The Arethusa class was a class of four light cruisers built for the Royal Navy between 1933 and 1937 and that served in World War II. It had been intended to construct six ships, but the last pair, Polyphemus and Minotaur, were ordered in 1934 as the 9,100-ton and . Design The Arethusas were a smaller version of the Amphion group of the earlier , having the unit machinery layout and two funnels of the former. The design was judged to be the minimum required for a "trade route cruiser" to counter the threat of the auxiliary cruiser over which, even with their reduced armament, they would enjoy a comfortable superiority. They were also to be capable of acting as a fleet cruiser (which was fortunate because, in the end, they were used almost exclusively with the fleet). Therefore, no reduction in speed (i.e. machinery) could be accepted, and savings had to be found in armament, size and protection; the Arethusas suppressed 'X' 6-inch gun turret and were shorter and displaced 1,250 tons less than the Leanders. The protection scheme was the same as the Leanders but was generally thinner to save weight. This allowed six Arethusas to be built for every five Leanders within the constraints of tonnage allowed under treaties then in force. Welded construction was widely used for the first time to save weight, over 250 tons being cut off the original specification. They were armed with six BL 6 inch Mark XXIII in three twin mountings Mark XXI in 'A', 'B' and 'Y' positions. Triple torpedo tubes were carried abreast the after funnel, the reduction in beam had reduced training space resulting in the omission of one tube vis-à-vis the Leanders. The secondary armament was four QF 4-inch Mark V on single mountings HA Mark III, controlled by a HACS director on the bridge. The 4-inch magazine was retained in the position of the Leander class well forward, but the guns themselves were moved well aft. As a result, the 4-inch shell and charge had to be transported along the ship to reach the guns. In Penelope and Aurora eight QF 4-inch Mark XVI on four twin mountings HA/LA Mark XIX replaced the single mounts, and a second HACS director was added aft. A shelter was added for the gun crews between each pair of guns as it was recognised that in wartime the crews would