The direction of evolution within the Polychaeta

The phylogenetic systematics of the polychaetes, i.e. the non-clitellate annelids, depend on which characters are regarded as belonging to the bauplan of the annelid stem species. The main competition is between two diametrically opposed hypotheses: the stem species was either (1) an errant, epibenthic organism with well-developed prostomium and prostomial appendages (antennae and palps), many homonomous segments, biramous and well differentiated parapodia, and numerous well-structured chaetae, or (2) a burrowing organism with small prostomium lacking appendages, which had many homonomous segments without parapodia, and only a few simple chaetae. (A third hypothesis, which is based primarily on certain morphological peculiarities and the presence of exclusively monociliary cells in Owenia, and which postulates a sessile stem species, is mentioned only peripherally for the present.) From a decision in favour of Hypothesis 2 it would follow that the Clitellata should be considered the most primitive annelids, so that the possession of parapodia and many extremely differentiated chaetae, for instance, would be interpreted as a highly derived character state. The consequence for the phylogenetic systematics of the Polychaeta is that oligochaete-like taxa would have to be considered more primitive than, for example, nereidid-like taxa. On the basis of Hypothesis 1, the evolution of these structures would have proceeded in the opposite direction, and polychaete systematics would have the reverse arrangement. The most important evidence for Hypothesis 2 comes from functional morphological considerations; namely the inference that metamerism has arisen from a burrowing mode of life. It is shown here that (1) this hypothesis rests partly on ignorance of the close relationship between reproductive biology and morphology in the clitellates, (2) the notion that metamerism, and hence the stem species of the Articulata, originated from a burrowing life in the marine environment is unconvincing, and (3) the origin of metamerism can be explained quite differently with reference to modern ultrastructural findings. According to these findings, septa, which are the fundamental structural elements for annelid segmentation, evolved as a morphological pre-requisite for the development of transversely running blood vessels; other purposes of septa (e.g. subdivision of the hydrostatic skeleton), therefore, have to be regarded as secondary. A highly complex blood vascular system may have been the consequence of the development of lateral parapodia-like appendages. Thus, parapodia are assumed to be part of the ground pattern of the Articulata and hence were present in the stem species of the Annelida. This is consistent with the traditional interpretation of annelid systematics, which places the errant polychaete taxa at the base of the system (Hypothesis 1).