ABSTRACT
Although still frequently described as radially symmetrical and diploblastic, there is a long-standing debate over the axes and the body layers of cnidarians and how it/they relate to those of Bilateria. Do some modern cnidarians illustrate an evolutionary stage through which bilaterians passed on the way to becoming overtly bilateral (Boero et al. 1998, Seipel and Schmid 2005), or are they an evolutionary offshoot that evolved from an already bilateral common ancestor? The standard textbook view has been that the Cnidaria are separated from the Bilateria by two major dichotomies. Firstly, they are considered to be radial, while Bilateria are bilateral. Secondly, they are considered to be diploblastic (have two body layers) in contrast to the triploblastic Bilateria. Further, it is commonly assumed that the oral end of Cnidaria, where the mouth lies, corresponds to the anterior end of Bilateria. Although these are widely prevalent views, none of them has gone undebated.
