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Vertebraes
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I. Class Amphibia All of the classes of vertebrates that we have examined so far are aquatic. The next class that we will discuss is the Amphibia (both + life). As the name Amphibia suggests, amphibians have a dual life characterized by an aquatic larval stage and a terrestrial adult stage. Modern amphibians are included in Lissamphibia, which is divided into Anura (frogs), Urodela (salamanders and newts), and Gymnophiona (caecilians). II. Challenges of Life on Land Amphibians were the first vertebrates to adapt to the challenges of life on land. Life on land presents different problems than life in the water and amphibians exhibit a series of modifications that allow them to meet the challenges of life on land. Aquatic versus Terrestrial Environments. The basic difference between life on land versus life in the water stems from the much greater density of water than air. Water is approximately 1000X more dense than air. Why is the difference between an aquatic medium and air important? (1) Support and Locomotion. A dense medium such as water helps to support the weight of the body. However, terrestrial vertebrates do not have this luxary and must develop modifications to support themselves on land. Terrestrial vertebrates must also develop an efficient method of locomotion. (2) Sensory Organs. Sense organs associated with vision, olfaction, and hearing/vibration detection must be modified in adapting to a terrestrial environment. For example, vibrations that are transmitted in the air are difficult to detect. This is because the inner ear is fluid-filled and it takes more energy to transmit vibrations in fluid than in air. In a terrestrial environment, there needs to be a mechanism to amplify force before it is transmitted to the inner ear. (3) Breathing Apparatus. Air also requires changes in the breathing apparatus. In water, gills are supported by water and exchange of gases can take place. However, gills do not function outside of water. When wet gills are placed in the air the surface tension of the water on the gills causes them to collapse and gas exchange cannot take place. Gills also dry out in a terrestrial environment. A terrestrial organism must develop a method of breathing in air.
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