Fluorescence light microscope footage of several Caenorhabditis elegans nematodes. This is a free-living nematode roundworm that typically inhabits moist soils in temperate regions, feeding mainly on bacteria. It is widely used as a model organism in biology, especially in neurology, as it is one of the simplest organisms with a nervous system, which comprises just 302 nerve cells. It was the first organism to have its genome sequenced, and also the first to have its \connectome\ (nerve wiring pattern) mapped. The fluorescent dyes here mark the muscle cells (red) and sensory neurons (green). C. elegans reaches a length of around one millimetre.