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Life in Freshwater

Flatworms (Turbellarians)

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Flatworms (Turbellarians)

IDENTIFICATION: As the name suggests these are very flattened organisms. They are small, typically around 5 mm. A common species like the dark Polycelis will be only 5 - 9 mm when extended. The majority are freshwater but some of the larger species are found on the seashore. The latter tend to be brightly coloured like Convoluta or the Candy-stripe Flatworm (see Freshwater varieties are drab. At the head end of the body there are two eyes. All flatworms contract to a jelly blob when taken out of water and they are not segmented like true worms. The freshwater forms are found in great abundance in streams and rivers. They also occur in static water of ponds and lakes. Marine species are less often seen but like all flatworms they can be found throughout Europe in a suitable habitat.

ECOLOGY: Flatworms have no blood transport system. To obtain oxygen and lose carbon dioxide they have to be very thin so diffusion can occur. They glide across the substrate by many cilia under the body. These beating hairs move the animal along. They can twist sideways (see image above) or contract into a blob. They are mainly carnivorous, feeding on small insect larvae and crustaceans that get caught up in their slim trails which they leave behind them. Some species extend the stomach out of the body to engulf and digest the prey. There is no anus. They are a primitive group and with great powers of regeneration. They can be cut up into several pieces and within a few weeks the pieces will regenerate. In the image above you can see that the end of one of them has a bit of a "tail" (extreme right). This is where the body is regenerating. Nitrogenous excretion is via unique flame cells which collect and channel waste out of the body.










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