A soil-dwelling nematode worm that is used experimentally as a model organism in genetics and developmental biology. It was the first multicellular organism to have its genome fully sequenced, in 1998. The genome contains an estimated 17 800 genes and amounts to about 97 megabase (Mb). Adults have a fixed number of body cells (959), and the developmental pathway of each cell can be traced. This has yielded valuable insights into mechanisms of development, including genetic control and the role of programmed cell death (apoptosis), and also the organization of the nervous system.