Marine worms, also known as Nereis, a worm with highly developed heads and many bristles arranged along the sides of the body. These are Polychaetes or "many-bristle") worms, Nereis being one and a common form along the seashore. The body presents the same segmented apprerance as the earthworm. The head is provided with numerous sense organs, among which are four eyes and several tentacles or feelers. A marine worm during normal seasons.