Caenorhabditis elegans
Caenorhabditis elegans is a tiny, transparent soil nematode about a millimetre long that became a model organism precisely because it is small and simple. Its body is see-through, so under a microscope you can watch individual cells divide and move inside a living animal — a level of access that larger organisms rarely allow.
The worm's most famous feature is that its development is almost completely invariant: in the hermaphrodite, every adult has the same fixed number of body cells, and the family tree of every cell — which cell divided to make which — has been mapped from egg to adult. This complete cell lineage made it possible to ask exactly which genes tell each cell when to divide, specialize or die.
Because of these traits, the worm was the first animal to have its genome fully sequenced and was central to discovering programmed cell death and RNA interference, a gene-silencing mechanism. It remains a powerful system for studying how genes guide cell fate, aging and the nervous system, whose every connection has been charted.
By tracking the worm's fixed lineage, researchers found genes that order specific cells to die on schedule, revealing that cell death is an active, gene-controlled program rather than mere damage.
A transparent worm made cell-by-cell genetics visible.
Most C. elegans are self-fertilizing hermaphrodites, which makes it easy to keep mutant strains pure and to obtain large numbers of genetically identical worms for experiments.