Section 6. The commissioner shall organize the department into divisions and bureaus and shall assign to said divisions their functions. The commissioner, with the approval of the board, may appoint a director to each division and a chief to each bureau thereunder, to have charge of the work of the division and the bureau and may, with like approval, remove such directors or chiefs at any time. The compensation of the directors and chiefs shall be fixed by the commissioner with the approval of the board. The commissioner, with the approval of the board, shall appoint such scientific experts as the work of the department may require, and may assign them to divisions, may transfer and remove them, and the provisions of chapter thirty-one shall not apply. He shall, subject to the provisions of said chapter, appoint such inspectors, clerks and other assistants as he may deem necessary.