Animal mesoderm forms by enterocoelic pouching, through invagination of the endoderm. The main deuterostome phyla are the Echinodermata and the Chordata. Echinoderms are exclusively marine and include starfish, sea urchins, and sea cucumbers. The chordates are dominated by the vertebrates (animals with backbones), which consist of fishes, amphibians, reptiles, birds, and mammals. The deuterostomes also include the Hemichordata (acorn worms). The Ecdysozoa are protostomes, named after their shared trait of ecdysis, growth by moulting. They include the largest animal phylum, the Arthropoda, which contains insects, spiders, crabs, and their kin. All of these have a body divided into repeating segments, typically
Bacterial phyla threshold definition by Hugenholtz and Pace: Since then several other candidate phyla have been identified:) Since then a candidate phylum called Poribacteria was discovered, living in symbiosis with sponges and extensively studied. (Note: the divergence of the major bacterial lineages predates sponges) Another candidate phylum, called Tectomicrobia, was also found living in symbiosis with sponges. And "Nitrospina gracilis", which had long eluded phylogenetic placement, was proposed to belong to a new phylum, Nitrospinae. Other candidate phyla that have been the center of some studies are TM7, the genomes of organisms of which have even been sequenced (draft), WS6 and Marine