A distinction between the _principales_ and _immunes_ of the cohorts may be based partly upon the principles already adopted for the ala, partly upon the analogy of the legion, the organization of which was clearly followed in several respects. On these grounds we may class as _principales_ the _imaginifer cohortis_, the _signifer_, _optio_, and _tesserarius_ of each century, and the _optio_ and _vexillarius_ of each turma in the _cohortes equitatae_. The case of the _optio_, who commanded, if necessary, in the place of the centurion or decurion, may be taken for granted. It may also be noted that both _optio_ and _vexillarius_ could be promoted to the position of decurion without any intervening step.[102] The _tesserarius_, whose main duty consisted in receiving from the centurion the orders and password for the day and transmitting them to the men, is found in charge of a detachment on special duty,[103] as is also the _imaginifer cohortis_.[104] The _signifer_,[105] lastly, can hardly have had a position inferior to that of the _vexillarius_ or _tesserarius_, and would indeed rank higher than the latter if the analogy of the legions holds good. As regards the _immunes_, the officer commanding a cohort possessed a smaller administrative staff than the _praefectus alae_, including only the _cornicularius_,[106] _actarius_,[107] _librarius_,[108] and _beneficiarius_.[109] The musicians possibly include the _cornicen_[110] as well as the _tubicen_[111] and the _bucinator_[112], and the post of _mensor_ seems to be confined to the cohorts.[113] At least no inscription has yet mentioned one among the _immunes_ of the ala.