Bacteria have three main shapes—spherical, rod-shaped, and spiral. Spherical bacteria, known as cocci, occur singularly in some species and as groups in other species. Cocci have the ability to stick together and form a pair (diplococci); when they stick together in long chains, they are called streptococci. Irregularly shaped clumps or clusters of bacteria are called staphylococci. Rod-shaped bacteria, called bacilli, occur as single rods or as long chains of rods. Spiral- or helical-shaped bacteria are called spirilla.