Figure 1. Equivalent cell lineages give rise to the motor neurons in the ventral nerve cord. There are 12 P precursor cells. P2, P9 and P10 have a similar lineage pattern, and give rise to four motor neurons, VA, VB, AS and VD. Similarly, P3 and P4 have a similar division pattern, and give rise to five motor neurons, four of which are the same types as those from the P2 lineage and one extra neuronal type, the VC cell; the equivalent VC cell in the P2 lineage undergoes programmed cell death. P5–7 cells give rise to five motor neurons of the same types as the P3 cell, as well as to the vulval lineages. Motor neurons of the same class have a similar morphology, projection pattern and neurotransmitter phenotype. An X indicates a cell that will undergo programmed cell death. (Modified from Sulston and Horvitz, 1977.)

Figure 2. Mechanosensory and chemosensory neurons in Caenorhaditis elegans. The mechanosensory neurons, the unpaired AVM and PVM cells and the paired ALM and PLM (only the left cells are shown), mediate response to light body touch. The amphid chemosensory neurons (red; only left cell of the pair is shown) send processes to the tip of the nose and one that goes into the nerve ring; these neurons are responsible for mediating chemosensation to water-soluble and volatile substances.