The external surface of the epitheca in modern and fossil corals is marked by tiny ridges lying parallel to the epithecal rim. These ridges have been assumed to be daily growth increments, and have been linked with supposed lunar and seasonal events recorded in the skeleton, to compute aspects of the history of the Earth's rotation. This communication presents structural and experimental evidence to show that the growth-ridges in the epithecae of modern hermatypic scleractinian corals, particularly Manicina areolata (Linnaeus), are formed as a result of daily changes in the shape of the tissues secreting the epithecae. The changes in shape of the tissues are an integral part of the mechanism by which the body wall of these corals is adjusted in position to accommodate for epithecal growth. This adjustment takes place in concert with a daily cycle of expansion and contraction of the animals. Because the epitheca is formed at the perimeter of the skeleton-secreting layer, its growth involves certain fundamental requirements. The presence of growth-ridges in all coral epithecae suggests that all corals meet, or met, these requirements with a similar mechanism to that which operates in the hermatypic species studied. However, the mechanism is not necessarily linked to a daily cycle of expansion and contraction.