Spiralian model systems.

The "Spiralia" represent one of the three major clades of bilaterian metazoans. Though members of this clade exhibit tremendous diversity in terms of their larval and adult body plans, many share a highly conserved early pattern of development involving a stereotypic cleavage program referred to as spiral cleavage. This group therefore represents an excellent one in which to undertake comparative studies to understand the origins of such diversity from a seemingly common ground plan. These organisms also present varied and diverse modes in terms of their ecology, development and life history strategies. A number of well established and emerging model systems have been developed to undertake studies at the molecular, genetic, cell and organismal levels. The Special Issue of the Int. J. Dev. Biol. entitled "Spiralian Model Systems" focuses on these organisms and here, I introduce this clade, pointing out different types of studies being undertaken with representative spiralian model systems.