A planarian is one of many flatworms of the Turbellaria class. 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.n asexual reproduction, the planarian detaches its tail end and each half regrows the lost parts by regeneration, allowing neoblasts (adult stem cells) to divide and differentiate, thus resulting in two worms. Some species of planaria are exclusively asexual, whereas some can reproduce both sexually and asexually.