World Animals
Animals (also
called Metazoa) are multicellular, eukaryotic organisms
in the biological kingdom Animalia. With
few exceptions, animals consume organic material, breathe oxygen, are able to move,
can reproduce sexually, and go through an
ontogenetic stage in which their body consists of a hollow sphere of cells,
the blastula,
during embryonic development. Over 1.5 million living animal species have
been described—of which around 1 million are insects—but
it has been estimated there are over 7 million animal species in total. Animals
range in length from 8.5 micrometres (0.00033 in) to 33.6 metres
(110 ft). They have complex interactions with each other and their
environments, forming intricate food webs.
The scientific study of animals is known as zoology.
Most living animal species are in Bilateria,
a clade whose
members have a bilaterally symmetric body plan. The
Bilateria include the protostomes, containing invertebrates such
as nematodes, arthropods,
and molluscs,
and the deuterostomes, containing the echinoderms and
the chordates,
the latter including the vertebrates. Life forms interpreted as early animals were
present in the Ediacaran biota of the late Precambrian.
Many modern animal phyla became clearly established in the fossil record as marine species during
the Cambrian explosion, which began around 542
million years ago. 6,331 groups of genes common to all
living animals have been identified; these may have arisen from a single common ancestor that
lived 650 million years ago.
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