This little crustacean, about a millimetre long, is Cyclops, with a single red eye-spot in the centre of its head - a minute freshwater counterpart of the monocular monster of Greek mythology. My garden pond is swarming with them at present, even though the ice has barely thawed. If you take a close look at the top end of the tail, near the body, you can just make out clusters of short-stalked objects attached to the animal's exoskeleton. At higher magnification these turn out to be....
.......... Vorticellids - single celled protists with beating cilia around their mouth, creating a whirlpool current that sucks in foot particles. You can see a movie of Vorticella in action here. At even higher magnification..........
..you can see their cilia and the contractile vacuoles that they use to expel waste (double-click for a larger image). Vorticellids attach themselves to all sorts of small pond animals, hitching a ride.
As Jonathan Swift (1677-1745) noted,
So, naturalists observe, a flea
Has smaller fleas that on him prey;
And these have smaller still to bite ’em;
And so proceed ad infinitum.