Cetaceans And Evolution :: essays research papers

CetaceansThere are many forms of cetacean life living in the worlds waters. How an why they came into existence is uncertain, that there keep up been many theories. Some say that dolphins are aquatic forms of goats, and others believe they evolved just like everything else, from a single cell, andnot from an actual multi-cellular beingness. Because intimately cetaceans have some vestigial structures such as under-developed os coxae, the former of these two theories remains prevalent in mostresearchers minds.The theory that cetaceans evolved from a goat-like being is not at all farcical. The similarities between the skeletal systems are more than coincidental. The only difference is that the cetaceans adapted to a marine lifestyle. The front limbs became modified as paddle- skeletal systemd flippers, the bones of which are still reminiscent of jointed limbs and digits, but the hind limbs were lost. The broad horizontal tail flukes that provide the main propulsive thrust turn off no anatomical connection to the lost hind limbs, but are a seperate and distint development. They contain no bone, and owe their firm and yet flexible shape to underlying fibrous elastic tissue. The body is enveloped in a thick layer of blubber that aids in bouyancy, helps to preserve body heat, and is a point of reference of stored energy. A cetaceans skin is free of sweat glands, oil glands, or hair, and feels much like smooth, wet rubber to the touch.Cetaceans, like other mammals, have lungs. They breathe air through a single nostril, or pair of nostrils, located on the top of the head but contrary to a frequent image, they do not spout water when they exhale. The visible spout, the size and shape of which is unique to many species, is simply water vapor in the lungs and a polished amount of water present in the depressionaround the blowhole, which is blown into the air as th cetacean exhales.A number of physiological adaptations modify goliaths to perform deep dives. First , they have a larger blood volume than land mammals of comparable size and weight, and they also have a greatly increase capacity to store oxygen in their blood and muscle tissue. Second, each breath provides an 80 to 90 percent renewal of air in the whales lung, compare with only 10 to 20percent in most land mammals. Third, cetaceans have a resistance to the metabolic by-product carbon-dioxide, the buildup of which in the tissues, rather than the lack of oxygen, triggers the involuntary breathing response of most mammals.