What does bilateral mean?

bilateral means symmetrical across the central axis or two sides. The word is used in international diplomacy as "between two countries" and can be used to describe geometric data, but this article focuses on the term bilateral, as used in biology.

Bilateral organisms are all members of the UNRANKED GROUP BILATERIA, scientific classification over the superfyl, but under the subset. Bilateral organisms include humans, fish, insects, flat worms, birds, reptiles, amphibians and many others. The bilateral nature of most animals is evolutionarily fixed - due to the way embryos develop, the descendants of bilateral animals will always be bilateral. The oldest known bilateral animal is Vernanimalcula, an oval animal between 0.1 and 0.2 mm across the one that lived 600 million years ago, during the early Ediacaran period, 60 million years before the most modern phylls, called Cambrian Explosion.I, jellyfish, corals and many flowers. As an adults, starfish and sea hedgehogs have five times radial symmetry (Pentamerism), but in their essence are bilateral animals, as shown by bilateral symmetry of their larvae. Some primitive animals, such as mushrooms, trichoplax and mesozoans, have no symmetry at all, which is referred to as asymmetric. It is assumed that the mushrooms form a group of STEM, from which all other animals originally developed.

Almost all bilaterans are tribroblastic, which means that their embryos develop in three layers called endoderm, mesoderm and ectoderm. Almost all bilaterans have complete digestive tracts with a separate rectum and mouth and inner body cavity called coeloma. Bilaterans can be either sessile (stationary) or mobile, albeit usually. Bilaterans colonize every -thinking environment on the ground, from Don the oceans to the peaks of Mt. Everest.

There are two main groups in bilaterals called Superfyles: deuterostomy and protostomes. The main difference between the two groups is that in the latter, the first hole that is formed in the embryo becomes mouth, while in the second it becomes an anus. In deuterostomes there is also less predetermine of cell sites during embryogenesis.