Cellular and molecular mechanisms of segmentation in annelids

The annelids are a diverse phylum of metamerically segmented animals. they are of special interest embryologically because of their highly stereotyped pattern of development, much of which is shared by other spiralian phyla. They are also of interest phylogenetically for what they can tell us about the evolution of segmentation, and the relationships of coelomate protostomes in general. In this paper we review the embryology of the different annelid groups, showing considerable conservation of the basic character of segmentation, with modifications associated with evolving life history strategies. We also describe the current knowledge of molecular mechanisms of segmentation in annelids. The most is known at a cellular and molecular level about the phylogenetically derived leeches, while information is just emerging on the more basal annelids, the polychaetes and oligochaetes. This review is intended to provide a framework for comparison of the different annelid groups, and to suggest avenues for future research that might help to illuminate the evolution of annelid developmental patterns.