The New Zealand flatworm, Arthiopostia triangulata. A Planarian flatworm, Planaria. These are not true annelid worms, and do not exhibit metameric segmentation.
Flatworms are dorso-ventrally flattened to varying degrees and elongate, with soft body tissues.
Flatworms range in length from a few millimetres to a massive 20 metres for the fish tapeworm. The lifestyle may be either free-living, or in the case of some orders, entirely parasitic. Structure Flatworms have a distinct posterior, anterior and head. The mouth of a flatworm is always located on the ventral surface of the body, but is not necessarily found on the animals head . Specialised cells, called Rhabdites are located in the animals epidermis, and secrete a layer of protective slime around the body.