Metamerism is the repetition of homologous body segments. This type of development can be seen in the Annelids, which are earthworms, leeches, tubeworm, and their relatives. It is also seen in a more advanced form in the Arthropods, or crustaceans, insects, and their relatives. Metamerism arises from teloblastic development, in which cells divide asymmetrically to form rings of small cells around the embryo. These cells proliferate into the many segments of worms and can be homonomous meaning very similar, or they can be differentiated into a wide variety of functions. These segments are called heteronomous. Metamerism has led to a great diversity of annelids, arthropods, and other segmented animals in the world. The simple segmented condition of annelids allows them to exist in every environment from the deepest parts of the ocean to the soil of some of the highest mountains.