament at the apex or by a lock - teeth, which are located along the upper edge of the cusps. Most representatives of the class of leaf have the same size. However, in some species lying at the bottom, they are different in size and shape.
The opening of the valves is due to the action of the connecting ligament, which is endowed with elastic properties. Rapprochement is provoked by the reduction of obturators. These powerful muscles bind both wings. The increase in the shell occurs due to the secretion of glands located in the underlying mantle. In winter, bivalves do not grow. As a result, annual stripes form on the wings. By their number, you can set 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. Commissures - nerve cords - bind all nodes.
The circulatory system of mollusks consists of blood vessels and the heart. 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 blood vessels that extend from the respiratory system. The heart is located on the dorsal side of the body.
In bivalves, the sense organs are rather poorly developed . Sensory cells are scattered in different parts of the body. On the gills there are organs of chemical senses - osfradia. Statocysts are located in the leg. These are the organs of balance. Some representatives of mollusks are distinguished by the presence of numerous eyes on the mantle edges.
The mouth is located above the base of the leg in the front of the body. There are two blades on the sides. Ciliated epithelium covers them - its cilia drive food particles to the mouth. The esophagus is short. It is connected to a small stomach into which the hepatic ducts open. The intestine leaves the stomach. It forms several loops, rises to the dorsal side, passes through the ventricle and pericardial sac back and ends in a cloacal siphon.
Shellfish are widely used in human activities. Many, for example, scallops, oysters, mussels, are eaten.