ABSTRACT
Annelida is one of the three phyla presenting a segmented body plan, composed of repeated morphological units, and the only lophotrochozoan group with this type of organization. Furthermore, while arthropods and vertebrates also show segmental organization, annelids are the only phylum where segment formation continues well beyond embryonic development and into adult stages, either in normal posterior growth, axial regeneration, or asexual reproduction. Annelids are also unique in that they can form segments through two quite different mechanisms: boundary-driven and lineage-driven segmentation. Boundary-driven segmentation, in which segments are defined by formation of molecular and fate boundaries between groups of cells, occurs broadly in embryonic development of arthropods and vertebrates, and during post-embryonic growth and regeneration of annelids. Lineage-driven segmentation, in which the components of each segment arise as the clonal descendants of individual cells generated from teloblastic growth, is known only from annelids and malacostracan crustaceans. Despite these outstanding features of annelid segmentation, very little is known about its developmental genetics or how much it varies across the phylum; however, the last decades have seen significant progress thanks to the emergence of new molecular techniques amenable for use in annelids. This chapter introduces the annelid body plan, summarize early embryonic development and segment formation in four species spanning the diversity of the phylum, and discuss what little is known about the molecular basis of annelid segmentation.
