Evolution of germ layers and gastrulation. Animal multicellularity, with the larval and juvenile/adult body plans of extant metazoans possessing multiple cell layers, is depicted above. The arrow represents metamorphosis from larval to juvenile/adult forms. Cell layers are coloured: orange, inner layer; green, outer layer; and blue, middle layer. In the case of cnidarians and bilaterians, these correspond to endoderm, ectoderm and mesoderm (bilaterians only), respectively. The phylogenetic relationship of these clades and the sister group to metazoans, the choanoflagellates, is shown below, along with the evolutionary origin of multicellularity. The origin of germ layers and gastrulation is debatable, occurring either before or after the divergence of sponges and eumetazoans.