A class of lower green plants called liverworts that belong to the division Bryophyta. The class is divided into approximately 225 genera and 8500 species. Although there is a great diversity of external form, most of the gametophytes (gamete-producing plants) are dorsoventrally differentiated. These plants are considered among the most primitive of the existing land plants. Fossil remains of liverworts have been found in both the Devonian and Carboniferous. Since the fossils found do not differ significantly from modern liverworts, they are of little value in ascertaining phylogenetic relationships. Liverworts are widely distributed over the world, but have their greatest diversity in the tropics of the Americas and East Indies.