Respiratory Gas Exchange in Echinoderms
J. Malcolm Shick
Abstract
Echinoderms have received little attention from respiratory physiologists compared to other invertebrate phyla. The lack of respiratory trees in some holothuroids has necessitated other adaptations to enhance oxygen delivery. Knowledge of structural features to promote gas exchange has advanced most in the echinoids. The echinoid tube foot has received considerable attention from anatomists, and functional adaptations to promote gas exchange in the tube foot-ampullar system have been described throughout the class. Adoption of an infaunal existence by irregular echinoids has required special respiratory adaptations. An adaptation to living within the substratum and a flattening of the test is the concentration of the respiratory ambulacra frequently in a petaloid arrangement, with their highly modified tube feet on the aboral surface, in closest proximity to the oxygen supply. Mechanisms for creating convection of the external medium are best developed in forms whose morphology or mode of life removes them from ready access to environmental oxygen.