Linkage systems are widely distributed in animals. The most thorough overview of the different types of linkages in animals has been provided by M. Muller, who also designed a new classification system which is especially well suited for biological systems. Linkage mechanisms are especially frequent and various in the head of bony fishes, such as wrasses, which have evolved many specialized aquatic feeding mechanisms. Especially advanced are the linkage mechanisms of jaw protrusion. For suction feeding a system of connected four-bar linkages is responsible for the coordinated opening of the mouth and 3-D expansion of the buccal cavity. Other linkages are responsible for protrusion of the premaxilla.