Chaetognath phylogenomics: a protostome with deuterostome-like development

Traditional textbook phylogeny splits bilaterians into protostomes and deuterostomes according to whether their mouth derives from the blastopore or not. This scheme has been largely confirmed by small subunit ribosomal RNA (SSU) molecular phylogeny. However, some phyla, such as the lophphorate phyla Phoronida and Brachiopoda as well as the Chaetognatha exhibit classical deuterostome embryological features such as formation of the mesoderm from the gut (enterocoely) and secondary opening of the mouth.