The word ‘animal’ is derived from the Latin word animalis which means ‘having breath‘. Animals are multicellular, eukaryotic organisms of the kingdom Animalia (also called Metazoa). The animal kingdom emerged as a basal clade within Apoikozoa as a sister of the choanoflagellates. Sponges are the most basal clade of animals. Most of the organisms are capable of locomotion, meaning they can move spontaneously and independently at some point in their lives. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their lives. All animals are heterotrophs, which are multicellular and lack cell wall. They depend directly or indirectly on plants for their food and they must ingest and digest other organisms or their products for sustenance.
Very few people realize how many different kinds of animals there are and how greatly they vary in size, structure, and habits of life. In order that different forms of animals may be known and definitely recognized, a scheme of grouping related kinds has been devised.
Aristotle divided the living world between animals and plants, and this was followed by Carl Linnaeus, in the first hierarchical classification. In Linnaeus’s original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, the Chordata, whereas the various other forms have been separated out.
In 1874, Ernst Haeckel divided the animal kingdom into two subkingdoms: Metazoa (multicellular animals) and Protozoa (single-celled animals). The secondary groups are phyla, and they in turn are divided into classes. The principal groups subordinate to the class are order, family, genus, and species.
The protozoa were later moved to the kingdom Protista, leaving only the metazoa. Thus Metazoa is now considered a synonym of Animalia.