Section 22. The registrars in cities and towns may appoint assistant registrars for the term of one year, beginning with April first, unless sooner removed by the registrars, and they shall equally represent the different political parties. Assistant registrars shall be subject to the same obligations and penalties as registrars. Registrars may remove an assistant registrar, and may fill a vacancy for the remainder of the term. Registrars may also appoint temporary assistant registrars from time to time to assist in the listing of persons seventeen years of age or over and in the registration of voters at registration sessions as provided for in this chapter, and they shall equally represent the different political parties. Except in Boston, persons appointed to serve temporarily as assistant registrars, or as temporary assistant registrars, shall not be subject to chapter thirty-one.