Wikipedia
Micro-animal
Micro-animals are animals so small that they can only be visually observed under a microscope. Microscopic arthropods include dust mites, spider mites, and some crustaceans such as copepods and the cladocera. Another common group of microscopic animals are the rotifers, which are filter feeders that are usually found in fresh water. Some nematode species are microscopic, as well as many loricifera, including the recently discovered anaerobic species, which spend their entire lives in an anoxic environment. The tardigrades, water-dwelling micro-animals, can survive extreme living conditions that they are not evolved for and have survived the hard vacuum of space, solar and UV radiation in an astrobiology experiment into space.