The oldest known examples of a bilaterian, bilateral animals. It means they had a a left and right side, a top and bottom, and a anterior-posterior organization. It lived some 580 to 600 million years ago, probably feeding on microbes as small vacuum cleaners on the seafloor. Vernanimalcula means small spring animal, referring to the period they lived in, which were after glaciers covered most of the planet (see also Snowball Earth). Not bigger than between a tenth and two-tenths of a millimeter across, roughly the width of one or two human hairs, it was not a big animal. It was surprisingly complex, and had paired coeloms, gut, paired external pits that could be sense organs, triploblastic structure, internal organs, anterior suctionlike and muscular mouth and a posterior anus. The structural complexity belongs to an adult rather than larval form. These anatomically details makes them more advanced than flatworms, which have a much simpler anatomy. This could mean the flatworms an other bilateral animals with no true coelom could have evolved from more complex animals through progenesis from the acoeolomate larvae and juveniles of a coelomate adult ancestor. The flatworms are protostomes, while Xenoturbella belongs to the deuterostomes. Also these deuterostomes lacks a coelom, plus a brain, gonads, gut and other inner organs. Since the larva has most of the features the adults lacks, this proves they evolved from more complex animals. Which again suppurts a theory that says the first bileteralian animals were quite advanced, relatively big, segmented organisms with a coelom. The Snowball Earth hypothesis attempts to explain a number of phenomena noted in the geological record by proposing that an ice age that took place in the Neoproterozoic was so severe that the Earths oceans froze over completely, with only heat from the planetary core causing some liquid water... A body cavity is an aspect of a number of basic animal body plans (phyla) that incorporate a central body cavity, known as a coelom. ... Classes Monogenea Trematoda Cestoda Turbellaria The flatworms (Platyhelminthes, Greek platy: flat; helminth: worm) are a phylum of relatively simple soft-bodied invertebrate animals. ... Xenoturbella is a genus of Bilaterian animals; it is a marine worm. ...