Multicellular, eukaryotic animals.  They generally possess digestive, circulatory, nervous, excretory, and reproductive systems.

Single-celled prokaryotic organisms that do not contain a nucleus and lack membrane-enclosed intracellular structures

Eukaryotic, single-celled microscopic organisms.  They possess at least one nucleus and numerous intracellular structures.

Eukaryotic organisms that possess a nucleus and intracellular structures.  They are important for the decomposition of organic matter.

Acellular entities that are composed of a nucleic acid and a few proteins.  They are very small and can't be seen with a light microscope.