 Amphibians have a skeletal system that is structurally homologous to other tetrapods , though with a number of variations . They all have four limbs except for the legless caecilians and a few species of salamander with reduced or no limbs . The bones are hollow and lightweight . The musculoskeletal system is strong to enable it to support the head and body . The bones are fully ossified and the vertebrae interlock with each other by means of overlapping processes . The pectoral girdle is supported by muscle , and the well @-@ developed pelvic girdle is attached to the backbone by a pair of sacral ribs . The ilium slopes forward and the body is held closer to the ground than is the case in mammals . 
