During this step, you will be assembling the ARMS units from the individual parts. The unit is a stack of alternating open layers and closed layers. The closed layers contain a cross-shaped structure dividing the ARMS in four compartments, providing 'cave-like' spaces for organisms. The very first space, at the bottom of the ARMS should always be a closed layer and the top layer will be open. Another important feature is that the first plate should lay flat on the base plate. No space should be created between the first plate and the base plate in order to avoid creating an extra space for colonization.