ABSTRACT
The position of the flatworms within the animal kingdom has long seemed fundamental to our understanding of the evolution of the Bilateria due to the widely held idea that they are the most basally branching triploblastic animals, and hence progenitors of all other animal body plans. The standard theory of the emergence of the triploblasts postulates the appearance of increasingly complex animals, starting with the evolution of a bilaterally symmetrical acoelomate worm from a radially symmetrical gastrula-like planuloid ancestor (represented by the cnidarian planula). From these acoelomate worms the pseudocoelomates were thought to have evolved and subsequently to have given rise to coelomate protostomes and deuterostomes. Although schemes differ in their particulars, the position of the flatworms as the most basal triploblasts is broadly accepted such that their apparent simplicity and lack of anus and body cavity is considered to be primitive. Any flatworm characters shared by other triploblasts were also considered primitive characters such as the spiral cleavage seen in the spiralian taxa. An alternative to this acoeloid scheme, championed by European workers (e.g., Slewing 1980a), reversed the logic and emphasized the apparently derived nature of flatworm embryogenesis, in particular the spiral mode of cleavage, and suggested that flatworms are related to the other spiralian taxa (molluscs, annelids, etc.) and hence have secondarily lost the coelom and anus. This distinction has enormous consequences for our understanding of the evolution of the Metazoa.
