Mollusks or spineless - type of invertebrate animals. Their body consists of soft fabrics, protective horn educations like a sink are quite often had. About ten Molluscs classes are known.
Instruction
1. The body of a mollusk is not divided into segments, has three departments: head, leg and trunk. Some classes have no one of departments. For example, the head at Two-fold is reduced. By means of a leg the mollusks move, the trunk serves as storage of internals. On a trunk there is a specialized fold which forms the cavity tied with the external environment. In this cavity there are channels of sexual, digestive and secretory systems.
2. Nervous system of mollusks is presented by a near-pharyngeal nervous ring and four trunks. Two of them innervate a leg, other two – internals. More developed representatives of type have nerve ganglions, the most developed from them is located in the head end of a body. The blood system which is mainly not closed that is blood streams from vessels directly in a body cavity. Then it gathers back in vessels and goes to respiratory bodies. In blood of mollusks there is a substance giving it bluish color. Only Cephalopods have the closed blood system.
3. Digestive system is also not closed, the anus leaves in a cloak cavity. In a drink of the majority of spineless the vocational education for crushing of food is had. This education is covered with rigid chitin. As a secretory system – kidneys which number can vary at different classes. The respiratory system of water mollusks is presented by gills, overland have a lung. This lung develops from a mantle cavity. Mollusks can be both diclinous, and hermaphrodites.
4. Mollusks are both predatory, and herbivorous, many of them parasitize on other beings. They can eat seaweed, protests, sponges, polyps, wood. There are even mollusks capable to photosynthesis by analogy with plants. They eat seaweed, and chlorolayers of seaweed collect in fabrics of a mollusk.
5. The most numerous class of mollusks is the class Gastropoda which representatives can live practically in all environments. The most known Gastropoda are snails and slugs. Other class of mollusks – Cephalopod, predatory inhabitants of sea depths. The leg of cephalopods turned into tentacles with suckers, and the head is especially large. Very rare of them have a sink. A class Two-fold – the inactive mollusks possessing the closed sink.