Mass. Gen. Laws ch. 152, § 72: Employee claiming or waiving rights against county, city, town or district

Section 72. A laborer, workman or mechanic entering the service of such county, city, town or district who would, if injured, have a right of action against the county, city, town or district by law, may claim or waive his right of action as provided in section twenty-four, and shall be deemed to have waived such right of action unless he claims it.