A Feeding Rotifer.
An actively feeding rotifer (1) extends from an organic mass consisting largely of bacteria. Only about a quarter of the creature is seen -- it is attatched within the mass at about the point 2. If disturbed, it will suddenly contract within the mass, and emerge when the disturbance has passed.
Below is a smaller rotifer (3) with its corona withdrawn.
Click here for more information on rotifers.