Polychaetes include such forms as sand worms, tube worms, and clam worms. Most have well developed, paired, paddle-like appendages (parapodia), well developed sense organs, and numerous setae (usually on the parapodia; "polychaete" means "many hairs"). Polychaetes usually have a well-developed head, often complete with well-developed eyes, antennae, and sensory palps. They lack any permanent sex organs (in contrast to other kinds of annelids); gonads appear as swellings during the breeding season. Gametes are shed into the coelom and carried outside the body through the nephridia or as a result of the body wall actually rupturing. Fertilization is external, and development proceeds indirectly through a trochophore larva.
Polychaetes are a large and extremely diverse group. Around 10,000 species have been described. Most are marine. Some, such as featherduster worms, are sedentary, living in tubes buried in sand or mud, and feed by trapping food particles in mucus or by ciliary action. Others, such as the clam worm, are active, mobile predators that capture prey in jaws attached to their pharynges. Still others, such as fireworms, graze on gorgonians and stony corals.
Polychaetes are extremely abundant in some areas. They play essential ecological roles, serving on one hand as predators on small invertebrates, and on the other as food for fish and large invertebrates.
Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.