triarii

Just watched this video. TeU8pXr0ucI In which the role of the Triarri is discussed. (at the 18:24 mark) He seems to think that they formed up as the rear guard to act both as a backstop to the enemy and to prevent any routed units from fleeing the battlefield. Forcing them back into the battle...

so just some time ago i read the Commentarii de Bello Gallico from Julius Ceasar... however in my translation of the book Ceasar kept reffering to the triarii. That while the Triarii were no longer jused after the marian reforms. So whats your explination for this? Is my version of the book...