Animal

Biologically, an animal is any member of the Animalia kingdom. Animals must eat plants or other organisms to survive. The plants are called "producers" and the animals are "consumers". Scientists call consumers heterotrophs. Animals have complex eukaryotic cells, unlike bacteria which have much simpler prokaryotic cells. Animal cells do not have cell walls, only structural proteins in the cell membrane; this allows the animal cell a certain amount of flexiblity.

Animals evolved from single-celled, colonial protists called choanoflagellates. The simplest of all animals are sponges, which do not move, and are instead sessile. Sponges have different cells for different tasks, but these cells are not organised further into tissues or organs. Sponges also do not exhibit symmetry, a characteristic found in all other animals.

The next simplest animals comprise a single species, placed between sponges and all other animals. This species, Trichoplax adhaerans, is very small, yet has differentiated cells organised into tissues, though not organs. This shows an improvement upon the sponge body plan.

All other animals are grouped into a subkingdom called Eumetazoa (true animals). All eumetazoans have tissues and organs, except for one phylum, Myxozoa, which have regressed to a single-celled form.