A displacement pump comprising a pump assembly and a cassette assembly. The pump assembly includes upper and lower housing portions that define a cavity, an arm disposed in the cavity, a roller attached to the distal end of the arm, and a motor attached to the proximal end of the arm for rotating the arm. The cassette assembly is removably disposed in the cavity and comprises upper and lower cassette housing portions that form an annular compression surface with a channel therein. A hollow compression tube having a flange extending along a length thereof is secured to the compression surface by the flange being engaged with the channel. As the motor rotates the roller arm, the roller presses the compression tube against the compression surface to create a moving occlusion of the compression tube for pushing fluid through the compression tube.