Arthropods, annelids and chordates all possess segments. It remains unclear
, however, whether the segments of these animals evolved independently or i
nstead were derived from a common ancestor. Considering this question invol
ves examining not only the similarities and differences in the process of s
egmentation between these phyla, but also how this process varies within ph
yla, where the homology of segments is generally accepted. This article rev
iews what is known about the segmentation process and considers various pro
posals to explain its evolution.