A diagram of of the nephridium of Nereis. Nereis is a genus of polychaete worms in the family Nereidae. It comprises many species, most of which are marine, including the sandworm (Nereis virens) and the common clam worm (Nereis succinea). Nereis possess setae and parapodia for locomotion. They may have two types of setae, which are found on the parapodia. Acicular setae provide support. Locomotor chaetae are for crawling, and are the bristles that are visible on the exterior of the polchaeta.