Planarians

Planarians, a class of worms known as the Turbellaria (q.v.). They are, as a rule, fairly small, and always consist of only a single segment. The skin or ectoderm is covered with the small vibratile processes known as cilia. The nerves consist of a pair of long cords passing back through the body from a pair of masses or ganglia at the front end of the body; in one group, the Acoela, the nerves are usually absent. There is a mouth, but no anus. They are nearly always hermaphrodite, but the male organs usually mature before the female. The class is divided into two divisions. The first, known as Rhabdocoelida, are small, and the intestine is either lobed, straight, or absent; the second, or Dendrocoelida, have a greatly branched intestine, and are large in size. The Planarians are carnivorous, and a few are parasitic, such as Anoplodium on the Sea-cucumbers. They live either in the sea, e.g. all the Polycladida and Accela, in fresh water, such as Plagiostoma. Planaria, etc., or on damp earth, such as the land Planarians, e.g. Bipalium. As they are soft-bodied, no fossil representatives are known.