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Organization of statocysts in the Otoplanidae (Plathelminthes): an ultrastructural analysis with implications for the phylogeny of the Proseriata

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Summary

The statocyst of otoplanids is enveloped by a bipartite capsule which consists of two different extracellular matrices. This capsule encircles three different types of aciliary cells: several peripherally located flattened parietal cells, one central statolith forming cell (lithocyte) and two clusters of accessory cells. Intracapsular lumina exist which are different from extracapsular intercellular spaces. The accessory cells most probably represent those structures that are mainly involved in nervous conduction. These cells extend cytoplasmatic processes towards different peripheral regions of the statocyst where processes of outer nerve cells penetrate the capsule. The statocyst does not seem to represent a more evolved equilibrium receptor system but may function as a relatively simple aciliary sense organ suitable for positive geotactic behaviour. The otoplanid statocyst corresponds to statocysts in other lithophorous proseriates but not to statocysts in other taxa of the free-living Plathelminthes. The monophyly of a taxon Lithophora within the Proseriata is corroborated by this autapomorphic characteristic.

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Abbreviations

ac :

accessory cell(s)

c :

capsule of the statocyst

ce :

cerebrum

ci :

cephalic intestine

co :

capsule opening

cp :

cell process(es) of accessory cell(s) and cell(s) containing filaments

ecm :

extracellular matrix

fc :

cell(s) containing filaments

ic :

intercellular spaces within the capsule

mc :

muscle cell(s)

n :

lobed nucleus of the lithocyte

nac :

nucleus (nuclei) of accessory cell(s)

nc :

nerve cell(s)

npc :

nucleus (nuclei) of parietal cell(s)

pc :

parietal cell(s)

s :

statolith

sc :

statolith cell (lithocyte)

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  1. II. Zoologisches Institut und Museum der Universität Göttingen, Berliner Strasse 28, D-3400, Göttingen, Germany

    Ulrich Ehlers & Beate Sopott-Ehlers

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  1. Ulrich Ehlers
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  2. Beate Sopott-Ehlers
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Ehlers, U., Sopott-Ehlers, B. Organization of statocysts in the Otoplanidae (Plathelminthes): an ultrastructural analysis with implications for the phylogeny of the Proseriata. Zoomorphology 109, 309–318 (1990). https://doi.org/10.1007/BF00803571

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