AnnelidAnnelida

Annelids, known as segmented worms from the Latin anellus meaning "little ring," are marine invertebrates characterized by distinctly segmented bodies, often bearing stiff bristles called setae. Many species construct permanent tubes from calcium carbonate, sand, or mucus, anchoring themselves to hard substrates or burrowing in sand. Sedentary forms, such as feather duster worms and Christmas tree worms, extend elaborate feathery crowns of radioles to filter plankton and suspended particles. These crowns display colours including reds, blues, yellows, and oranges, and retract quickly when disturbed. Mobile annelids, like fireworms, typically have more robust, flattened bodies and crawl across the substrate, hunting small invertebrates or scavenging detritus; some possess stinging bristles for defence.

Annelids range in size from under 1 cm to over 30 cm (12 in) in length. They are commonly observed on coral reefs, rocky outcrops, and sandy patches, from the intertidal zone to depths exceeding 40 m (130 ft). Sedentary tube worms are conspicuous fixtures on hard substrates, while mobile forms are found among rubble and under ledges.