Die Entwicklung ophiocephaler Pedizellarien vonSphaerechinus granularis (Echinodermata, Echinoidea)

Summary

The development of the ophiocephalous pedicellariae ofSphaerechinus granularis was investigated by means of a continuous series of regenerating pedicellariae with the aid of a scanning electron and light microscope. The anlagen of the three valves appears in the form of isolated triradiate skeleton rudiments; the central axis arises from their center. At the deepest level, the basal plate develops from this horizontal structure, which at last forms the handle segment of the valve. Two more horizontal plates are built above this basal plate, which are supported by spool-shaped elements; around these skeletal elements the connecting collagenous fibers are arranged. Starting from the central axis, five parallel skeletal rods arise under regular ramification and arrangement; they are connected to one another by a meshwork forming the central stem (axis tube). In a similar way, the muscle baskets on both sides of the axis tube are built up, arising from the level of the first horizontal plate. As a median support of the axis tube against the horizontal plate, a vertical, perforated plate develops with the differentiation of the central articulations at its free margin. This middle segment of the valve, as well as the handle segment, proceeds in its development, while the distal, biting segment is more retarded in its increase. Both facts can be proved by function. The final differentiation of the margins of the distal valve segment with regard to the precise meeting of the gripping teeth takes place under the influence of the beginning movements of the valves. Therefore, full differentiated articulations and adequate muscle systems are necessary.

Coincidently, reorganization takes place as long as the central articulations and the distal, biting valve segment are joined to a functional unit, with the muscle baskets for the Mm. adductores at both sides. In the same way, the handle segment, the superstructure of the insertions of the Mm. abductores at the outside of the valve, and the supporting structures of the distal segment of the valve all evolve into a functional unit. The primary, rather descriptive formation of a valve into a distal, middle, and handle-bearing segment gets partly lost from this point of view.

This reconstruction also becomes clear by the decrease of the perforated plate structure for the benefit of a framework construction by apposition of compact material at stronger loaded zones; highly stressed structures as articulations receive a layer of polycrystalline calcite. A similar allometry, also based on functional conditions, can be seen in the three segments of the entire pedicellariae — head, neck, and stalk — because the neck, which is the essential part for the later high mobility of the head, does not start growing before the final organization of both other segments.

Zusammenfassung

Die Entwicklung ophiocephaler Pedizellarien beiSphaerechinus granularis wurde anhand einer kontinuierlichen Reihe von Regeneraten rasterelektronenoptisch und lichtmikroskopisch studiert. Nach Anlage der skelettogenen Grundelemente zeigt sich eine deutliche Wachstumsallometrie der 3 Klappenabschnitte, indem der Bügelabschnitt und besonders der mittlere Klappenteil in der Entwicklung gegenüber dem distalen beißenden Teil vorauseilen. Die Enddifferenzierung der Klappenränder mit der genauen Passung ihrer Zähnchen, das Einschleifen der Gelenke und die endgültige Ausgestaltung der Bügel erfolgen unter dem formenden Einfluß der beginnenden Bewegung. Die ursprüngliche deskriptive Gliederung in distalen, mittleren und bügeltragenden Abschnitt wird mehr und mehr durch funktionelle Einheiten ersetzt; gleichzeitig wird auch die anfängliche Lochplattenstruktur von einer Rahmenkonstruktion abgelöst. Eine gleichfalls funktionell begründbare Allometrie ergibt sich in der Entwicklung von Köpfchen, Hals und Stiel des gesamten Pedizellars.