Flatworms, also called “planarians,” are found just about everywhere there is water. There are many different species of flatworms, some live here, in the Andaman Sea off the coast of Thailand.
Some flatworms can be more than 10 m long, the longest one have been 30 m. They have no body cavity and no circulatory and respiratory organs. With no restriction in shape means the worms can grow flat which gives them their name.
This flattened shape allows oxygen and nutrients to pass through by diffusion. The digestive cavity has only one opening for both the intake of nutrients and removal of undigested wastes, which means that the food cannot be processed continuously.
Flatworms are one of the simplest animal groups with a basic central nervous system and eyes that can only see the difference between light and dark. Some worms lack eyes as they are completely parasitic living in or on another animal.
They are hermaphrodite which means they “choose” their gender during mating. This procedure is quite particular as flatworms use their “penis” to fight each other. The first one to penetrate the other will inject sperm and become male. For the flatworms, this contest is serious business. Mating is a fight because the worm that assumes the female role then must expend considerable energy caring for the developing eggs.