In some biological taxonomy schemes, protists were a large group of diverse eukaryotic, mainly unicellular microorganisms, that do not form tissues. Formerly, these were assigned to the now-obsolete kingdom Protista. However, in modern taxonomy the Protista are understood to be paraphyletic (not a clade), so the term remains in use only for convenience, similar to "invertebrate". An equivalent term Protoctista is used for these organisms by various organisations and institutions. Molecular analyses in modern taxonomy have been used to redistribute former members of this group into diverse and sometimes distantly related phyla. When used, the term “protists” is now considered to mean similar-appearing but diverse phyla that are not related through an exclusive common ancestor, and which have different life cycles, trophic levels, modes of locomotion, and cellular structures. Besides their relatively simple levels of organization, the protists do not have much in common.