C. elegans explores its environment and moves to favorable surroundings by chemotaxis, thermotaxis, and aerotaxis and escapes from harmful and noxious stimuli by avoidance/escape behaviors. The perception of environmental cues, including mechanical stimuli, temperature, many water-soluble and volatile chemicals, noxious substances, ambient osmolarity, oxygen levels, pH, and light, is accomplished through 24 sensillar organs and various isolated sensory neurons (NeuroTABLE 1) (Bargmann, 2006; Bergamasco and Bazzicalupo, 2006). Sensillar neurons perform most of the sensory functions. However, some sensory functions, including oxygen sensation and mechanosensation, are performed by nonsensillar neurons. Each sensillum contains ciliated endings of one or more neurons and often two types of glia: the socket cells and the sheath cells. Except for posterior deirids and phasmids, all sensilla are located in the head (see Neuronal Support Cells and Introduction; IntroTABLE 1).