When excluding internal parasites, free living species in terrestrial environments are represented by the following ten phyla : Flatworms ( Planarians ), Nemertea ( ribbon worms ), Nematoda ( roundworms ), Rotifers, Tardigrada ( water bears ), Onychophora ( velvet worms ), Arthropods, mollusks ( gastropods : land snails and slugs ), Annelida and Chordata ( tetrapods ).
