A multiwell assembly includes a microplate and a cover. The microplate includes a set of wells. Each well defines an opening. The cover includes a body and a shutter. The body of the cover is disposed over the microplate. The shutter is mounted to the body such that the shutter is movable over a range of travel between a first position, in which the shutter occludes the openings of the set of wells, and a second position, in which the shutter is in offset relationship to the openings of the set of wells to permit access to the set of wells through the respective openings.