 C. elegans is unsegmented, vermiform, and bilaterally symmetrical. It has a cuticle (a tough outer covering), four main epidermal cords, and a fluid-filled pseudocoelom (body cavity). It also has some of the same organ systems as larger animals. About one in a thousand individuals is male and the rest are hermaphrodites. The basic anatomy of C. elegans includes a mouth, pharynx, intestine, gonad, and collagenous cuticle. Like all nematodes, they have neither a circulatory nor a respiratory system. The four bands of muscles that run the length of the body are connected to a neural system that allows the muscles to move the animal's body only as dorsal bending or ventral bending, but not left or right, except for the head, where the four muscle quadrants are wired independently from one another. When a wave of dorsal / ventral muscle contractions proceeds from the back to the front of the animal, the animal is propelled backwards. When wave of contractions is initiated at the front and proceeds posteriorly along the body, the animal is propelled forwards. Because of this dorsal / ventral bias in body bends, any normal living, moving individual tends to lie on either its left side or its right side when observed crossing a horizontal surface. A set of ridges on the lateral sides of the body cuticle, the alae, are believed to give the animal added traction during these bending motions.
