Continent world ultimately persisted with these being regarded as separate continents. From the late 18th century, some geographers started to regard North America and South America as two parts of the world, making five parts in total. Overall though, the fourfold division prevailed well into the 19th century. Europeans discovered Australia in 1606, but for some time it was taken as part of Asia. By the late 18th century, some geographers considered it a continent in its own right, making it the sixth (or fifth for those still taking America as a single continent). In 1813, Samuel Butler wrote of Australia
Smallest organisms on the archaeon "Ignicoccus"; it must be in contact with the host organism to survive. Prasinophyte algae of the genus "Ostreococcus" is the smallest free-living eukaryote. The single cell of an "Ostreococcus" measures only 0.8 μm across. Several species of Myxozoa (obligately parasitic cnidarians) never grow larger than 20 µm. One of the smallest species ("Myxobolus shekel") is no more than 8.5 µm when fully grown. The shell of the nut clam "Condylonucula maya" grows only 0.54 mm long. The smallest water snail (of all snails) is "Ammonicera minortalis" in North America, originally described from Cuba. It measures 0.32 to