The phylum Priapulida is a small group of marine worms that is allied with the nematodes, kinorhynchs, loriciferans and nematomorphs in a clade called the Cycloneuralia or Introverta. Together with the arthropods they are generally considered to comprise the Ecdysozoa, a clade of moulting animals. A number of recent priapulid species possess features that resemble the predicted Ecdysozoan ancestor. In addition, recent molecular studies have also shown that they are basal within the Ecdysozoa/Cycloneuralia (Garey 2001, Webster et al. 2006). Their putative basal position thus makes priapulids highly interesting research objects for understanding the evolution of Ecdysozoa.
Earlier investigations of the early embryology of the priapulid Priapulus caudatus are critically revised with the aid of modern techniques and equipment, confirming earlier studies that the early cleavages are highly symmetrical, total, subequal, radial and stereotypical. New results show that up to the sixth cleavage, the spindles are oriented along the animal/vegetal axis at both poles. This unique cleavage pattern has only limited similarities to other animals. During the sixth cleavage two cells move inwards and gastrulation commences. If the mesoderm is derived from both cells, its origin differs from that of many other protostomes.
Two previously undescribed larval stages of P. caudatus; the light bulb shaped hatchling and the first lorica larva are described. The second lorica larva superficially resembles the previously described type 2 lorica larva (Higgins et al 1993). Differences between the second lorica larva and the type 2 lorica larva, with respect to possible ecophenotypical variation and sub-specialization, are described.
Preliminary data are presented on musculature development of P. caudatus. Preliminary data have also been obtained on the early development of a second priapulid, Halicryptus spinulosus. Comparison of Halicryptus and Priapulus may help to resolve developmental ground pattern of the priapulids.