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The hydrocoel (from Greek hydor = 'water' and Greek koilos = 'hollow') denotes a section of the coelom in echinoderms . It is the part that develops from the mesocoel , i.e. the middle section of the coelom. It forms the animal's ambulacral system , which consists of a central ring and five radial canals. The name is derived from the German term 'water vessel system', which is no longer in use today.