570 Biowissenschaften; Biologie 590 Tiere (Zoologie) 593 Wirbellose Tiere des Meeres und der Meeresküste
The study presented here shows the development of Antedon bifida starting with the doliolaria larva until juvenile stage based on ultrastructural and lightmicroscopical data. 3d-reconstructions of a doliolaria and of three young cystidean stages as well as a juvenile have been done. The related data have been complemented by serial sections of older cystidean and pentacrinoid stages. Special attention has been given to the development of the “ventral structures” in interradius CD, to the development of the axocoel and the fate of both the left and the right somatocoel. The fusion processes of these three coeloms as well as their potential contribution to the formation of the axial sinus (=axial coelom) and its associated hemal organs of the adult are investigated.
Part of the “ventral structures” in interradius CD are the primary stone canal, which is sent out by the hydrocoel, as well as the axocoel and the small ventral coelom, whose formation is traced in this thesis. Furthermore, there is a ventral mesenteric formation, which derives from the horizontal mesentery and from whose coelothelia the potential primary gonadal cells (PGCs) are formed. Enclosed in the ventral mesenteric formation resides the ventral lacuna, which is partially formed by cell protrusions sent out by the small ventral coelom’s coelothel and also bordered by the nearby coeloms. Structural and positional characteristics of the PGCs, the small ventral coelom and the ventral lacuna support the assumption that these are parts of a primary gonad anlage, whose fate in the adult is, however, unclear.
In contrast to former findings in Antedon mediterranea, the axocoel in the cystidean larva of A. bifida forms a process around the esophagus that may even become annular. The possibility is discussed in this thesis that this axocoelomic process takes part in fusion processes and that it is involved in formation of the periesophageal lacuna that extends on the right side of the esophagus from the ventral lacuna to the oral end of the glandular axial organ. The intimate proximity of the axocoelomic periesophageal process to the oral end of the glandular axial organ’s hemal sinus corroborates the widely accepted assumption that the latter is homologous to the axial organ of asterioids, ophiuroids and echinoids (AOE) and also supports an homology of the AOE axial organ to the Reichensperger’s organ of certain sea lily species (Isocrinida, Calamocrinus diomedae).
The primarily formed larval lacunae can be traced back to the mesenteries dissolved during the fusion processes. In this thesis, marker structures are defined, that allow tracing the development and the fate of the coeloms up to juvenile stage and may possibly also give an orientation in the adult. The detailed investigation of mesenteric remnants in older larval stages and in the juvenile furthermore shows that the left somatocoel extends aborally within the innermost intestinal coil (axial compartment). This is probably enabled by an inward directed tilting of the gut during intestinal outgrowth. Where the left and the right somatocoel come to lie against each other, a new mesentery forms, from whose extracellular matrix the hemal sinus originates. The present data support the heretofore proposed initial formation of the epithelial tubule of the glandular axial organ as a delamination of a cell strand from the coelothelium of the right somatocoel in intimate proximity to the proximal end of the prospective chambered organ in the doliolaria. However, a continued delamination from the right somatocoelothel, as assumed until now, is highly unlikely. The present data suggest that the tubule achieves an epithelial organization in the oralmost part of the prospective chambered organ and grows successively orally into the axial mesentery as well as aborally into the stalk.
Based on the origin discussed for both the periesophageal lacuna and the glandular axial organ it can be shown, that the axial sinus is largely formed by the left somatocoel. Based on the present results, the coelothelium that rests on the hemal space of the spongy body, also derives from the left somatocoel.
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