(P20) The body plan of dicyemid mesozoans which consisting of only 10 to 40 cells is amongst the simplest of all multicellular animals. They lack body cavities and almost all recognizable organs such as a gut or nervous system, moreover, tissues that characterize animals. Because of their extreme body plan simplicity, dicyemids have long been the subject of phylogenetic controversy; whether their simple body plan reflects their primitiveness or degeneration from progeny of higher metazoa. Dicyemids were at first considered to be an intermediate between protists and metazoa. Alternatively, they have been suggested to be derived from parasitic platyhelminths, because of their morphological and complex life cycle resemblances with trematode larvae, or placement within the protists. Recent 18S rDNA data suggest that dicyemids are related to triploblasts, although their phylogenetic placement is still unclear. Here we use Hox gene sequences to obtain insight into the phylogenetic status of dicyemid mesozoa. We isolated genomic and cDNA clones for Hox genes from the dicyemid mesozoan Dicyema orientale. The homeodomain sequences clearly indicate that Doxlab, DoxC, Doxpost2 are a member of the anterior, central, posterior group of Hox genes, respectively. Of phylogenetic significance is the presence of a diagnostic peptide motif for spiralian protostomes in DoxC. The existence of spiralian peptide firmly establishes that dicyemid mesozoa are not basal and primitive animals, rather, they are members of the Lophotrochozoa. We conclude that dicyemids are secondarily degenerated from higher protostome animals, with a body plan enormously reduced as a result of their endoparasitic lifestyle. They represent one of the most extreme cases of secondary reduction of body-plan complexity.