Annelids share the common characteristics of size and symmetry and have body walls and coeloms. Although they vary widely in habitat area and live in different environments around the world, annelids have structured nervous systems, sense organs, circulation and respiratory structures and segmented organs.
Annelids are bilaterally symmetrical and range in size from under 1 millimeter to over 3 millimeters. The largest species reside in South America, while smaller annelids live in temperate and more northern locales. All annelids have coeloms, which are fluid-filled cavities between the outer body wall and digestive organs. Coeloms are used as storage areas for gametes and function as hydrostatic skeletons, which facilitates movement and locomotion. Annelids have bodies covered by thick external cuticles, called body walls, which are never shed or molted. These walls are situated just beneath the outermost shell layer on annelids and protect them from damage caused by attacking predators. Annelids also share parapodia, which are unjointed extensions of their body walls. These features act as cartilage and help annelids move on land and swim through water. Annelids also have complex nervous systems, made complete with a brain and sensile organs. Annelids also have six sensory organs along with closed circulatory systems and respiratory components.