Why does cutting a flatworm into pieces not kill it?

In gereral, the more poorly differentiated (the fewer complex structures that have specific functions) an organism is, the greater its ability to live and re-grow after being cut apart. Worms have this ability to re-grow in entirety, as do lower organisms like amoebas and fungi. In flatworms, their cells have the ability to differentiate into any type of tissue that the worm might need. These types of cells are called stem cells, and are the subject of ongoing research in medicine as to their applications in humans. There is a limit, however. a tiny slice of flatworm would not be able to develop into another worm.