Rotifers are rather large 0.2mm to 0.5mm microscopic organisms found in fresh (i.e. not salt) water. A drop of typical scummy pond water will contain quite a few, which can be easily observed in a microscope at 100x magnification. The following images show a fairly large specimen which I observed swimming around and feeding on other microscopic organisms for quite a while.
The rotifer is at the bottom of the image. The long strand across the middle is a piece of alge. Although I followed the rotifer for many minutes it was not until enough water had leaked out from under the coverslip to pin him to the slide that I was able to take images. The magnification here is 40x.
The same Rotifer at 100x. Although unable to swim at this point, it was still moving around a bit. The head is at the top and the tail is at the bottom. I am pretty sure that the yellowish portion is the digestive tract.
Sadly, as the water continued leaking out from under the coverslip the pressure on the rotifer increased until he burst apart.