Caenorhabditis elegans (full size) is a free-living, transparent nematode (roundworm) which measures about 1 millimetre (0.039 in) in length. Males, such as the one pictured here, comprise a small minority of the species—just 0.05 percent—and can be differentiated from hermaphrodites by the adult male's smaller size, single rather than double-armed gonad, vas deferens, and fan-like tail specialized for mating.Diagram: K.D. Schroeder