Zoology. A free-living, 1 mm long nematode which is a good model organism for genetics and developmental biology since 1965. Its genome, consisting of only six chromosomes, was the first one of a multicellular organism to be completely sequenced in 1998. C. elegans has always exactly 959 cells at full-grown size. It has a short generation time of about three days and is easy to grow at high densities (up to 10,000 worms on one Petri dish).