Phylum: Euglenophyta   Family: Euglenaceae

Euglena mutabilis  F. Schmitz

A common species in acidic waters

References: 
John, D.M., Whitton, B.A. & Brook, A.J. (2011) The Freshwater Algal Flora of the British Isles
Pentecost (1984) Introduction to Freshwater Algae