Which one of the circuits does not have valves?
Ayre's T-piece was invented by Phillip Ayer in 1937. This consists of a light metal tube 1 cm in diameter, 5 cm in length with a side arm. Used as such, it functions as a non-rebreathing system. Fresh gas enters the system through the side arm and the expired gas is vented into the atmosphere and there is no rebreathing.
Magills circuit
Bains circuit
Closed circuit
Mapleson E
Anesthesia Circuit