What is the unique ability of some species of a starfish in respect of its arms?

Starfish are marine invertebrates. They typically have a central disc and usually five arms, though some species have a larger number of arms. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, grey, or brown. Starfish have tube feet operated by a hydraulic system and a mouth at the centre of the oral or lower surface.

Some species of starfish have the ability to regenerate lost arms. Regenerating their own arms is perhaps one of the most useful things a starfish can do. This attribute can be used in many different ways. Arms can be lost when a starfish comes in contact with a predator, or it may be amputated in order to hide or escape from one. Arms can take months, even years to fully regenerate, so it has to be a pretty serious situation to lose one. Incredibly, if the severed arm is not harmed, it can heal itself and even regenerate - resulting in a genetically identical starfish. Single arms that regenerate a whole individual are called comet forms.