Section 57. Continuous recruitment for certain positions

Notwithstanding
  any other provisions of this chapter or any other law, the civil service
  department or a municipal commission may establish a continuing eligible
  list for any class of positions for which it finds inadequate numbers of
  well qualified persons available for recruitment.   Names  of  eligibles
  shall  be  inserted  in  such  list  from time to time as applicants are
  tested and found qualified in examinations held at such intervals as may
  be prescribed by the civil service department  or  municipal  commission
  having  jurisdiction.  Such  successive  examinations  shall,  so far as
  practicable, be constructed and rated so as to be  equivalent  tests  of
  the  merit  and  fitness  of  candidates.  The name of any candidate who
  passes any such examination and who  is  otherwise  qualified  shall  be
  placed  on the continuing eligible list in the rank corresponding to his
  final  rating  on  such  examination.  The  period  of  eligibility   of
  successful  candidates  for  certification  and  appointment  from  such
  continuing eligible list, as a result of any such examination, shall  be
  fixed  by  the  civil  service  department  or municipal commission but,
  except as a list may reach an announced terminal date, such period shall
  not be less than one year; nor shall such period of  eligibility  exceed
  four  years.  Subject  to  such  conditions and limitations as the civil
  service department or municipal commission may  prescribe,  a  candidate
  may take more than one such examination; provided, however, that no such
  candidate  shall  be certified simultaneously with more than one rank on
  the continuing eligible list. With respect to any candidate who  applies
  for  and  is  granted  additional  credit  in  any such examination as a
  disabled or  non-disabled  veteran,  and  for  the  limited  purpose  of
  granting such additional credit, the eligible list shall be deemed to be
  established on the date on which his name is added thereto.