Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. The including of the Protozoa into Animalia is questionable and was already abandoned by most of scientists. However, it is still educated at some schools and may be considered as a traditional. Therefore, the Virtual Museum stil keep this older, although already invalid, approach. The body plan of Metazoans 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 are also heterotrophs, meaning they must ingest other organisms or their products for sustenance. Most known current animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago. It is unsure, if so called Ediacaran fauna of uppermost Proterozoic age represents animals in modern concept (very probably, it is an entirely blind evolutionary branch).