Comparative molecular, developmental and morphogenetic

analyses show that the three major segmented animal groups-

Lophotrochozoa, Ecdysozoa and Vertebrata-use a wide range of

ontogenetic pathways to establish metameric body organization.

Even in the life history of a single specimen, different mechanisms

may act on the level of gene expression, cell proliferation,

tissue differentiation and organ system formation in individual

segments. Accordingly, in some polychaete annelids the first three

pairs of segmental peripheral neurons arise synchronously, while

the metameric commissures of the ventral nervous system form in

anterior-posterior progression. Contrary to traditional belief, loss

of segmentation may have occurred more often than commonly

assumed, as exemplified in the sipunculans, which show remnants

of segmentation in larval stages but are unsegmented as adults.

The developmental plasticity and potential evolutionary lability of

segmentation nourishes the controversy of a segmented bilaterian

ancestor versus multiple independent evolution of segmentation in

respective metazoan lineages.