So how did vertebrates move from the water onto land?
There had to be some major modifications. Modifications in how the animal moves, how the animal breathes, and modifications in the animals skin.
Structure and Function in Amphibians
Amphibians are vertebrates that exist in two worlds: they divide their time between freshwater and terrestrial habitats. They share a number of features with air-breathing lungfish, but they also differ from lungfish in many ways. One way they differ is their appendages. Modern amphibians include frogs, salamanders, and caecilians, as shown Figure below.
Examples of Living Amphibians. In what ways do these three amphibians appear to be similar? In what ways do they appear to be different?
Amphibians are the first true tetrapods, or vertebrates with four limbs. Amphibians have less variation in size than fish, ranging in length from 1 centimeter (2.5 inches) to 1.5 meters (about 5 feet). They generally have moist skin without scales. Their skin contains keratin, a tough, fibrous protein found in the skin, scales, feathers, hair, and nails of tetrapod vertebrates, from amphibians to humans. Some forms of keratin are tougher than others. The form in amphibian skin is not very tough, and it allows gases and water to pass through their skin.
Amphibians are ectothermic, so their internal body temperature is generally about the same as the temperature of their environment. When it’s cold outside, their body temperature drops, and they become very sluggish. When the outside temperature rises, so does their body temperature, and they are much more active. What do you think might be some of the pros and cons of ectothermy in amphibians?
Amphibian Organ Systems
All amphibians have digestive, excretory, and reproductive systems. All three systems share a body cavity called the cloaca. Wastes enter the cloaca from the digestive and excretory systems, and gametes enter the cloaca from the reproductive system. An opening in the cloaca allows the wastes and gametes to leave the body.
Amphibians have a relatively complex circulatory system with a three-chambered heart. Their nervous system is also rather complex, allowing them to interact with each other and their environment. Amphibians have sense organs to smell and taste chemicals. Other sense organs include eyes and ears. Of all amphibians, frogs generally have the best vision and hearing. Frogs also have a larynx, or voice box, to make sounds.
Most amphibians breathe with gills as larvae and with lungs as adults. Additional oxygen is absorbed through the skin in most species. The skin is kept moist by mucus, which is secreted by mucous glands. In some species, mucous glands also produce toxins, which help protect the amphibians from predators. The golden frog shown in Figure below is an example of a toxic amphibian.
Toxic Frog. This golden frog is only about 5 centimeters (2 inches) long, but it’s the most poisonous vertebrate on Earth. One dose of its toxin can kill up to 20 humans!
- Amphibians are ectothermic vertebrates that divide their time between freshwater and terrestrial habitats.
- Amphibians are the first true tetrapods, or vertebrates with four limbs.
- Amphibians breathe with gills as larvae and with lungs as adults. They have a three-chambered heart and relatively complex nervous system.
- What is a tetrapod?
- How does the temperature of the environment affect the level of activity of an amphibian?
- What is the cloaca? What functions does it serve in amphibians?
- Describe the different ways that amphibians may obtain oxygen.