Conservation and innovation in spiralian development

It is clear that the spiralian developmental program represents a highly fl exible platform for the generation of diverse larval and adult body plans. The widespread occurrence of this pattern of early development attests to i ts tremendous evolutionary success. Despite the large degree of conservatio n in the spiral cleavage pattern and other basic aspects of early developme nt, changes in cell fate maps and in the mechanisms of blastomere specifica tion have arisen. While we have learned a great deal about this mode of dev elopment, a number of important questions remain to be answered. To what ex tent do these conditions apply to the lesser studied spiralian phyla? What constraints have led to the preservation of the early spiral cleavage progr am? How has this developmental program been adapted for the construction of the various spiralian body plans (e.g. the segmental body plans of annelid s or to the potential secondary loss of segmentation)? Are most changes ass ociated with the elaboration of these different larval and adult body plans restricted to the late period of development? What molecular/genetic proce sses underlie this developmental program? Clearly, the spiralian phyla repr esent an important group of organisms for further studies on development an d evolution.