Acoelomorpha

The Acoelomorpha are a group of animals with features that were once put in the phylum Platyhelminthes (flatworms).
In 2004 molecular studies showed this was wrong.
Now it is clear that their closest relatives are the Xenoturbellida, and the two groups make up the proposed new phylum, the Xenacoelomorpha.
Most researchers believe them to be basal among the bilateria, slightly more derived than the Cnidaria. Recent results suggest that they (along with "Xenoturbella") may lie near the base of the deuterostomes. An ongoing collaborative research project has "the researchers ... confident that they can reach an agreement about where acoels fit in evolutionary history".
Acoels are almost entirely marine, living between grains of sediment, swimming as plankton, or crawling on algae. Acoels have a statocyst, which presumably helps them orient to gravity. Their soft bodies make them difficult to classify.