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The article focuses on evolutionary biology. Despite the comforting certainty of textbooks and 150 years of argument, the true relationships of the major groups of animals remain contentious. In the late 1990s, a series of controversial papers used molecular evidence to propose a radical rearrangement of animal phylum. Traditional trees group together phyla of bilaterally symmetrical animals that possess a body cavity lined with mesodermal tissue, the coelom, as Coelomata. Those without a true coelom are classified as Acoelomata and Pseudocoelomata.