Immunocytochemically determined regulatory proteins, troponin, calponin and caldesmon, may occur together in the musculature of a Gordian worm (Ecdysozoa, Cycloneuralia, Nematomorpha).

Based on an electron microscopic immunocytochemical study of the body wall musculature in a Gordian worm, the three regulatory muscle proteins troponin, calponin and caldesmon appear to be present. Given that no caldesmon homolog has been identified in the genome of invertebrates to date, the result seemed surprising. Moreover, troponin and calponin/caldesmon should exclude each other. However, in view of the fact that troponin has recently also been found in non-striated invertebrate muscle and the smooth muscle proteins calponin and closely related proteins can occur in invertebrate oblique muscle types, it seems possible that in the body wall of Gordian worms a muscle type occurs that combines the features of smooth and striated muscle. Similarities to the <italic>Caenorhabditis elegans</italic> muscle underscore the phylogenetic relationship between the ecdysozoan phyla Nematoda and Nematomorpha. [ABSTRACT FROM AUTHOR]