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Alien parasite. Computer artwork of a parasite on an alien moon. Its head is at upper centre, seen against the planet that the moon orbits, while its tail is at lower right. While alien biology may be completely different from that on Earth, there are some concepts which are thought to be universal or extremely widespread. Having a head and a tail may not be universal, but parasites may be. A parasite is an organism that lives on and feeds off a host organism. The spike on this parasite's head may be used to penetrate its host, or it may have another function altogether. (KEYSTONE/SCIENCE PHOTO LIBRARY/VICTOR HABBICK VISIONS)