A honeycomb extrusion body has multiple cells extending along a common direction from a first end of the body to a second end of the body. The cells are separated by cell walls, and the body has at least one fluid path defined within a plurality of said cells. The fluid path includes one or more apertures, through respective cell walls between cells of one or more respective pairs of said plurality of cells. Each aperture has an aperture width measured perpendicular to the common direction of 90% or less of a cell wall width of the respective cell wall measured perpendicular to the common direction. Optionally one or more of the plurality of cells has at least two cell walls each having an aperture at the same position in the common direction. As a further option, these apertures may be offset from the respective centers of their respective walls in the same rotational direction about a central axis of the cell.