On a shell-deformation of Littorina littorea (Linné) caused by Balanus

Deformations of molluscan shells, both recent and fossil, have been described many times. These deformations result from various causes and they may show very different characters. Sometimes, as in the case of certain divergences of the axis of gastropod spires, or in those instances, where gastropod shells with otherwise normally coiled spire (whorls in contact) show separated whorls, we cannot attribute the anomalies to ordinary causes, such as influences of the substratum or damage of the shell-secreting mantle, but then the anomaly may be of a pathological character. One of the most striking examples of deformation is the abnormal development of a shell caused by commensalism, but it is not often noted in mollusca.