Flatworms are flattened, leaf-like forms living in damp places on land, in freshwater streams of ponds, or along the seacoast while a variety of other species are parasitic. The free forms are usually small, barely reaching a length greater than five or seven cetimeters (2 to 3 inches). Some of the parasitic species attain the great length of six to thirteen meters (20 to 40 feet). This is a fresh water flatworm. This is an anatomy of freshwater flatworm (Planaria), showing the excretory system, with flame-cell. The alimentary canal is stippled.
Winchell, Alexander Sketches of Creation (New York, NY: Harper & Brothers, 1870)