Hidden Animals

Aristotle divided the living world between animals and plants, and this was followed by Carolus Linnaeus (Carl von Linné), in the first hierarchical classification. Since then biologists have begun emphasizing evolutionary relationships, and so these groups have been restricted somewhat. For instance, microscopic protozoa were originally considered animals because they move, but are now treated separately.
In Linnaeus's original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Reptila, Aves, and Mammalia. Since then the last five have all been subsumed into a single phylum, the Chordata, whereas the various other forms have been separated out. These information represent our current understanding of the group, though there is some variation from source to source.