What Does Bilateral Mean?
Bilateral means symmetrical across a central axis, or concerning two sides. The word is used in international diplomacy to mean “between two countries,” and can be used to describe geometric figures, but this article will focus on the term bilateral as it is used in biology. Bilateral organisms are all members of the unranked group Bilateria, a scientific classification above superphylum but below subkingdom. Bilateral organisms include humans, fish, insects, flatworms, birds, reptiles, amphibians, and many others. The bilateral nature of most animals is evolutionarily fixed — because of the way embryos develop, the descendants of bilateral animals will always be bilateral. The oldest known bilateral animal is Vernanimalcula, an oval-shaped animal between 0.1 and 0.2 mm across that lived 600 million years ago, during the early Ediacaran period, 60 million years before the appearance of most modern phyla, called the Cambrian explosion. The primary alternative to bilateral symmetry in or
