Two Rotifers and Paramecia.
This picture shows a dense cluster of feeding Paramecia, with one rotifer in extended feeding mode at the upper middle (1), and another (below the plane of focus) at the bottom left (2).
The Paramecia are single-celled organisms, whilst the rotifers are more complex, and are composed of around a thousand cells. Paramecia are frequently found in large numbers in waters which are heavily polluted with decaying organic matter and densely populated with bacteria, since bacteria are their principal food.
For more information on Paramecium, click here.
Click here for more information on rotifers.