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Flatworms (Platyhelminthes)

Characteristics of Flatworms

Flatworms have a soft body which is bilaterally symmetrical which can from less than 1 mm to more than 30 m long. They have structures at the front end for catching prey near the only body opening which is mouth and anus. Flatworms’ organs (e.g. for osmoregulation, digestion) are composed of tissues and organised into systems, but they don’t have blood, lungs or a heart. In contrast to the Cnidaria, flatworms have three layers of tissue (= triploblastic): endoderm, mesoderm and exoderm. The nervous system of flatworms consists in the simplest form of light-sensitive eyespots connected to a cluster of nerve cells in the head (= brain) and ventral, longitudinal nerve cords, but can also be a lot more complex and similar to the nerve net of the Cnidarians. Free living flatworms can detect chemicals, food, objects and water movement with tentacles on the sides of their heads. Reproduction in flatworms can be sexually or asexually and some flatworms can even regenerate full worms out of small sections. Flatworms normally have male (= testes) and female (= ovaries) sexual organs (= hermaphrodites). Ribbons of eggs are laid in cocoons or stuck to stones.

Importance of Flatworms

Flatworms can glide over surface by sweeping cilia of which every cell possesses several through secreted mucus. Flatworms can house symbiontic algae, but most turbellarians are feeding on other animals alive or dead or dead plant matter (= detritus). Through there large surface compared to their volume they are able to exchange gases like carbon dioxide, oxygen and ammonia across the body surface. Flatworms can adapt to many conditions and are therefore able to colonise an enormous variety of habitats from bat guano to the mantle fold of limpets. Many live as parasites within other animals, often vertebrates. They can survive at temperatures from –50 to + 47ºC. Of the free living flatworms some live in moist soil, some are marine, but most live in freshwater. In contrast to the soil living flatworms which are mainly tropical, the aquatic forms are more common in temperate than in tropical waters. Of the free-living aquatic species most live on the bottom (are benthic), others swim actively or get driven within the water column. Collection is easy when baiting with meat. The more than 20 000 species are divided into three classes: Turbellaria (including brightly-coloured free living forms, commendal and parasitic forms), Trematoda (flukes) and Cestoda (tapeworms).
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Editors: Lydia King
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added: 13 June 2003
updated: 13 June 2003
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