Planaria have a huge regenerative power because of the presence of adult stem cells called neoblasts. If you cut Planaria into pieces, each piece can regenerate into a complete organism and that’s a form of asexual reproduction. Cells which are located on the wound site proliferate to form a blastema-mass of cell that will grow into a new organ. Those cells will differentiate into new tissues and regenerate the missing parts.
The similar thing is happening with starfish.