C. elegans is a nematode - a member of the phylum Nematoda which incorporates roundworms and threadworms, a phylum of smooth-skinned, unsegmented worms with a long cylindrical body shape tapered at the ends, including free-living and parasitic forms both aquatic and terrestrial. C. elegans is small, growing to about 1 mm in length, and lives in the soil - especially rotting vegetation - in many parts of the world, where it survives by feeding on microbes such as bacteria. It is of no economic importance to man. C. elegans is about as primitive an organism that exists however it shares many of the essential biological characteristics that are central problems of human biology. The worm is conceived as a single cell which undergoes a complex process of development, starting with embryonic cleavage, proceeding through morphogenesis and growth to the adult. It has a nervous system with a 'brain' (the circumpharyngeal nerve ring). It exhibits behavior and is even capable of rudimentary learning. It produces sperm and eggs, mates and reproduces. After reproduction it gradually ages, loses vigour and finally dies. Embryogenesis, morphogenesis, development, nerve function, behaviour and aging, and how they are determined by genes are some of the most fundamental mysteries of modern biology. C. elegans exhibits these phenomena, yet is only 1 mm long and may be handled as a microorganism - it is usually grown on petri plates seeded with bacteria. All 959 somatic cells of its transparent bodyare visible with a microscope, and its average life span is a mere 2-3 weeks. Thus C. elegans provides researchers with the ideal compromise between complexity and tractability. There are two sexes, a self-fertilizing hermaphrodite and a male. The adult essentially comprises a tube, the exterior cuticle, containing two smaller tubes, the pharynx and gut, and the reproductive system. Most of the volume of the animal is taken up by the reproductive system. Of the 959 somatic cells of the hermaphrodite some 300 are neurons. Neural structures include a battery of sense organs in the head which mediate responses to taste, smell, temperature and touch and although C. elegans has no eyes, it might respond slightly to light. Among other neural structures is an anterior nerve ring with a ventral nerve cord running back down the body. (There is also a smaller dorsal nerve cord.) There are 81 muscle cells. C. elegans moves by means of four longitudinal bands of muscle paired sub-dorsally and sub-ventrally. Alternative flexing and relaxation generates dorsal-ventral waves along the body, propelling the animal along. The development and function of this diploidorganism is encoded by an estimated 17,800 distinct genes.