The successive subdivisions along the length of the body axis in bilaterally symmetrical animals; also called somites or segments. Common examples are the muscles and spinal nerves in the human body and in the body and tail of many mammals, snakes and lizards, salamanders, and fishes. It also occurs in other chordates, and in arthropods and annelid worms. It never involves reproductive organs, and thus differs from strobilization in tapeworms and certain jellyfish. This serial repetition of parts (metamerism or segmentation) arises either from a bilateral series of coelomic pouches which form the segmental muscles, kidneys, and body cavities of lower forms, or from mesoblastic somites which form the skeletal and muscular segments of vertebrates. Repetitive features of the nervous system are acquired secondarily through the influence of mesodermal metameres upon adjoining ectodermal tissues. Several primitive embryonic somites become fused in the heads of adult arthropods and vertebrates.