Polychaeta is a class of annelid worms characterized by their segmented bodies and numerous bristle-like structures called chaetae. These marine organisms are found in various habitats, from shallow coastal waters to deep-sea environments. Polychaetes play essential roles in the ecosystem, including nutrient cycling and serving as a food source for many marine animals. Most polychaetes have a well-developed head with sensory organs and are often active predators or scavengers. They exhibit a wide range of body forms and sizes, with some species being brightly colored. Reproduction can be sexual or asexual, with many polychaetes releasing gametes into the water for external fertilization.