Phylum: Euglenophyta   Family: Euglenaceae

Strombomonas acuminata (Schmarda) Deflandre

Very similar to Euglena except they build a protective external casing called a lorica. The lorica is extremely variable and can have a pointed posterior or not. The shape also differs. Usually found in alkaline conditions

References: 
John, D.M., Whitton, B.A. & Brook, A.J. (2011 second edition) The Freshwater Algal Flora of the British Isles