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A planarian is one of many flatworms of the Turbellaria class.〔(【引用サイトリンク】title=Planarian (flatworm) – Britannica Online Encyclopedia )〕 It is also the common name for a member of the genus ''Planaria'' within the family Planariidae. Sometimes it also refers to the genus ''Dugesia''. Planaria are common to many parts of the world, living in both saltwater and freshwater ponds and rivers. Some species are terrestrial and are found under logs, in or on the soil, and on plants in humid areas. Some planarians exhibit an extraordinary ability to regenerate lost body parts. For example, a planarian split lengthwise or crosswise will regenerate into two separate individuals. Some planarian species have two eye-spots (also known as ocelli) that can detect the intensity of light, while others have several eye-spots. The eye-spots act as photoreceptors and are used to move away from light sources. Planaria have three germ layers (ectoderm, mesoderm, and endoderm), and are acoelomate (they have a very solid body with no body cavity). They have a single-opening digestive tract; in Tricladida planarians this consists of one anterior branch and two posterior branches. Planarians move by beating cilia on the ventral dermis, allowing them to glide along on a film of mucus. Some also may move by undulations of the whole body by the contractions of muscles built into the body membrane. Triclads play an important role in watercourse ecosystems and are often very important as bio-indicators. The most frequently used planarian in high school and first-year college laboratories is the brownish ''Girardia tigrina''. Other common species used are the blackish ''Planaria maculata'' and ''Girardia dorotocephala''. Recently, however, the species ''Schmidtea mediterranea'' has emerged as the species of choice for modern molecular biological and genomic research due to its diploid chromosomes and the existence of both asexual and sexual strains. Recent genetic screens utilizing double-stranded RNA technology have uncovered 240 genes that affect regeneration in ''S. mediterranea''. Many of these genes have orthologs in the human genome.〔(【引用サイトリンク】title=Regeneration in S. mediterranea )〕 ==Anatomy and physiology== The planarian has very simple organ systems. The digestive system consists of a mouth, pharynx, and a gastrovascular cavity. The mouth is located in the center of the underside of the body. Digestive enzymes are secreted from the mouth to begin external digestion. The pharynx connects the mouth to the gastrovascular cavity. This structure branches throughout the body allowing nutrients from food to reach all extremities.〔 Planaria eat living or dead small animals that they suck up with their muscular mouths. Food passes from the mouth through the pharynx into the intestines where it is digested by the cells lining the intestines. Then its nutrients diffuse to the rest of the body. Planaria receive oxygen and release carbon dioxide by diffusion. The excretory system is made of many tubes with many flame cells and excretory pores on them. Also, flame cells remove unwanted liquids from the body by passing them through ducts which lead to excretory pores, where waste is released on the dorsal surface of the planarian. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「planarian」の詳細全文を読む スポンサード リンク
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