THERE IS NO SUPPORT FOR JENSENS HYPOTHESIS OF NEMERTEANS AS ANCESTORSTO THE VERTEBRATES

Nemerteans (phylum Nemertea) have been viewed by most zoologists as de scended from, or closely related to, the flatworms. This view is based mainly on their supposedly acoelomate body. Their ancestry, however, is a point of controversy and there is evidence for a coelomate, proto stomous origin. Notwithstanding these different views, most zoologists consider nemerteans to be phylogenetically distant from the chordates . Four authors (Hubrecht, Macfarlane, Jensen, Willmer), however, have postulated that nemerteans instead are closely related to the chordate s and that they share a most recent common ancestor with the vertebrat es. We argue that this view is based on a flawed view of homology and of seeing evolution as a series of progressions, which has no support in modern evolutionary thinking. Since there are no morphological syna pomorphies supporting a Chordata-Nemertea clade, these authors instead guess what characteristics in extant nemerteans gave rise to characte rs observed in recent chordates. For example, they propose that the ne mertean proboscis sheath has evolved into the notochord. This is mere speculation, lacking testable propositions and is hence void of inform ation, and thus becomes futile in our view. However, the idea of a nem ertean-vertebrate sister relationship as such is a testable hypotheses , and we test it by applying the parsimony criterion to a set of morph ological characters, and a set of molecular (the 18S rRNA gene) charac ters. Both tests reject the hypothesis.