Modifications among the vertebrate classes. Amphibians

Modern amphibians are characterized by the flexibility of their gaseous exchange mechanisms. Amphibian skin is moistened by mucous secretions and is well supplied with blood vessels. It is used for respiration to varying degrees. When lungs are present, carbon dioxide may pass out of the body across the skin, but in some salamanders there are no lungsand all respiratory exchanges occur via the skin. Even in such animals as frogs, it seems that oxygen can be taken up at times by the skin, under water for example. Therefore, regulation of respiration occurs within a single species, and the relative contribution of skin and lungs varies during the life of the animal.

The amphibian heart is generally of a tripartite structure, with a divided atrium but a single ventricle. The lungless salamanders, however, have no atrial septum, and one small and unfamiliar group, the caecilians, has signs of a septum in the ventricle. It is not known whether the original amphibians had septa in both atrium and ventricle. They may have, and the absence of septa in many modern forms may simply be a sign of a flexible approach to the use of skin or lung, or both, as the site of oxygen exchange. In addition, the ventricle is subdivided by muscular columns into many compartments that tend to prevent the free mixing of blood.

The conus arteriosus is muscular and contains a spiral valve. Again, as in lungfishes, this has an important role in directing blood into the correct arterial arches. In the frog, Rana, venous blood is driven into the right atrium of the heart by contraction of the sinus venosus, and it flows into the left atrium from the lungs. A wave of contraction then spreads over the whole atrium and drives blood into the ventricle, where blood from the two sources tends to remain separate. Separation is maintained in the spiral valve, and the result is similar to the situation in lungfishes. Blood from the body, entering the right atrium, tends to pass to the lungs and skin for oxygenation; that from the lungs, entering the left atrium, tends to go to the head. Some mixing does occur, and this blood tends to be directed by the spiral valve intothe arterial arch leading to the body.

Blood returning from the skin does not enter the circulation at the same point as blood fromthe lungs. Thus, oxygenated blood arrives at the heart from two different directions—from the sinus venosus, to which the cutaneous (skin) vein connects, and from the pulmonary vein. Both right and left atria receive oxygenated blood, which must be directed primarily to the carotid arteries supplying the head and brain. It is likely that variable shunting of blood in the ventricle is important in ensuring this. A ventricular septum would inhibit shunting; it is at least possible that its absence in amphibians is not a primitive feature but a secondary adaptation to variable gas-exchange mechanisms.

The amphibian venous system shows various features that are characteristic of land vertebrates. The posterior cardinal veins are replaced by a posterior vena cava, but they are still visible in salamanders. There is a renal portal system, and an alternative route back to the heart from the legs is provided by an anterior abdominal vein that enters the hepatic portal vein to the liver.

Amphibian larvae and the adults of some species have gills. There are four arterial arches in salamanders (urodeles) and three in frogs (anurans). These are three through six of the original series, the fifth disappearing in adult frogs. There is no ventral aorta, and the arterial arches arise directly from the conus—an important feature, given that the conus and its spiralvalve control the composition of blood reaching each arterial arch. The names given to the three arterial arches of frogs are those used in all land vertebrates, including mammals. They are the carotid (the third), systemic (the fourth), and pulmonary (the sixth) arches. Blood to the lungs (and skin in frogs) is always carried by the sixth arterial arch, which loses its connection to the dorsal aorta. All land vertebrates supply their lungs with deoxygenated blood from this source.