Your argument that the curious Xenoturbella flatworm represents a “crucial intermediate stage of animal evolution” (Nature 470, 161–162; 2011) perpetuates a popular misconception, stemming from a presumption that the features of such ancient living relics are intermediate between those of other extant creatures.

Today's organisms are all at the twig tips of one large tree of life, with no knowable connections between primitive and higher forms. Reproductively isolated populations of species, such as chimpanzees and humans, are not modifications on a 'ladder' of descent — thus living chimpanzees are not our ancestors, but a sister species adapted to a different habitat (tropical forests versus savannah).

Xenoturbella has largely maintained its internal structure and body shape over millions of years of evolution, during which stabilizing selection removed descendants that were less-well adapted to their environment than their parents.

Such 'living fossils' have always occupied a narrow ecological niche, apparently without ever experiencing much competition from more complex organisms, and so may serve as models for reconstructing crucial steps in animal evolution. But they do not represent 'intermediate' evolutionary forms in the way that some of the famous fossils from the Mesozoic, such as the feathered dinosaurs or ancient snakes with hind legs, are viewed as earlier, extinct connecting links in the tree of life.