Introduction

The pseudocoelomates have always been the most trying group of invertebrates as far as phylogeneticists are concerned, and are certainly destined to retain this status for the foreseeable future. All the animals concerned are small (in diameter at least, though some are very elongate), and all are more or less soft-bodied, so there is little possibility of obtaining evidence from the fossil record. About eight distinct groups of animals can be included in this assemblage, and at some time or other almost every possible permutation of affinities between them has been proposed. There is not even a consensus about their rank – do they collectively constitute one phylum, should some of the animals be set apart as one phylum and the rest placed elsewhere, or is each group of phyletic status in its own right? Are any of them related to each other at all, in fact?

Classically, a sub-set of these phyla have often been united as the phylum Nemathelminthes, or more recently Aschelminthes. Hyman (1951b) included six groups in her aschelminth assemblage: the gastrotrichs, rotifers, kinorhynchs, nematodes, nematomorphs, and priapulids. She set the acanthocephalans and entoprocts apart as separate groups. Other authors and many subsequent text-books have followed similar schemes, though the priapulids have sometimes been taken out since there has been controversy about the nature of their body cavity. However, there have also been recent attempts to ally the priapulids with the acanthocephalans (Conway Morris & Crompton 1982), or the rotifers with the acanthocephalans (Lorenzen 1985), the latter author reinstating priapulids with a kinorhynch/gastrotrich/nematode assemblage.