In order to better understand such a large number of organisms, it is necessary to arrange them in groups. This arrangement presents a great deal of information in an orderly way. In the traditional two-kingdom system of classification (not followed these days) the multi-cellular animals were referred to as Metazoa to distinguish them from one-celled Protozoa. In modern five kingdom classification scheme the Protozoa belongs to kingdom Protoctista whereas the true animals are placed in kingdom Animalia. A true animal is now defined as a Eukaryotic, multi-cellular, heterotrophic organisms which is diploid and developed from an embryo formed by the fusion of two different haploid gametes, a large egg and a small sperm. Most of animals are motile, a few, however, are sessile.
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