Planarian

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A planarian (also known as a triclad; plural planarians or planaria) is a free-living flatworm of the order Tricladida. Planarians occur in marine, freshwater, and terrestrial environments, and the order is divided into the suborders Maricola, Cavernicola, and Continenticola. Planarians are bilaterian flatworms that lack a fluid-filled body cavity and have a three-branched intestine with one anterior and two posterior branches. They lack circulatory and respiratory systems, absorbing oxygen through the body wall, and move using cilia and muscular contractions. Depending on the species, planarians reproduce sexually, asexually through fission, or by both methods. Planarians are best known for their ability to regenerate missing tissues and organs, a process enabled by abundant adult stem cells called neoblasts. Neoblasts can self-renew and generate the cell types needed for regeneration and tissue maintenance. Because of these regenerative abilities and their relatively simple anatomy, planarians are widely used as model organisms in studies of regeneration, stem cells, aging, toxicology, genetics, and memory.