These single-celled protists are so small that some live symbiotically within two other organisms found on this list: radiolarians and forams. A loriciferan’s body contains adorable small-scale versions of parts seen in a larger animals, including “a brain, digestive and excretory systems, specialized appendages, sense organs, musculature and locomotory functions, separate sexes, and a protective external cuticle.” As zoologist Dr. Ross Piper puts it, “For an animal so small and with only around 1,000 cells in its entire body, this structure is amazingly complex; an assemblage of muscles, ligaments, and toothed plates (trophi) all working together to macerate the food before it’s digested.”