(1) A type of anatomic structure exhibiting serially homologous metameres; in primitive forms, such as the annelids, the metameres are almost alike in structure; in vertebrates, specialization in the cephalic region masks the underlying metamerism, which is still clearly evident in serially repeated vertebrae, ribs, intercostal muscles, and spinal nerves, and in young vertebrate embryos. (2) In chemistry, rarely used synonym for structural isomerism.