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The job of getting oxygen into the body and removing waste gases such as carbon dioxide, is carried out by the tube-feet. Circulating fluid passes constantly from the internal ampullae to the external tube-feet and back into the ampullae in a one-way system, and gases diffuse across the thin walls of both external and internal organs. For sea urchins living bathed in surface currents gaseous exchange is relatively efficient, even with standard cylindrical-walled tube-feet. However, for those living infaunally, or for epifaunal echinoids living in environments that can become a little dysoxic at times, more efficient tube-feet are needed. Specialised respiratory tube-feet are extremely thin-walled and also highly flattened to provide a large surface area. In some there may be lateral flanges to further increase the effective surface area for gaseous exchange. In irregular echinoids these tube-feet are situated in the adapical part of the ambulacra (nearest the point at which water currents are drawn into the burrow) and form distinctive structures termed petals. |