A Muscular Double Helix in Gastrotricha
Studies on the muscular system of gastrotrichs revealed a new muscle arrangement. Helicoidal muscles, in right- and left-hand spirals, were found in all 9 species of Gastrotricha examined. The macrodasyidans Acanthodasys aculeatus, Tetranchyroderma papii and Thaumastoderma heideri had helicoidal muscles along the entire digestive tract (pharynx and intestine), whereas Macrodasys caudatus and Turbanella cornuta displayed helicoidal muscles mainly along the pharynx. Representatives of the order Chaetonotida, Chaetonotus aculifer, Draculiciteria tessalata, Lepidodermella squamata, and Xenotrichula
intermedia, all had helicoidal muscles on the intestine, while pharyngeal helicoidal muscles were not observed in C. aculifer or L. squamata. Helicoidal muscles are the thinnest-diameter muscles in the gastrotrich body and generally surround splanchnic circular muscles and longitudinal bands. The proposed functions of helicoidal muscles include antagonization of radial dilations of the digestive tract during feeding, pressurization of the intestine and closure of the pharyngeal-intestinal valve in chaetonotidans, and prevention of intestinal kinking during changes in body shape. The presence of this unique muscle orientation may be a synapomorphy for Gastrotricha.