(also referred to as metazoa) are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals
consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from 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 kingdom Animalia includes humans, but in colloquial use, the term animal
often refers only to non-human animals
. The scientific study of animals
is known as zoology.
Most living animal
species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan. The Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes, arthropods, and molluscs—and the deuterostomes, containing both the echinoderms as well as the chordates, the latter containing the vertebrates. Life forms interpreted as early animals
in the Ediacaran biota of the late Precambrian.