(_i_) Cholera spirillae.

(_j_) Typhoid bacilli.

(_k_) Tetanus bacillus; _i_, _j_, _k_ are actively motile, motion being effected by the small attached threads.

(_l_) The screw-shaped spirochite which is the cause of syphilis.] The bacteria (Fig.

17) are unicellular organisms and vary greatly in size, shape and capacity of growth.