Segmented worms (Annelida) are a relatively large group of primarily free-living aquatic and terrestrial forms, displaying a fantastically large diversity in their morphology, physiology, and ecology. They play a critical ecological role, especially in the seas’ benthic and terrestrial detritivore communities. Annelids are characterized by a body structure consisting of numerous identical sections, the segments. A collagenous cuticle covers their epidermis, and they possess a coelom and a closed blood vascular system.