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==Characteristics==
Animals have several characteristics that set them apart from other living things. Animals are [[eukaryote|eukaryotic]] and usually [[multicellular]] (although see [[Myxozoa]]), which separates them from [[bacteria]] and most [[protist]]s. They are [[heterotroph]]ic, generally digesting food in an internal chamber, which separates them from [[plant]]s and [[alga]]e. They are also distinguished from plants, algae, and [[fungus|fungi]] by lacking [[cell wall]]s. All animals are motile, if only at certain life stages. Embryos pass through a [[Blastula|blastula stage]], which is a characteristic exclusive to animals.
===Structure===
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The [[Ecdysozoa]] are protostomes, named after the common trait of growth by moulting or [[ecdysis]]. The largest animal phylum belongs here, the [[Arthropoda]], including [[insect]]s, [[spider]]s, [[crab]]s, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the [[Onychophora]] and [[Tardigrada]], are close relatives of the arthropods and share these traits.
The ecdysozoans also include the [[Nematoda]] or roundworms, the second largest animal phylum. Roundworms are typically microscopic, and occur in nearly every environment where there is water. A number are important parasites. Smaller phyla related to them are the [[Nematomorpha]] or horsehair worms, which are invisible to the unaided eye, and the [[Kinorhyncha]], [[Priapulida]], and [[Loricifera]]. These groups have a reduced coelom, called a pseudocoelom.
The remaining two groups of protostomes are sometimes grouped together as the [[Spiralia]], since in both embryos develop with spiral cleavage.
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