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If you're going for sci-fi realism, you need to recognise that alien species wouldn't share similar cell structures. No DNA, possibly not even any carbon. If animalia is just a kingdom that is a sub-group of a domain that is in turn a subgroup of 'life', then when we discover aliens, 'life' will need to become 'earth life', and any other planets will get their own hierarchy.Naming moving multicelled organisms by a vernacular 'animal' is probably the better way to describe something that looks like it could fit into the animalia kingdom, yet in fact has nothing in common, or in fact no part in Earth-based taxonomic rank.