The Wikipedia article for "Symmetry in Biology" claims: "Animals with bilateral symmetry are classified into a large group called the Bilateria which contains 99% of all animals (comprising over 32 phyla and 1 million described species)." This claim that almost all animals are Bilateria can be found in other sources too, e.g. in "A new paradigm for animal symmetry" by Gábor Holló. In both cases, however, no source is provided. I'm no biologist so I don't no exactly how self-evident this claim is.
Does someone know a source corroborating these claims?