The most ancient animal groups (phyla) include the Porifera (sponges), Placozoa, Cnidaria, and Ctenophora (comb jellies). The sponges are extremely simply built, and have no organs. The placozoans ...

Three evolutionary biologists weigh up the evidence. There are five major branches of animals: Porifera (sponges), Cnidaria (jellyfishes, corals and related species), Ctenophora (comb jellies ...