Segmented worms, such as earthworms and leeches, have bodies made up of connected sections. To move, they send waves of contractions down their body through the sections. Circular muscles contract to stretch part of a worm forward, followed by longitudinal muscle contractions that shorten the worm, bringing another part along with it. Suckers or stiff hairs keep each section of the worm anchored so that it does not move backwards as the contractions passes through it. The nervous system of a worm includes long nerve cords running its length, interacting with clusters of nerve cells (ganglia) in each section. This long system of nerves, with instructions from two ganglia in the worm's head, controls the muscle contractions that cause movement. Each body section is full of fluid that makes it rigid enough for the muscles to change its shape. Some worms also have little appendages they use like paddles to row or walk themselves along.

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.