Cycloid scale from a common carp (Cyprinus carpio) 插畫檔

Focus (Center): The small, central circular area is the "focus," which is the first part of the scale to develop when the fish is young.Circuli: The concentric rings radiating from the center are called circuli. Similar to tree rings, these represent periods of growth.Annuli: When growth slows down (typically during winter or spawning), the circuli become more crowded, forming a darker band known as an annulus. Biologists count these bands to determine the age of the fish.Radii: The vertical lines extending from the focus toward the edges are "radii." These channels help provide flexibility to the scale so it can bend with the fish's movements without breaking.