Article: The water-vascular system in living and fossil echinoderms [The Fourteenth Annual Address, delivered 3 March 1971]

All Recent echinoderms possess extensile tube-feet, and probably all extinct groups had them too. In the most primitive of the living groups, the Crinozoa, the tube-feet are extended by muscular contractions of the water-vascular canals that link together all tube-feet in the animal; most probably, all fossil crinozoans share this method. In the Asterozoa, as well as the agency of the canals, various accessory structures, such as bulbs and ampullae, also become involved. The Homalozoa appear to have most resembled the asterozoans in the operation of their tube-feet. In the Echinozoa, either the accessory ampullae assume the dominant role in tube-foot protractions or, as in aspidochirote holothuroids, protraction is brought about solely by retraction of adjacent tube-feet; some extinct groups could have adopted this method too. The evolution of the water-vascular system is discussed.