The Echinoderm Immune System Characters Shared with Vertebrate Immune Systems and Characters Arising Later in Deuterostome Phylogeny

As is now widely appreciated, the deuterostomes, of which echinoderms, hemichordates, tunicates, and all higher chordates are the major extant groups, constitute a separate branch of the animal kingdom (see, e.g., Refs. 1-4). On structural, embryological, biochemical, and molecular bases, deuterostomes are regarded as a monophyletic group of common ancestry. They are the sister group of the coelomate protostomes, the major taxa of which are the annelids, molluscs, and arthropods. Thus, in considering the origins of the higher vertebrate immune system, it is essential to keep in mind that it is among the extant lower deuterostomes that the most relevant homologous immune system elements are to be sought.