A cell was the functional basic unit of a living organism. The study of cells is known as cellular biology. The Human body was made up of millions of cells. (TOS: "The Immunity Syndrome" ) Cells developed an internal clock based on a 24 hour day. (TNG: "Clues" ) In 2267, Spock suggested that each individual Denevan neural parasite was a single cell that comprised a larger organism, adding that although not physically attached to each other, they nevertheless worked together as one. (TOS: "Operation -- Annihilate!" )