Understanding the origins and evolution of the highly diverse body plans of the ~30 distinct phlya that make up the animal Kingdom is a major programme in biology. Recent research in my lab has demonstrated the existence of a new phylum of animals with a unique bodyplan - the Xenacoelomorpha - whose evolutionary origins are particularly surprising and highly contentious.Our work has elevated these very simple worms from a lowly position at the base of the animal tree to a place right within a group of highly complex animals called the deuterostomes. The deuterostome ‘super-phylum’ includes three other major animal body architectures: the phyla Chordata (including Humans), Echinodermata (starfish and sea urchins) and Hemichordata (acorn worms).All species within the phylum Xenacoelomorpha appear to have lost almost all features that define them as deuterostomes implying a history of extreme evolutionary simplification and loss. The research I propose aims to understand the extraordinary evolutionary path that led to this bizarre phylum of worms and give deep new insights into the origins of phyla.We will use diverse techniques to study the morphology, ultrastructure, embryogenesis and developmental genetics of the Xenacoelomorpha to get a much clearer picture of the extent to which their phenotype is simplified.In parallel we will undertake ab initio sequencing of the genomes of three species of xenacoelomorphs and perform comparative genomics across all deuterostome phyla to discover the genotypic correlates of their phenotypic innovations.In a reversal of the standard evolutionary approach which moves from morphology to genetics, we will use our genomes to discover the genetic novelties associated with all groups of deuterostome and then use data on gene function to link these novelties to their novel biological functions.Our ultimate goal is to uncover the links between genotypic and phenotypic novelty in the evolution of the deuterostomes.