Amoeba

Amoebas are all over the place in soil, water, and even inside other organisms, but the reason you've probably never seen one is that they're so tiny— just a single cell. They're surrounded by a thin membrane, and filled with a fluid called cytoplasm. They move by squirting the cytoplasm around to form pseudopods, projections from their body which they use to drag themselves around and capture their food. They reproduce by splitting in two, so the first amoeba that ever lived is still out there, sort of. The species Amoeba proteus is big for an amoeba, measuring a whopping 0.001 millimetres!