The Nemerteans show the most advanced development of all the acoelomate phyla, in some books they are also referred to as the Rhynchocoela, Nemertinea, Nemertina or Nemertini. They are a quite successful phylum (900 named species)of mostly marine worms that are generally characterised by being extremely long and thin. Lineus longissimus has been recorded being 30 metres long while still only a few millimetres wide and some scientists estimate they may reach 60 metres in length, if a specimen this long is ever found it will become the longest animal in the world. Nature likes to experiment however so not all Nemerteans are long thin marine worms, some live in freshwater Potamonemertes sp. and Dichonemertes sp., a few are even terrestrial in damp habitatsGeonemertes sp., while others are quite chubby, Dinonemertes investigatoris can reach 5.6cm or 2.5 ins in width. The Nemertea are believed to be an ancient order with their origins in the Cambrian period over 500 millions years ago, however because of their soft bodies fossil Nemerteans are nonexistent as far as I know