As you have already seen, terrestrial vertebrates face challenges that are far different from those faced by aquatic vertebrates. In this section, you will learn about some of the ways amphibians meet the challenges of living on land.
The skin of an amphibian serves two important functions— respiration and protection. The skin is moist and permeable to gases and water, allowing rapid diffusion of oxygen, carbon dioxide, and water. Numerous mucous glands supply a lubricant that keeps the skin moist in air. This mucus is what makes a frog feel slimy. The skin also contains glands that secrete foul-tasting or poisonous substances that provide protection from predators.
However, the same features that allow efficient respiration also make amphibians vulnerable to dehydration, the loss of body water. Therefore, amphibians live mainly in wet or moist areas on land. Many species are active at night, when loss of water through evaporation is reduced. Although some species of frogs and toads survive in deserts, they spend most of their life in moist burrows deep in the soil. Only after heavy rains do these amphibians come to the surface to feed and reproduce.
Amphibians are affected by pollution. Chemicals present in water can be absorbed by amphibian skin. As a result, amphibians can serve as indicators of the health of an ecosystem.
While water supports the body of an aquatic vertebrate against the force of gravity, terrestrial vertebrates must rely on the support of their strong internal skeleton. The vertebrae of the spine interlock and form a rigid structure that can bear the weight of the body. Strong limbs support the body during walking or standing. The forelimbs attach to the pectoral girdle (the shoulder and supporting bones), while the hind limbs attach to the pelvic girdle (the "hips"). The pectoral and pelvic girdles transfer the body's weight to the limbs. The cervical vertebra at the anterior end of the spine allows neck movement.
The frog skeleton in Figure 40-7 shows several specializations for jumping and landing. In frogs, the bones of the lower forelimb are fused into a single bone, the radio-ulna. The bones of the lower hind limb are fused into the tibiofibula. Frogs have few vertebrae, and the vertebrae at the posterior end of the spine are fused into a single bone called the urostyle. The pectoral girdle has thick bones that are braced to absorb the impact of landing.
The skeleton of the frog has adapted to absorb shocks when the frog jumps and lands.
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