Cycliophora is a very recently described phylum of acoelomate metazoans with a complex life cycle and a phylogenetic position that has been under debate ever since its discovery in 1995. Symbion americanus, which lives attached to the mouthparts of the American lobster, Homarus americanus, represents the second species described for the phylum. Aiming to increase the morphological knowledge about this cryptic clade, the present study describes the muscle arrangement of the feeding stage, the attached Prometheus larva with the dwarf male inside, the free living male, the Pandora larva, and the chordoid larva of S. americanus using actin staining and confocal laser scanning microscopy. 3D reconstructions of the muscular systems are presented. In the feeding stage, circular muscles compose the buccal funnel aperture. In addition, a pair of muscles runs longitudinally in the buccal funnel. A complex sphincter was found just proximally to the anus, and six longitudinal muscles run from the trunk constriction ("neck") in basal direction. The musculature of the larval stages and the dwarf male is very complex and includes longitudinal muscles that run dorsally and ventrally. In addition, we found dorso-ventral muscles. The male has a complex posterior muscle apparatus in the vicinity of the penis. In this stage, X- and V-shaped structures were identified on the dorsal and the ventral side, respectively. Pandora and chordoid larvae possess additional circular muscles. We discuss our findings with respect to muscle elements of other metazoan groups and the chordoid larva of Symbion pandora. Copyright 2008 Wiley-Liss, Inc.