Rotifers  (multicellular)
Examples of genera left to right below:
Philodina, Collotheca, Brachionus

Further details:
Smallest page on the web - rotifers

Name (phylum): Rotifers

Size : Wide range amongst species
25 µm - 1 mm

Where to find them : Some species are planktonic, others are attached to plants, other organisms, stones or creeping on plants, mud surface etc.

Notes : A particularly fascinating group for the pond dipper with a microscope. Older books call them wheel animalcules. The head has a crown of cilia (the 'wheel organ'), which in some species looks as if it is rotating like a wheel, hence their common name.

Rotifer species show a wonderful variety of forms. Some move and contract like a leech (e.g. Philodina far left), others build 'houses'.

Classification :
Kingdom - Animalia, Phylum - Rotifera