Invertebrate Reproduction and Development, 50:2 (2007) 57–66 57 Balaban, Philadelphia/Rehovot 0168-8170/07/$05.00 © 2007 Balaban Embryogenesis and larval development of Phoronopsis harmeri Pixell, 1912 (Phoronida): dual origin of the coelomic mesoderm ELENA N. TEMEREVA* and VLADIMIR V. MALAKHOV Biological Faculty, Invertebrate Zoology, Moscow State University, Moscow 119992, Russia Tel. and Fax: +8 (495) 939-4495; email:
[email protected] Received 12 December 2006; Accepted 26 March 2007 Summary Phoronids are marine invertebrates which great significance from the perspective of comparative anatomy. Phoronids have traditionally been described as deuterostomes because they exhibit developmental and larval traits similar to those traits found in echinoderms and hemichordates. However, molecular phylogenetic evidence has consistently identified the phoronids and brachiopods as a monophyletic group within the lophotrochozoan protostomes. The nature of egg cleavage, gastrulation and the origin of coelomic mesoderm can help to resolve this contradiction. These questions were investigated with Phoronopsis harmeri. Embryos and actinotroch larvae were cultivated in the laboratory, and embryonic and larval development was followed with SEM, light and video microscopy. Egg cleavage is radial, although the furrows of the 4th and succeeding divisions are oblique, which allows for blastula formation by the 16-cell stage. The ciliated blastula hatches 10 h after the onset of cleavage and acquires the apical tuft. Gastrulation proceeds by invagination. Coelomic mesoderm derives from two, the anterior and posterior, precursors. The anterior precursor forms by cell migration from the anterior wall of the archenteron, the posterior one by enterocoelic outpouching of the midgut. The lining of preoral coelom and tentacle coelom is derived from the anterior precursor, whilst the lining of trunk coelom originates from the posterior one.