Animals are
multicellular,
eukaryotic organisms of the
kingdom Animalia or
Metazoa. Their
body plan eventually becomes fixed as they
develop, although some undergo a process of
metamorphosis later on in their life. Most animals are
motile, meaning they can move spontaneously and independently. All animals must ingest other organisms or their products for
sustenance (see
Heterotroph).
Most known animal
phyla appeared in the fossil record as marine species during the
Cambrian explosion, about 542 million years ago. Animals are divided into various sub-groups, including
birds,
mammals,
reptiles,
fish and
insects.