A kit of parts is provided for forming a device for performing anastomosis a first gastro-intestinal tract. The kit of parts comprises an inner ring, comprising an outer surface generally facing away from the passage, and an outer ring, comprising an inner surface. When the inner ring and the outer ring are in an assembled state, the inner ring is at least partially positioned in the reception volume of the outer ring, at least part of the outer surface of the inner ring faces towards the inner surface of the outer ring, and the outer surface and the inner surface together define an annular space between them, wherein at least part of the annular space diverges in the generally axial direction
