Abstract
Muscular systems of euopisthobranchs comprise body muscles and the muscular tissue of internal organs. Body wall musculature is stronger in species without a shell or with a reduced shell. Most euopisthobranchs have a muscular ventral foot for benthic locomotion. However, in pteropods, the foot is highly modified into wing‐like parapodia used for swimming, and other euopisthobranchs also possess muscular parapodia. In euopisthobranchs the musculature is markedly developed around the buccal cavity, in the esophagus, and in the gizzard in species that have a gizzard. Muscle cells contain a contractile system made of thin and thick myofilaments. Thin myofilaments consist of actin associated with other proteins, and thick myofilaments are formed by a paramyosin core coated with myosin molecules. Electron‐dense bodies are points of attachment for thin myofilaments, which are also linked to the cell membrane. Smooth muscle fibers are the most common type of muscle cell in euopisthobranchs. In these cells, the electron‐dense bodies are scattered in the cytoplasm, and a striated pattern is not formed. Caveolae are very abundant in the cell membrane of these muscle fibers. As in other mollusks, transversely striated muscle fibers are not common in euopisthobranchs, being present in the heart and a few other muscles. In these cells, a transverse striation is created by the alignment of contractile units formed by thin and thick myofilaments that resemble the sarcomeres of vertebrates, but with discontinuous Z‐lines.