Muscles work against hydrostatic skeleton, which is fluid inside coelom. Setae are bristles that extend from each segment and anchor the worm. Nervous system has simple brain of paired ganglia, and ventral nerve cord. Closed circulatory system. Swim free, burrow in sand, or live in tubes cemented with mucus.
Parapodia are paddle-like appendages extending from each segment.
Function in locomotion and gas exchange. One species releases sperm and eggs within 2 hours of each other. Cuticle is thin, transparent and keeps body from drying out. Cuticle secreted by epidermal cells. Mucus secreted by epidermal gland cells also protects from desiccation.