Extension and Retraction of the Tube-Feet of Ophiuroids

THE greater part of the basic anatomy of ophiuroids was worked out in the latter half of the last century and the results summarized in Bronn 1 . Iri the water-vascular system, the absence of tube-foot ampullæ comparable with those of asteroids or echinoids is striking. It has been generally recognized that the feet are less important in locomotion than the arms, and they are frequently attributed only sensory and adhesive functions. In several common genera the tube-feet are small and show only slight changes of shape, although protraction has been recorded 2,3 . Perhaps for these reasons there has been little discussion of possible mechanisms of protraction and it seems often to have been tacitly assumed that none is present, and that retracted feet resume the extended condition only on relaxation of their musculature (which is wholly longitudinal), by the general pressure of the cœlomic fluid in the water-vascular canals or by the elasticity of the podial wall. Ludwig 4 was the first to describe the valve at the head of the podium, the lips of which prevent water from flowing back into the radial canal. The presence of this valve has been remarked upon by all subsequent reviewers of the group; but its significance, namely, that pressure is developed within the foot, has been overlooked. Comparisons between active and anæsthetized specimens of various species suggest that in a number of cases podial extension is far in excess of that which could be achieved by relaxation alone.