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Water bear (Echiniscus granulatus). Coloured scanning electron micrograph (SEM) of the head of a water bear, or tardigrade (phylum Tardigrada). Water bears are small, water-dwelling, segmented micro-animals with eight legs that live in damp habitats such as moss or lichen. They are classed as extremophiles as they can survive dry conditions by changing into a desiccated state, in which they can remain for many years. Whilst in this form (known as a tun) they can withstand the most extreme environments and can tolerate radiation levels as high as x1000 more than other animals, including humans. They have even been brought back alive after spending 10 days in the vacuum of space. Magnification: x100, when printed at 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/STEVE GSCHMEISSNER/SCIENCE PHOTO LIBRARY)