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Keys are documents that ask you questions about the species provide alterntives e.g. if animal has wings it is a bird, if has gills it is a fish. Specific keys let you identify the organism from phylum, class, order, family, genus and species. Some keys are illustraded or offer photos e.g. here is a photographic key for Daphnia http://cfb.unh.edu/cfbkey/html/anatomy/daphnia/daphnia.html