This is a rotifer - a tiny aquatic animal about one tenth of a millmetre long. Twenty, placed end to end, would stretch across the head of a dressmaker's pin. Rotifers feed by using two circles of rhythmically beating cilia ('wheel organs') on their head, which create a vortex that sweeps food particles into their digestive tract, into the path of constantly grinding jaws called the mastax, which you can see below the wheel organs. The animal attaches itself with a tail and, in some species, when it lets go of its anchorage the wheel organs can become propellers. Rotifers are every where where there is water - in puddles, in wet mosses and even in a vase of flowers that has been standing on the windowsill for a few days - which is where this one came from.