Their skin structure has certain characteristics typical among all terrestrial vertebrates. These include the extreme cornification of the outer layers of skin which is renewed sporadically through molting.
Their permeable skin allows for fluid and gas exchange into their bodies, making it possible for the adult animals to breathe underwater. The mucous glands located mainly on the backs, heads and tails of Amphibians secrete fluids to keep their delicate and thin skin adequately moist. Additionally, many species have granular poison-producing glands like the paratoids.
There are three pigment cell layers called chromatophores that are responsible for the skin coloration of these animals. The layers are comprised of melanophores, guanophores and lipophores. The characteristic color changing ability of some Amphibians comes from certain hormone secretion from pituitary gland. The vivid skin coloration of some species indicates their toxicity.
Their skeletal system is homologous to various other tetrapods. Most of the animals have four limbs with the exception of the legless Amphibian caecilians as well as certain salamander species. The lightweight, hollow bones are fully ossified with the vertebrae being interlocked with one another in an overlapping manner. Both the head and body are supported by the strong musculoskeletal system. The sloping ilium holds their bodies closer to ground compared to mammals.
The circulatory system for their juvenile and adult stages is different from each other. It resembles the circulatory system of a fish during the juvenile phase with a two-chambered heart for pumping blood through gills to be oxygenated. After reaching adulthood, their heart becomes divided into three chambers – 1 ventricle and 2 atriums.
Their digestive system consists of a pancreas along with a liver and gall bladder. The liver generally has two lobes with its size depending on its function as fat and glycogen storage unit. The adipose tissue located in the abdomen is an important energy storage site. This tissue can also be found under their skin and in the tail (in certain salamanders).
The two kidneys of Amphibians are located dorsally close to the roof of body cavity. The main function of the kidneys is filtering the blood of all metabolic waste and transporting the urine through the urinary bladder to be stored as urine before being excreted through cloacal vent. Nitrogen is excreted from the body as ammonia in dilute urine by the juveniles and many aquatic Amphibians. But the terrestrial species are known to pass the waste materials as urea which is comparatively less toxic.
Their lungs are fairly primitive than those of amniotes, having few large alveoli and internal septa. Due to this reason, the lungs have a slow oxygen diffusion rate through the bloodstream. Buccal pumping is responsible for ventilation in these animals. The permeable property of the skin also helps with respiration. Some Amphibians, like the plethodontid salamanders breathe primarily through skin as they do not have lungs or gills. Various species carry out respiration through gills during the larval stage.
The basic anatomy of the nervous system is quite similar to that in other vertebrates. It comprises of a central brain, spinal cord and numerous nerves throughout their bodies. Their brains are not as developed as those of mammals, reptiles and birds. Rather, they bear similarities in function and morphology to the brains of fish. The brains are divided into three equal parts – cerebrum, mesencephalon (midbrain) and cerebellum.