ABSTRACT
Among the invertebrates, echinoderms are a world of their own. Not only do most of them switch from bilateral to radial symmetry after the larval stage, they also have physiological properties not found in other clades:
1. A water vessel system that communicates with the ambient sea water by a porous valve (madreporite plate, because the pore arrangement resembles brain corals). It is called the ambulacral system, because its dead-end surficial extensions serve primarily as hydraulically operated tube feet (often with a terminal suction disc), but they may also serve a respiratory function.
