THE FUNCTIONAL-MORPHOLOGY OF STARFISH TUBE FEET - THE ROLE OF A CROSSED-FIBER HELICAL ARRAY IN MOVEMENT

The morphology and mechanics of the tube feet, ampullae, and lateral a nd radial canals of the water vascular systems of Luidia clathrata and Astropecten articulatus (Echinodermata, Asteroidea) were analyzed. Hi stological methods, based on embedding in both paraffin and glycol met hacrylate, were used to document the arrangement of muscle and connect ive tissue. The tube foot wall includes longitudinal muscles and conne ctive tissue fibers, the latter arranged in a crossed-fiber helical ar ray, with a fiber angle of about 67 degrees in elongated tube feet. No evidence was found for the circular rings of connective tissue report ed in earlier studies; the appearance of rings is probably an artifact of folding. The ampullae are bilobed and include circumferentially ar ranged muscle fibers and connective tissue fibers aligned 90 degrees t o the muscle. The lateral canals are short and equipped with one-way f lap valves similar to those described for other echinoderms. The radia l canal is thin-wailed, nonmuscular, and enclosed in the connective ti ssue and ossicles of the ambulacrum. Frame-by-frame video analysis of both intact animals and animals with ''windows'' cut in the arm wall w as used to document the movements of the tube feet and ampullae. No ev idence was found for the previously suggested role of the radial canal in protracting the tube feet. The ampullae protract the tube feet and antagonize the tube foot musculature. The fiber angle of the connecti ve tissue allows protraction and prevents dilation of the tube feet, a nd limits elongation of the ampullae.