Flatworm

Flatworms (Platyhelminthes, Greek "platy"': flat; "helminth": worm) are a phylum of invertebrates. They are relatively simple animals. They have soft bodies. 

With about 25,000 known species, they are the largest phylum of animals without a body cavity.  Flatworms can be found in marine, freshwater, and even damp terrestrial environments.Walker J.C. & Anderson D.T.(2001. The Platyhelminthes. In Anderson D.T. (ed) Invertebrate Zoology. Oxford University Press. pp. 58–80. ISBN 978-0-19-551368-4 

A troublesome terrestrial example is the New Zealand flatworm, Arthurdendyus triangulatus.  It is an invasive species which colonized large areas of Ireland and Scotland. It was brought there by accident in the 1960s. Since then, it has destroyed most of the indigenous earthworms.

Most free-living flatworms found in Britain are very small ranging from 5mm to less than 1 cm. They are usually black and can easily be mistaken for debris. These are mostly found in freshwater rivers or streams and are of the Turbellaria class.

Most flatworms (over 50%) are parasitic on other animals. There are four classes: 
Trematoda (Flukes): internal parasites.
Cestoda (Tapeworms): internal parasites.
Monogenea: external parasites.
and 
Turbellaria: free-living flatworms, sometimes called 'Planarians'. Planaria is actually a genus.

Flatworms are one of the invertebrate groups which are studied in school biology. The main reason for this is that they may infect humans, pets and farmyard animals. Some do enormous harm to humans and their livestock. Schistosomiasis, caused by one genus of trematodes, is the second most devastating of all human diseases caused by parasites, surpassed only by malaria.  Effective treatments are now available for these infections. Also, in first-world countries, regulations on domestic cattle prevent infected meat from reaching the human hosts.

 Related pages 
 List of animal phyla
Fasciola hepatica
Planaria

 References 

Category:Flatworms