Transparent, free-living nematode worm. Unsegmented body plan with full set of differentiated tissues (neural, endoderm, ectoderm and muscle). Genome size approximately 97 Mb, as five autosomes and one X sex chromosome. Fully sequenced genome, which comprises approximately 20 000 predicted genes. Defined cell lineage. Has made major contribution to studies of development, cell-to-cell signalling, cell ageing and cell death processes. Large-scale gene deletion, microarray analysis of gene expression and two-hybrid protein interaction analysis projects under way. Comparative studies mainly with C. briggsae, but also with other free-living and parasitic nematodes.