Looks like the stalked freshwater Vorticella (campanula) however the photo is unsharp and the animal does not look in good shape. Was it a live (unstained) sample? Could you fix it with e.g. formalin and post another picture? Best regards, Jan
Yes it's an unstained sample Dr Jan. Unfortunately I have only photographs and no sample. The photo was took over a year ago and this one was remained unidentified.
In my opinion this is not a rotifer. All sessile genisotrochids (like Collotheca or Floscularia) are relatively huge (up to 1 mm long). This 'creature' also lacks mastax or any other anatomical feature (generaly it looks "empty") upon which it could be assigned to rotifers. This is rather aforementioned Bicosoeca or Dinobryon, maybe Stentor (but as it was obtained from phytoplankton collection the latter is less possible).