Resembling a discarded purple sock, Xenoturbella has baffled scientists since its early 20th century discovery on the seafloor off the coast of Sweden. Its classification among the many creatures in the animal kingdom has shifted like the tides as each new bit of information about it has surfaced. Furthermore, Xenoturbella's proper placement on the tree of life has long been a matter of debate among evolutionists. Now, analysis of four newly discovered species from the Pacific Basin has refined its taxonomic position and led evolutionists to declare with confidence that Xenoturbella can help us learn how animals and humans evolved their internal organs.
What Are They Like?
With no brain and no intestinal tract or other organs, Xenoturbella was initially thought to be a sort of flatworm. Like us, it is bilaterally symmetrical, but it has only a single opening through which food enters and later returns. Flatworms, such as tapeworms, flukes, and planaria, likewise only have one gut opening. But then a DNA analysis revealed Xenoturbella had mollusk DNA, so scientists thought perhaps it was a mollusk that had lost a lot of features. Later, scientists realized that Xenoturbella probably eats mollusks, illustrating an amusing problem with the aphorism "You are what you eat" and removing Xenoturbella from the mollusk part of the animal kingdom. Some thought the animal was somehow related to acorn worms or perhaps sea urchins and starfish, but its lack of gill slits casts a shadow on that possibility. Many evolutionists wondered whether Xenoturbella was a de-evolved, super-simplified descendant of a more complex creature, maybe the descendant of a "deuterostome" that had originally had a digestive tract with two openings. That was one of the leading thoughts on the matter until the authors of the recent genomic study concluded that Xenoturbella has probably always been as simple as it appears.
Ranging in color from pale pink to purple, orange, and bright pink, this gutless wonder has no excretory system, no gonads, and only a diffuse scattering of nerves. In short, it has almost no internal anatomical structures at all, making it even simpler than flatworms. Like many invertebrates, Xenoturbella does have a sac-like statocyst, a balance receptor containing flagellated cells and a little stone-like structure that bumps the flagellae when the animal changes its position, functioning much like the simple level from your toolbox.
Xenoturbella have been found in water of varying depths. The known species range in size from about 1.5 to 8 inches in length. Each has an oval or diamond-shaped mouth on its underside. No one has ever seen Xenoturbella eat, and because it doesn't have teeth or scraping mouthparts or even a suctioning proboscis, scientists aren't sure how it eats. When scientists cut them open, they don't find any mollusk bits inside. But the earlier DNA analysis and Xenoturbella's frequent association with clam beds, whether near a cold seep, a hydrothermal vent, or an undersea whale carcass, have led to the thought that Xenoturbella probably preys on mollusks.