Reference : Biol. Bull., 136: 288-300. (April, 1969) LEG EXTENSION IN LIMULUS DIANA VALIELA WARD Department of Zoology, Duke University, Durham, North Carolina 27706 The ancestors of arthropods were probably worm-like animals with hydrostatic skeletons. With the evolution of a rigid exoskeleton the hydraulic system lost im-portance as a means of support and leg extension and has apparently disappeared in most adult arthropod forms. Nevertheless hydraulic systems seem to persist in a few groups ; such animals typically show relatively large membranous areas where the integument is not stiff, and lack extensor muscles in one or more of their leg joints. Spiders, various myriapods, scorpions, and Limulus show both of these char-acteristics. These characteristics alone, however, are not sufficient evidence for the presence of a hydraulic system. Only in spiders is the existence of such a system well substantiated (Parry and Brown, 1959a, 1959b). In centipedes, millipedes, and scorpions the evidence for the presence of hydraulic leg extension is equivocal (Manton, 1958a). We have no evidence other than the above characteristics that a hydraulic system exists in Limulus. The problem is especially interesting in Limulus since the horseshoe crab is considered important in discussions of arthropod evolution. The femoro-patellar joints of all the walking legs of Limulus (Limulus poly-phemus) are hinge joints lacking extensor muscles (see Fig. 1). Nevertheless, in the power strokes of normal walking the femoro-patellar joints of Limulus perform up to 120 degrees of extension from the flexed state. This is the greatest angular displacement occurring at any joint in the legs. The mechanism of extension of these joints is not known. These extensions are important in performing walking motions of the legs, which propel the animal over intertidal mud flats and through surface sediments of the ocean floor. In this paper I present data concerning femoro-patellar extension in Limulus. I propose and provide evidence for a mechanical scheme of extension of these extensorless hinge joints. I conclude that the mechanism is neither hydraulic nor elastic. EXPERIMENTAL ANIMALS Live specimens of Limulus were obtained commercially from Panacea, Florida, and collected at Beaufort, North Carolina. The animals used for pressure measure-ments ranged from 20.0 to 23.5 cm in length from the front of the carapace to the base of the telson. Larger or smaller animals were used for other work. Some animals arrived in poor condition, showed below normal activity and died a few days later. These animals also showed below normal blood pressures that were as low as zero in some cases ; data from such animals are not included here. In one Limulus the blood pressure was very low but the animal appeared otherwise normal ; these data are therefore included. All animals included became quite active when picked up and they lived in the laboratory for three or more weeks 288
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