What characteristics define an animal?
Animals have eukaryotic cells (very simply, they have a nucleus surrounded by a membrane). They are multicellular. Their cells lack cell walls. They are heterotrophs (meaning they can't make their own food but must take it in from outside.). They can move around, at least in some part of their life cycle.
Heterotrophic (how it obtains energy)
Eukaryotic (what type of cell)
Multicellular (number of cells)
Cell walls (lacks this structure)