The world's oldest living species?

Although Triops cancriformis is the world's oldest known living animal species, it is not necessarily the world's oldest living animal species per se.  The likely situation is that the world's oldest living animal species is some species of primitive worm or maybe even a jellyfish or other soft-bodied invertebrate.  Many of these orders of animal life were around much earlier than arthropods but lack hard structures necessary to leave the sort of fossil records required for species identification.   As such, whether really deserving of the title or not, the award goes to T. cancriformis.