Much of the anatomical variation in amphibians is represented by body shape and skeletal anatomy. In anurans, skeletal modifications are present to permit saltatory locomotion (hopping and leaping). Frogs lack cervical vertebrae and ribs, though elongated transverse spinal processes of the—five to eight vertebral bodies are visible and can be mistaken for ribs. The urostyle is an elongated coccyx bone that typically articulates with the sacrum, and along with an elongated ischium and ilium, are the biomechanical underpinnings for jumping. To accommodate the forces of landing, the bones of the limb, including the radius/ulna as well as the tibia/fibula/tarsal bones, are fused. There are four digits on the forelimbs and five digits on the hindlimbs. The anuran skull is composed of small, light bones.

In caudates, the spinal column is elongated and arranged in poorly defined cervical, trunk, sacral, caudal sacral, and caudal segments. Unlike in anurans, there is no fusion of the bones of the limbs. Limbs are reduced in many genera, and in the aquatic sirens (Family Sirenidae), there is complete absence of the pelvic limbs and pelvic girdle with markedly reduced forelimbs. The caudate skull is composed of larger and thicker bones than that of anurans. Lastly, in the caecilians, the spinal column is composed of a single atlas and up to almost 300 trunk vertebrae, generally in the absence of sacral or caudal segments. All limbs are absent, there is no pectoral girdle, and caecilian skulls are compact and heavily ossified, consistent with their natural fossorial behavior.

The heart of all amphibians has three chambers, two atria and a single ventricle, and the right atrium is generally larger than the left. A small number of caudates (sirens and mudpuppies) have a poorly developed ventricular septum. All amphibians have both renal and hepatic portal systems. Lymph hearts are small structures that beat synchronously independent of the cardiac contraction rate; their location and number is variable in amphibians.

Amphibian lungs are variably developed. In most anurans, they are thin, simple, grossly translucent structures (Figure 17.1). In aquatic frogs, such as Xenopus, they are more developed. In caudates, lung development is variable; some caudates, including obligate aquatic species such as hellbenders, have prominent lungs (Figure 17.2). In others, such as the plethodontid salamanders, the lungs are absent entirely. Amphibians rely on multiple modes of respiration in addition to pulmonic, including buccopharyngeal, cutaneous, and branchial; the relative importance of each method varies by species. All amphibians lack a diaphragm, and pulmonary respiration occurs under positive pressure. The trachea is very short and composed of complete cartilaginous rings. Obligate aquatic caudates rely on external gills for respiration. Caecilian lungs extend nearly the entire length of the coelomic cavity (Figure 17.3).

The hematolymphopoietic tissues of amphibians include the thymus, spleen, bone marrow, mucosal-associated lymphoid tissue, and the liver. All amphibians lack lymph nodes, but an analogous structure, lymphomyeloid organs, can be seen microscopically in anurans; these structures receive afferent blood flow from veins and arteries rather than lymphatics. The amphibian thymus is present throughout life; while generally not visible grossly, they can be seen microscopically bilaterally at the caudal angle of the jaw.

Amphibian kidneys are unique in that they are mesonephric rather than metanephric and contain two types of nephrons microscopically. Coelomostomic (also called nephrostomic) nephrons are located ventrally in the kidney and contain a primitive glomerulus; their primary function is to drain fluid from the coelomic cavity rather than to filter blood. More typical glomerular, blood-filtering nephrons are present dorsally. The kidneys are paired and located in the caudodorsal coelomic cavity. A urinary bladder is present that receives urine from the cloaca rather than the kidneys and serves fluid storage and osmoregulatory functions. The urinary bladder is very thin and can often be very challenging to discern grossly in all but the largest of species (Figure 17.4).

Most amphibians are highly fecund, and breeding behavior can be explosive. In reproductively active females, the bilateral ovaries and oviduct will occupy much of the coelomic cavity (Figure 17.5). The testes are located ventromedial to the kidneys in male frogs and cranial to the kidneys in caudates and caecilians (Figure 17.4). In some male anurans, vestigial ovarian tissue (Bidder’s organ) will be present at the craniomedial pole of the kidney and can be evident grossly. In caudates, prominent glands surrounding the cloaca are present in both males and females to permit the deposition and reception of spermatophores (aggregates of protein and spermatids). Phalli are absent in anurans and caudates, but male caecilians have a phallodeum (pseudophallus).