The locomotory muscle of fish is likely to vary structurally and physiologically due to the great diversity in form and habitat of the living fish. The locomotory muscle constitutes a major proportion of the body musculature and body weight in most fishes. The fish body trunk muscle is mostly segmented. Each segment, known as a myomere, is a disk and constitutes a separate muscle. Myomeres increase phylogenetically in complexity of shape from amphioxus to the higher teleosts. The axial musculature is divided into dorsal and ventral portions by the horizonal ligament—found in the central part of the trunk, which divides the superficial musculature and part of the deep musculature. Muscle fibers are innervated by motor terminations derived from axons arising from the spinal medulla in two adjacent nerve roots. Fish muscle does not contain muscle spindles, an important sensory organ, but has sensory nerve attachments as in higher vertebrates.