Musculature of Gnathostomula armata Riedl 1971 and its ecological significance

When stained with a phalloidin-linked fluorescent marker and viewed with co nfocal scanning laser microscopy, the musculature of the gnathostomulid Gna thostomula armata Riedl 1971 can be seen to be dominated by longitudinal fi bers and to have circular fibers only in the posterior half of the body, no t the anterior half. Such an arrangement can be related to the animal's abi lity 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 ele ments may then pull the rest of the body through. Pharynx muscles visible i n such preparations al e positioned to move the cuticularized mouthparts in ways appropriate to scraping and grasping the bacteria and fungal hyphae o n which gnathostomulids are believed to feed. Two arcs of muscles can protr act and tilt the basal plate in scraping motions, and a pair of large diduc tor muscles on the jaws could open it against a springlike closing mechanis m, while two pairs of muscles at either end of the jaws could tilt them inw ard in a pulling motion. The rest of the pharynx comprises a bulb of strong muscles arranged to compress its lumen.