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The muscular system of Nemertoderma westbladi and Meara stichopi (Nemertodermatida, Acoelomorpha)

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Abstract

Nemertodermatida is a small taxon of marine worm-like animals; its position in the tree of life is highly contested. The musculature of Nemertoderma westbladi and Meara stichopi is studied here in detail using fluorescent phalloidin and confocal microscopy. In both species, the musculature is composed of an outer layer of circular and an inner layer of longitudinal musculature, diagonal muscles form a distinct layer in N. westbladi, but in M. stichopi these fibres connect to both other layers. The supraterminally opening male pore and antrum are formed by invagination of the whole body-wall in both species, and the seminal vesicle is lined by a thin net of musculature only in full male maturity. Modifications of the ventral body-wall adjacent to the mouth are small and transient in N. westbladi including no extra musculature, whereas it consists of additional strong U-shaped musculature in M. stichopi. Myogenesis in N. westbladi is not finished in hatchlings and will be completed dorsally in juvenile specimens and ventrally in male mature ones, after the loss of the mouth. Musculature between the two species differs considerably and might give insights into the internal relationships of Nemertodermatida and might prove to be useful in studies investigating their phylogenetic position. More data of other species and developmental changes are needed.

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Abbreviations

bwm:

Body-wall musculature

cl:

Cilia

cm:

Circular musculature (blue in sketches)

cw:

Cell web

db:

Dorsal bands of stronger longitudinal fibres in M. stichopi

dm:

Diagonal musculature (green in sketches)

eg:

Epidermal glands

ef:

Epidermal fibres

ep:

Epidermis

int:

Intestine

lm:

Longitudinal musculature (red in sketches)

m:

Mouth

ma:

Male antrum

mo:

Male opening

mco:

Male copulatory organ

mv:

Microvilli

ov:

Ovaries

pm:

Parenchymal musculature

st:

Statocyst with two statoliths

stm:

Statocyst musculature

sv:

Seminal vesicle

U:

U-shaped musculature (orange in sketches)

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Acknowledgments

We wish to thank the crew at the Sven Lovén Centre for Marine Sciences (Kristineberg), Sweden, and the marine station in Espegrend, Norway. The work was funded by the Swedish Research Council. OR received financial support from the Royal Swedish Academy of Sciences and Russian Basic Research Foundation grants 09-04-01309 and 13-04.02002. We are also grateful for constructive critique and advice from the two anonymous reviewers.

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Authors and Affiliations

  1. Department of Invertebrate Zoology, Swedish Museum of Natural History, Box 50007, 104 05, Stockholm, Sweden

    Inga Meyer-Wachsmuth & Ulf Jondelius

  2. Zoological Institute of the Russian Academy of Sciences, Universitetskaya emb., 1, 199034 St., Petersburg, Russia

    Olga I. Raikova

  3. Chair of Invertebrate Zoology, Saint-Petersburg State University, Universitetskaya emb., 7/9, 199034 St., Petersburg, Russia

    Olga I. Raikova

Authors
  1. Inga Meyer-Wachsmuth
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  2. Olga I. Raikova
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  3. Ulf Jondelius
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Correspondence to Inga Meyer-Wachsmuth.

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Communicated by A. Schmidt-Rhaesa.

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Meyer-Wachsmuth, I., Raikova, O.I. & Jondelius, U. The muscular system of Nemertoderma westbladi and Meara stichopi (Nemertodermatida, Acoelomorpha). Zoomorphology 132, 239–252 (2013). https://doi.org/10.1007/s00435-013-0191-6

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  • DOI: https://doi.org/10.1007/s00435-013-0191-6

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