Comparative ultrastructure of the epidermal ciliary rootlets and associated structures in species of the Nemertodermatida and Acoela (Plathelminthes) |
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Authors: | K Lundin |
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Institution: | Department of Zoology, University of G?teborg, Medicinareg. 18, S-41390 G?teborg, Sweden Fax: +4631 416729; e-mail: k.lundin@zool.gu.se., SE
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Abstract: | The fine morphology of epidermal ciliary structures in four species of the Nemertodermatida and four species of the Acoela
was studied, with emphasis on Meara stichopi (Nemertodermatida). The cilium of M. stichopi has a distal shelf and is proximally separated from the basal body by a cup-shaped structure. The bottom of the cup consists
of a bilayered dense plate, or basal plate. The basal body consists of peripheral microtubule doublets continuous with those
of the cilium. In the upper part of the basal body, the doublets are set at an angle and are anchored to the enclosing cell
membrane by Y-shaped structures. The lower part of the basal body tapers eventually. The striated main rootlet arises on the
anterior face of the basal body, initially like a flattened strap, and continues along the basal body shaped as a tube which
further down becomes solid. The hour-glass-shaped posterior rootlet arises on the posterior face of the basal body. Contrary
to the main rootlet, the striations in the proximal part of the posterior rootlet run parallel to the microtubule doublets
of the basal body. A pair of microtubule bundles lead from the posterior rootlet to the two main rootlets in the hind ciliary
row, and follow these to their lower tip. In the other species of the Nemertodermatida studied, the structure of the ciliary
basal body and the ciliary rootlets is similar to that of M. stichopi. Structural differences in the species of the Acoela are that the lowermost end of the basal body is narrow and bent forwards,
the proximal part of the main rootlet is trough-shaped, the main rootlet is accompanied by a pair of lateral rootlets and
the posterior rootlet with associated microtubule bundles is thin. The epidermal ciliary structures in species of the Nemertodermatida
and Acoela have a number of shared characters which are unique within the Plathelminthes. However, almost all of these characters
are found in Xenoturbella bocki (Xenoturbellida), and some even in species of other ”phyla” of the ”lower” Metazoa. Hence, these characters cannot be considered
apomorphic for the Acoelomorpha. A character seemingly present only in species of the Nemertodermatida and Acoela is the bilayered
dense plate. This feature might represent an autapomorphic character state for the Acoelomorpha.
Accepted: 7 March 1997 |
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