Animalia

From EvoWiki

Animals are one of the major kingdoms on earth. They are a group of multicellular, heterotrophic eukaryotic organisms.

Early definitions of "animals" were along the lines of any living thing that gains its nutrients from the bodies of other living things, as opposed to generating its own nutrients via exploitation of unliving chemicals and energy in its immediate vicinity. Unfortunately, this criterion yields a 'false positive' when applied to carnivorous plants meaning it incorrectly identifies a living thing as an animal! Thus, a better distinguishing characteristic, or perhaps even a set thereof, would be nice to have.

Fortunately, there is a better candidate for a criterion to distinguish animals from plants: The presence of a cell wall, a sturdy shell made of cellulose. All plant cells have them, and almost no animal cells do, especially since virtually all known animals, from placozoans to sponges to vertebrates lack the ability to manufacture cellulose (most animals even lack the ability to digest cellulose). One exception to this rule is that some tunicates ^[1], such as Ciona intestinalis, are the only animals known to produce cellulose.

Fungi are distinguished from animals also through the presence of a cell wall. However, fungal cell walls are composed of chitin, whereas plants' cell walls are composed of cellulose.