Polychaetes are little bristly worms (polychaeta = "with lots of hair"). Typically, each of their body segments has two appendages with tufts of bristles at the end. In some species, the bristles can be extended or retracted. While they help the polychaete get traction to move along a surface, the bristles may also be hollow and venom-filled for defense. The fleshy appendages (parapodia) that hold the bristles are used for walking or swimming around. A variety of other appendages are adaptations for a variety of functions. Polychaetes often live in burrows or tubes they make of mucus and sand, usually in saltwater habitats. Antennae and sensory tentacles on their heads allow them to feel their way around, while they see light and dark with cells that are organized into eye-like structures on their heads or other parts of their bodies. Feathery appendages (branchiae) act as gills that supply the oxygen they need from the seawater.