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Respiration
In advance of transitioning to a land existence, aquatic animals had to find a means of obtaining oxygen from the air. They developed lungs and discarded their gills to retrieve oxygen directly from the air. The first lungs may have developed in early amphibian forms by modifying the swim bladder, the air-filled organ that stabilizes fish in water. Amphibians continue to live close to water since they partly rely on their gills and skin for gas exchange. Birds, reptiles and mammals have transitioned completely onto land with more sophisticated lungs and respiratory systems.
Limbs
The fossil record points to the coelacanth and the lungfish as precursors of modern land animals. Scientists theorize that these lobe-finned fish crawled onto land using their broad fins. The swampy environment would have selected those individuals equipped with fins strong enough to support their weights. A recently discovered ancestor of tetrapods, the extinct Tiktaalik, had fins whose bone structures closely resembled those of modern land animals. As the bones of the limbs and digits continued to develop, animals became more comfortable with their land environments.
Skeletal Structure
The fluidity and buoyancy of water allowed aquatic animals to develop limbs that need not support much weight. As land creatures relied more on their limbs to support their masses, these limbs had to develop further. The bones of the limbs and torso strengthened to lessen strain on the skeleton caused by the development of massive muscles used for land locomotion. The bones of the skull decreased in number and became more rigid in structure to protect the brain from the more jarring movements of walking and running. In addition, the neck bones exhibited a wider range of motion, allowing land animals to move their necks up and down to feed.
Minimizing of Water Loss
Land animals were no longer in close contact with water sources, so they had to find a way of dealing with desiccation, or drying out. Land animals typically exhibit tough skin protected by a layer of wax or keratin to prevent water from escaping into the environment. Terrestrial organisms either seek cooler, more humid habitats or limit their primary activities to nighttime conditions.
Animals adapted to a terrestrial lifestyle have an additional secret weapon to prevent unnecessary water loss: a kidney that reabsorbs much of the water and nutrients and concentrates wastes. It also maintains the concentration of essential salts in the internal environment, releasing more water or salts into the urine as needed.
Internal Fertilization
Many terrestrial animals employ internal fertilization as their means of reproduction. With the external fertilization exhibited in many sea creatures, the innumerable sperm released by the male travel through the water to reach the eggs. Internal fertilization provided organisms an avenue for sperm to find a sure way to eggs without relying on water. This method also ensured that the fertilized eggs would develop safely within females.
Amniotic Egg
Although earlier tetrapod specimens and modern amphibians lay water-dependent eggs similar to fish, tetrapods such as reptiles and birds evolved specialized amniotic eggs. The amniotic egg has a tough outer covering to protect the developing embryo from drying out. Three sets of membranes--the chorion, amnion and allantois--allow the embryos to take in oxygen and discard carbon dioxide. The yolk inside the egg provides nourishment to the developing young. In most mammals, the placenta and umbilical cord replaced the shell and yolk for protection and nourishment of young within the mother's body.