Turbellarians: Turbellaria

Turbellarians (ter-buh-LAIR-ee-uhns) are free-living flatworms. Free-living means they are not parasites (PAIR-uh-sites), which are animals or plants that live on or in other animals or plants without helping them and usually harming them. Turbellarians have three tissue layers and bilateral symmetry (bye-LAT-er-uhl SIH-muh-tree), meaning the right and left halves of the body match each other. These animals have a complex but incomplete digestive tract, meaning they have no anus (AY-nuhs) and all waste leaves the body through the mouth. Turbellarians have a brain and nerve cords that form a ladderlike nervous system. They have numerous sense organs at the front end of the body and touch receptors all over the body, especially around the mouth, and have organs for eliminating waste and controlling the salt balance in their cells. Turbellarians have no circulatory system, a factor that restricts the size and shape of the animals. Each turbellarian makes both eggs and sperm. The outer layer of turbellarians is covered with hairlike fibers and contains mucus-secreting cells and structures that can produce mass quantities of mucus to prevent the animal from drying out. Most turbellarians have eyelike structures for detecting light. Some species have a pair of these light-detecting structures on their front end, but larger species may have numerous pairs along the body.