The taxonomy, habitat and distribution of the freshwater triclad Planaria torva (Platyhelminthes: Turbellaria) in Britain

Planaria torva was described originally by O. F. Müller in 1774 under the generic name Fasciola. The synonyms arising since have been critically evaluated and the many varied morphological descriptions of this species are reviewed. The identity of this species in Britain has been confirmed by detailed examination of external and internal anatomy and it is shown that Planaria torfrida, described from Britain by Perkins (1928), is a synonym of Planaria torva. Dugesia lugubris (Schmidt, 1862) is the species most likely to be confused with Planaria torva and convenient methods of separating these species are described. The validity of all available British records has been examined and new localities described. Planaria torva occurs in England, Scotland and Ireland but has a markedly discontinuous distribution. There are real concentrations of habitats in the Edinburgh and Glasgow areas where the species also forms a larger proportion of the triclad population than usual. The species is markedly eurytopic and occurs in a wide range of lotic and lentic habitats, although most of them are calcium rich by British standards. It is not yet possible to offer a sound explanation for these facts because the ecology of Planaria torva is insufficiently known.