Chaetognaths, whose name means “bristle-jaw,” can be found all over world, swimming in brackish estuaries, tropical seas and above the deep dark ocean floor. Also known as arrow worms, the creatures have been around since the Cambrian Period, but their precise place in evolutionary history has long eluded scientists. Now, researchers at the Okinawa Institute of Science and Technology Graduate University (OIST) have learned where arrow worms wiggle on the Tree of Life, and their results could reveal important trends in the evolution of bilateral organisms.