Animalia (multicellular animals) form the largest group of eukaryotic life with over a million described species. They are heterotrophic organisms and often show behavior such as reactions to exterior stimuli via sensory systems. Animals range in size from a few dozen micrometers (e.g. Tardigrada = water bears) to more than 20 meters (e.g. blue whale) and are present in all, terrestrial, limnic and marine ecosystems. Unlike plants and fungi, animals do not posess a cell wall of any kind.