Ultrastructure of the nuchal organ and cerebral organ in Onchnesoma squamatum (Sipuncula, Phascolionidae) |
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Authors: | G Purschke F Wolfrath W Westheide |
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Institution: | Spezielle Zoologie, Fachbereich Biologie/Chemie, Universit?t Osnabrück, D-49069 Osnabrück, Germany Fax 0541 969/2870 e-mail purschke@cipfb5.biologie.uni-osnabrueck.de, DE
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Abstract: | The ultrastructure of the nuchal organ and cerebral organ is described for the first time in a species of the Sipuncula,
Onchnesoma squamatum. The nuchal organ is an unpaired structure lying outside and dorsal to the tentacular crown; furrows give the organ a paired
appearance. The cerebral organ is an unciliated pad anterior to the nuchal organ. The nuchal organ consists of ciliated supporting
cells, non-ciliated supporting cells and bipolar primary sensory cells. The cerebral organ is composed of unciliated supporting
cells and numerous bipolar sensory cells. This clearly favours the hypothesis that this structure has a sensory function in
adults rather than being a vestige of a larval organ. The sensory cells are similar in both organs and exhibit features indicative
of chemoreception. Since the density of the sensory cells is low in the nuchal organ, an exclusively sensory function is questioned.
There is some evidence that the two organs represent a functional unit. The present findings do not support the view that
the nuchal organs of Sipuncula and ”Polychaeta” are homologous, but instead suggest that they are convergent structures.
Accepted: 18 September 1996 |
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