The Polychaeta are a huge group of marine worms, also known as the bristle worms or polychaetes. They are among the most common marine organisms.
Polychaetes are multi-segmented worms living in all environments in the world's oceans. Most polychaete body segments bear a pair of parapodia (flat, lobelike outgrowths) with a dense cluster of tiny bristles and hooks called chaetae. The name polychaete actually means 'many bristles'.
Polychaetes are divided informally into two groups; the errantia, or free-moving forms, and sedentaria, or tube-dwelling forms which are normally in a fixed location (imobile).
Serpulidae (fanworms) are very well-known polychaetes owing to their colourful radiolar crown. The crown is covered in densely packed microscopic projections that look like tiny hairs in a feather-like feeding and respiratory structure. This structure is attached to a lobe surrounding the mouth. Adult Serpulidae range in size from 2mm to well over 100mm.
Spaghetti worms are a marine relative of the earthworm, growing up to 15 cm in length. These worms have long, cylindrical bodies that are divided into many similar sections called segments. Some structures, like muscles, kidneys, and nerves are repeated in each segment. Body segments have appendages on each side which have small bristles protruding. They have many long tentacles radiating from the head, close to the mouth, which are used for finding and collecting food. Tentacles may be up to 1 m long (100 cm).