AKA - an Annelid ally alienated:
Eeckhaut, I., D. McHugh, P. Mardulyn, R. Tiedemann, D. Monteyne, M.
Jangoux, & M. C. Milinkovitch. 2000. Myzostomida: a link between
trochozoans and flatworms? - Proceedings of the Royal Society London, B
267(1451):1383-1392.
Abstract: Myzostomids are obligate symbiotic invertebrates associated with
echinoderms with a fossil record that extends to the Ordovician period. Due
to their long history as host-specific symbionts, myzostomids have
acquired a unique anatomy that obscures their phylogenetic affinities to
other metazoans: they are incompletely segmented, parenchymous,
acoelomate organisms with chaetae and a trochophore larva. Today, they
are most often classified within annelids either as an aberrant family of
polychaetes or as a separate class. We inferred the phylogenetic position
of the Myzostomida by analysing the DNA sequences of two slowly evolving
nuclear genes: the small subunit ribosomal RNA and elongation factor-
1alpha. All our analyses congruently indicated that myzostomids are not
annelids but suggested instead that they are more closely related to
flatworms than to any trochozoan taxon. These results, together with recent
analyses of the myzostomidan ultrastructure, have significant implications
for understanding the evolution of metazoan body plans, as major
characters (segmentation, coeloms, chaetae and trochophore larvae) might
have been independently lost or gained in different animal phyla.
--
Geoff Read <g.read at niwa.cri.nz>
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