The musculature of larvae of Gordius aquaticus was investigated by laser-scanning microscopy and compared to transmission electron microscopic data for the larva of Paragordius varius. In the anterior portion of the body, the preseptum, four different muscle groups can be distinguished: (1) 12 anterior parietal muscles in the body wall, (2) six oblique muscles that function as retractors of the introvert, (3) six proboscideal muscles, which function as retractors for the proboscis, and (4) six muscles associated with spines of the outermost of the three rings of spines. The posterior portion of the body, the postseptum, possesses four pairs of longitudinal muscle strands in G. aquaticus, the postseptal parietal muscles, that are located dorsolaterally and ventrolaterally. These are not clearly visible in P. varius, where instead three pairs of parietal muscles are present. Additional small muscles are associated with the terminal spines and with the duct running from the pseudointestine to the body wall. All fibers show a cross-striated pattern although this striation is less obvious at the ends of the fibers.
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We would like to thank Ben Hanelt for his permission to use larvae of Paragordius varius from his laboratory life cycle at the University of Nebraska at Lincoln and Fernanda Zanca for her help in collecting and culturing specimens of Gordius aquaticus. We are grateful to Eward E. Ruppert and an anonymous reviewer for valuable comments, and to Mechthild Krabusch for rearranging the TEM plates.
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Müller, M.C.M., Jochmann, R. & Schmidt-Rhaesa, A. The musculature of horsehair worm larvae (Gordius aquaticus, Paragordius varius, Nematomorpha): F-actin staining and reconstruction by cLSM and TEM. Zoomorphology 123, 45–54 (2004). https://doi.org/10.1007/s00435-003-0088-x