Lynn Margulis classified the Chaetognaths, known as arrow worms, as deuterostomes. Deuterostomy is characterized by  several developmental characteristics including radial, indeterminate cleavage, a posterior position of the blastopore (deuterostomy=”second mouth”), enterocoelous coelom formation and a tripartite adult body plan with a post-anal tail. At least this is what I was taught “growing up”. Three . . . → Read More: What in Darwin’s Name Are Chaetognaths?!