Abstract
When stained with a phalloidin‐linked fluorescent marker and viewed with confocal scanning laser microscopy, the musculature of the gnathostomulid Gnathostomula armata Riedl 1971 can be seen to be dominated by longitudinal fibers and to have circular fibers only in the posterior half of the body, not the anterior half. Such an arrangement can be related to the animal’s ability to move through the small pore spaces in its environment, namely fine‐grained, poorly sorted sediments. The circular muscles may be used to push the anterior body half through restricting spaces, and the longitudinal elements may then pull the rest of the body through. Pharynx muscles visible in such preparations are positioned to move the cuticularized mouthparts in ways appropriate to scraping and grasping the bacteria and fungal hyphae on which gnathostomulids are believed to feed. Two arcs of muscles can protract and tilt the basal plate in scraping motions, and a pair of large diductor muscles on the jaws could open it against a spring‐like closing mechanism, while two pairs of muscles at either end of the jaws could tilt them inward in a pulling motion. The rest of the pharynx comprises a bulb of strong muscles arranged to compress its lumen.