Flatworms

Containing the most primitive flatworms, the class Turbellaria consists of nine orders and a total of about 3,000 species, most of which are free-living. While some species live in moist, dark areas on land, most live at the bottom of marine water. These flatworms are found in all seas. While the aquatic species seldom grow more than 0.4-0.8 in (1-2 cm) long, some land varieties can reach lengths of 19.7 in (50 cm). The aquatic species have relatively flat, leaf-shaped bodies and are usually gray, brown, or black, although some species have a green tint. The turbellaria's head possesses one or more pair of eyes and tentacles. These flatworms are covered by microscopic hairs (cilia) that they beat continuously, creating turbulence in the water-an activity that gave them their name. Their cilia are important in their locomotion; they also crawl along the ground gripping it with sticky secretions from their glands.

Turbellarians are hermaphrodites, possessing the complex reproductive apparatus of both male and female. The fertilized eggs usually produce a small worm, although sometimes larvae are produced. The majority of turbellarians are carnivores.