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 s tereotyped pattern of development, much of which is shared by other sp iralian phyla. They are also of interest phylogenetically for what the y can tell us about the evolution of segmentation, and the relationshi ps of coelomate protostomes in general. In this paper we review the em bryology of the different annelid groups, showing considerable conserv ation of the basic character of segmentation, with modifications assoc iated with evolving life history strategies. We also describe the curr ent knowledge of molecular mechanisms of segmentation in annelids. The most is known at a cellular and molecular level about the phylogeneti cally derived leeches, while information is just emerging on the more basal annelids, the polychaetes and oligochaetes. This review is inten ded to provide a framework for comparison of the different annelid gro ups, and to suggest avenues for future research that might help to ill uminate the evolution of annelid developmental patterns.