ABSTRACT
The developmental biology of annelids has been actively studied for more than 100 years. Early in this history it was recognized that annelids are spiralians, sharing basic developmental patterns with a number of other protostome phyla, notably the mollusks. They are also, along with the vertebrates and arthropods, a major phylum of segmented eucoelomate animals. Recent revisions in metazoan phylogeny have placed the annelids as the most intensively studied, in terms of development, among the phyla of the Lophotrochozoa (Halanych et al 1995; Tessmar-Raible and Arendt 2003). This recognition of a superphylum Lophotrochozoa has made study of annelid molecular development patterns important as an evolutionary comparison with the model systems from the other two major triploblastic clades, the Edysozoa, containing Drosophila, and Deuterostomia containing the chordates.
