Embryonic development of the priapulid worm Priapulus caudatus L. and its phylogenetic significance.
Uppsala University, Department of Earth Sciences, Palaeobiology, Norbyvägen 22, SE-752 36 Uppsala, Sweden.
Priapulida is a relatively poorly known phylum that is now classified as a member of the group Cycloneuralia (Ehlers et al. 1996) together with nematodes, kinorhynchs, loriciferans and nematomorphs. The reasonable fossil record of the priapulids, with stem-group members known from the early Cambrian, makes them interesting for studies of how body plans have evolved. In addition, priapulids are the only cycloneuralians that are likely to have a primitively large body size, and potentially at least, a relatively unmodified development. With this project the embryonic development of Priapulus caudatus has been monitored from the first cell stage until the "loricate" larva stage for the first time. The developmental series may help us to resolve the early evolution of Ecdysozoa (cycloneuralians plus arthropods). Still, important questions such as the origin of the body cavity, the nature of mesoderm formation and whether any remnant segmental features can be detected remain unanswered. One particularly important question that can be addressed with the help of this study is whether or not the segmental and coelomate features of the arthropods have been lost in the cycloneuralians or have been secondarily gained in the arthropods themselves. Resolving these questions would have an important bearing on our views of the bilaterian ancestry of these important morphological features, and thus on our understanding of the early fossil record of animals.
Ehlers, U., Ahlrichs, W., Lemburg, C. & Schmidt-Rhaesa, A., 1996: Phylogenetic systematization of the Nemathelminthes (Aschelminthes). Verh. Dtsch. Zool. Ges. 89 (1), 8.