Ultrastructure of the preseptal part of metanephridia in Nais variabilis and Dero digitata (Annelida, Clitellata) |
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Authors: | D Bunke |
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Institution: | (1) Zoologisches Institut, Lehrstuhl für Spezielle Zoologie, Auf der Morgenstelle 28, 72076 Tübingen, Germany e-mail: dieter.bunke@uni-tuebingen.de Tel.:+49-7071-2972617, Fax: +49-7071-294634, DE |
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Abstract: | The composition and arrangement of cells in the preseptal region of metanephridia have been examined by ultrastructural methods
in two naidid species, Nais variabilis and Dero digitata. Within this region special attention has been paid to the portion around the orifice and the region where the metanephridium
penetrates the septum. In N. variabilis, the preseptal region is composed of four cells and, in D. digitata, three cells are present. In both species three cells correspond in position and ultrastructural details and, hence, are interpreted
as homologous. These are the mantle cell, the flame cell, and the canal cell. The mantle cell covers the preseptal region
and surrounds the opening. The margin around the orifice is endowed with cilia, which extend into the coelomic space and beat
irregularly. They do not enter the orifice and, thus, are not part of the internal ciliary flame. Posteriorly, in D. digitata, the mantle cell originates from the septal wall, i.e., its extensions spread in the plane of the frontal coelothelium of
the septum. In N. variabilis, the mantle cell is continued by a further cell, enwrapping the posterior region of the preseptal part. This cell, called
the septal cell, is anchored in the septal wall like the mantle cell in D. digitata. Both cells are interpreted as mesodermal components of the metanephridium. The flame cell lies beneath the mantle cell.
In front, on its dorsal wall, many cilia are inserted which extend posteriorly into the nephridial canal forming a flame.
In D. digitata, the caudal extension of this cell was examined in more detail; it originates from an intraseptal position. The canal cell
lines the anterior lumen of the nephridial duct. While the mantle cell and flame cell enclose the organ from a dorsal position,
the canal cell lies opposite embracing the lumen from a ventromedial position. Behind, it extends into the postseptal region
for a certain distance. It is concluded that metanephridia in the Clitellata have a coelothelial component and, probably,
are not just descendants of a single cell, the nephridioblast. The results further indicate that a flame cell and a mantle
cell or some corresponding coelothelial cells may be constitutive elements of the ground plan of the clitellate metanephridium.
Phylogenetic consequences for non-clitellate Annelida are discussed.
Accepted: 21 December 1999 |
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