Metamorphosis is a remarkable process in echinoderms, transforming a bilaterally symmetrical planktonic larva into a radially symmetrical benthic adult. This shift in habitat involves functional and anatomical changes in virtually every organ system (Bury, 1895; MacBride, 1914; Okazaki, 1975). Although metamorphosis is a crucial process in echinoderm development, we know relatively little about it. Furthermore, most of what we do know concerns sea urchins, and even less information is available about metamorphosis in other echinoderms. We have examined the expression of regulatory and structural genes during metamorphosis in several different echinoderm species (Lowe & Wray, 1997 and unpublished results). These data, together with those from several recent studies concerning additional genes (reviewed in Wray & Lowe, 2000), are beginning to shed new light on this complex and important process in echinoderm development.

The overt transformation from swimming larva to settled juvenile is quite rapid in echinoderms (Cameron & Hinegardner, 1978), requiring less than half an hour in many species. The complete process of metamorphosis takes much longer, however (Okazaki, 1975; Gosselin & Jangoux, 1998). Extensive preparations begin several days to weeks before settlement (MacBride, 1914; Okazaki, 1975), depending upon the species and upon environmental conditions (Strathmann et al., 1992). During this preparatory phase, initially small populations of ectodermal and mesodermal cells fated to become the adult proliferate, differentiate and undergo complex morphogenetic movements to form the imaginal rudiment (MacBride, 1914; Okazaki, 1975). The rudiment is more complex than the larva in several important ways: it contains a greater number of cell types, it is the first place where true tissues form, and it contains the first well-organised nervous system (Okazaki, 1975; Chia & Burke, 1978).