Legion, a constituent part of the ancient Roman army, the organisation of which differed in different epochs. During the republican period it contained 4,500 men - viz. 1,200 hastati, 1,200 principes, 600 triarii, 1,200 velites (skirmishers), and 300 equites (cavalry), recruited from the richer class. The velites were the "youngest and poorest," the hastati "those next them," the principes the "most vigorous in years," the triarii the "oldest." The hastati, principes, and triarii formed three lines, ranged one behind the other in the order named. Each line contained 10 maniples, consisting in the first two of 120, in the third of 60 men apiece. The officer commanding a maniple was called a centurion. To every legion were attached six "military tribunes," who took the command in rotation, each for two months. Under Marius the three lines were amalgamated, and the whole legion divided into 10 cohorts, each containing 3 maniples. During the civil wars the tribunes were replaced by a permanent commander called a legatus. Under the early Empire the total number of men was raised to 6,000, exclusive of cavalry and velites. Various other changes were subsequently introduced.