Micro-animals are animals that are microscopic and thus cannot be seen with the naked eye. Mostly these microorganisms are multicellular but none are vertebrates. Microscopic arthropods include dust mites, and spider mites, while microscopic crustaceans include 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 recently discovered anaerobic species, which spend their entire lives in an anoxic environment.

Although most microscopic animals are multicellular, myxozoa, jelly fish relatives from the phylum Cnidaria, are unicellular.