The animal kingdom is divided into Prokaryotes and Eukaryotes.
Prokaryotes lack a cell nucleus or other membrane-bound organelles. In general, they seem to be much simpler than Eukaryotes. Prokaryotes are divided into Archaea and Bacteria.
Eukaryotes, however, are much more complicated, containing a nucleus. Humans, mammals, insects, fishes and birds are all examples of eukaryotes.
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