Class Polychaeta consists of mostly marine annelids. Polychaetes are the largest group of annelids, and have hard bristles that allow the worms to wriggle and move. However, only one type of polychaetes actually move. The Errantia are active and mobile, while the Sedentaria are immobile, spending their entire lives burrowed in sediment. The appendages also are used in exchange of oxygen and wastes by increasing surface area. Many polychaetes, especially the group Sedentaria, live in tubes that they make from sticky proteins secreted near the mouth. Feathery appendages extending from the tubes trap food in the water. An example of a tube-building polychaete is a Christmas tree worm. Polychaetes usually have at least one set of eyes, one set of sensory organs, and are of one sex.

Polychaetes are in the phylum Annelida, made up of segmented wormss. Body segmentation, a hallmark of annelids, was a major step in the evolution of animals. Annelids are protosomes, meaning they have a coelom made from cell masses. This coelom is divided into a series of repeated parts. Except for the head and tail region, each with an opening of the digestive tract, making it a complete tract, each segment in an annelid is ringlike and very similar. Segmentation allows for flexibility and mobility because annelids can bend at segmented parts Other hallmarks of the annelids are soft bodies that are round in cross section, repetition of organs in the segemented parts, and a body that is much longer than it is wide. There are three main classes in the phylum Annelida.