A CLADISTIC-ANALYSIS OF PSEUDOCOELOMATE (ASCHELMINTH) MORPHOLOGY

We investigated possible relationships among pseudocoelomates (aschelm inths) by using cladistic analysis to uncover all most-parsimonious tr ees from a data set comprising 45 morphological characters of 11 phyla (3 acoelomate, 7 pseudocoelomate, and 1 coelomate). Analysis of our m atrix yielded 2 most-parsimonious trees, differing only in relative pl acement of 3 taxa (Kinorhyncha, Loricifera, and Priapulida). Tree topo logy suggests that the pseudocoel either evolved twice, or that it evo lved once and was then independently lost in Gastrotricha and in Gnath ostomulida. Tree topology also is consistent with derivation of the ps eudocoel from a coelom. From our analysis and that of other morphologi cally-based studies, we conclude that there are 2 main lines of pseudo coelomate evolution: (1) a minor clade (Acanthocephala + Rotifera) def ined principally by a syncytial epidermis, intracytoplasmic lamina, an d sperms with anterior flagella; (2) a major clade (the 5 remaining ph yla), defined primarily by a cuticle that is molted. Within the major clade are 2 subclades (Nematoda + Nematomorpha; Kinorhyncha + Loricife ra + Priapulida). Using morphological data, the chief issues remaining to be resolved are placement of Gastrotricha and Gnathostomulida, and evolution of the pseudocoel. No analysis based solely on morphologica l data has so far produced a tree congruent with those based on molecu lar data. Nevertheless, there is a general consensus that (1) the pseu docoel is of little phylogenetic significance and (2) the pseudocoelom ates are probably polyphyletic.