Die vorliegende Arbeit befasst sich mit Aspekten der frühen Entwicklung der Phoronida („Hufeisenwürmer“). An drei Arten wird der Furchungsprozess untersucht (Phoronis pallida, Phoronis muelleri, Phoronis vancouverensis). Dies erfolgt sowohl mithilfe der 4D-Mikroskopie als auch anhand von immunocytochemischen Markierungen der Mitosespindeln und konfokaler Laser-Scanning-Mikroskopie. Verschiedene morphologische Merkmale des Furchungsprozesses werden quantitativ erfasst und innerhalb sowie zwischen den Arten verglichen. Die Ergebnisse zeigen eine weitgehend übereinstimmende Furchung bei P. pallida und P. muelleri Embryonen: Ab dem dritten Zellzyklus teilen sich die Blastomeren meist schräg – und alternierend dextral und sinistral – zur animal-vegetativ Achse. Dieses Muster zeigt überraschende Übereinstimmungen mit dem Muster der Spiralfurchung. Dies kann als morphologische Unterstützung molekular-phylogenetischer Befunde einer Stellung der Phoronida innerhalb der Spiralia/Lophotrochozoa interpretiert werden. Die Furchung bei P. vancouverensis unterscheidet sich von der Furchung der anderen beiden Arten; sie weist jedoch auch Unterschiede zu einer Radiärfurchung auf. Generell zeigt die Furchung aller drei Arten einen gewissen Grad an Variabilität. Anhand von in-vivo Einzelzellmarkierungen untersucht die Studie darüber hinaus das Schicksal der Blastomeren früher P. pallida Embryonen bis zu späten Gastrulationsstadien. Diese Analysen zeigen, dass die ersten beiden Furchungsteilungen durch die spätere Achse Blastoporus-Apikalplatte, jedoch in keinem konstanten Orientierungsverhältnis zur Ebene der Bilateralsymmetrie der Gastrula verlaufen. Dies unterscheidet sich von der Situation, wie sie von spiralfurchenden Tieren bekannt ist. Die Unterschiede und die beobachtete Variabilität des Furchungsprozesses werden im Licht unterschiedlicher Mechanismen der Spezifizierung von Zellschicksalen und Körperachsen bei verschiedenen Taxa der Spiralia und den Phoronida diskutiert. / This study addresses aspects of the early development of Phoronida (“horseshoe worms”). The cleavage process is analyzed for three species (Phoronis pallida, Phoronis muelleri, Phoronis vancouverensis). These investigations are performed using 4D-microscopy as well as immunocytochemical stainings of the mitotic spindle apparatuses in combination with confocal laser-scanning microscopy. Different morphological features of the cleavage process are quantified and compared within as well as between the species. The results reveal a highly consistent cleavage of P. pallida and P. muelleri embryos: from the third cell cycle onward, the blastomeres divide mostly obliquely – and alternatingly dextral and sinistral – with respect to the animal-vegetal axis. This cleavage pattern shows surprising correspondences to the pattern of spiral cleavage. The finding can be interpreted as morphological support for recent molecule-based phylogenies, which indicate a position of Phoronida within the Spiralia/Lophotrochozoa clade. The cleavage of P. vancouverensis differs from the cleavage in the other two species; however, it also shows differences to a radial cleavage pattern. In all three species, the cleavage process also involves some degree of variability. Furthermore, the study traces the cell fates of early P. pallida embryos up to the state of late gastrulation, by the use of fluorescent in-vivo single cell markings. These analyses reveal that the first two cleavage divisions both pass through the later axis blastopore-apical plate of the gastrula, yet they do not pass in a constant relationship with respect to the later plane of bilateral symmetry. This differs from the situation known from spiral cleaving animals. The differences and the encountered variability of the cleavage process are discussed with respect to different mechanisms of the specification of cell fates and body axes in different taxa of the Spiralia and the Phoronida.

The evolutionary origin and the phylogeny of higher metazoan taxa is still under debate although considerable progress has been made in the past 20 years. Metazoa represents a monophyletic group of highly diverse animals including Bilateria, Cnidaria, Porifera, Ctenophores, and Placozoa. Bilateria comprises the majority of metazoans and consists of three major clades: Deuterostomia, Spiralia (= Lophotrochozoa sensu lato), and Ecdysozoa, whereas the sister group taxa Spiralia and Ecdyzozoa form the monophyletic clade Protostomia. Molecular data have profoundly changed the view of the bilaterian tree of life. One of the main questions concerning bilaterian phylogeny is the on-going debate about the evolution of complexity in Bilateria. It was assumed that the last common ancestor of Deuterostomia, Ecdysozoa and Spiralia had a segmented and coelomate body organization resembling that of an annelid. On the contrary, the traditional view is the evolution of Bilateria from a simple body organization towards more complex forms, assuming that the last common ancestor of Bilateria resembles a platyhelminth-like animal without coelomic cavities and segmentation. To resolve this question, it is necessary to unravel the phylogenetic relationships within Bilateria. By using mitogenomic and phylogenomic data, this thesis had a major contribution to clarify phylogenetic relationships within problematic metazoan taxa: (1) the phylogeny of Deuterostomia, (2) the questionable monophyly of Platyzoa, and first assumptions concerning the phylogeny of Gnathostomulida, Gastrotricha and Polycladida, (3) phylogenetic relationships within annelid taxa, especially Terebelliformia, Diurodrilidae, and Syllidae, with new insights into the evolution of mitochondrial gene order, and (4) new insights into the evolution of annelids, especially the interstitial ones, as well as the colonization of the interstitial realm.