Cetaceans And Evolution :: essays research papers

CetaceansThere are umpteen forms of cetacean life living in the worlds waters. How an why they came into existence is uncertain, hardly there substantiate 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 echt multi-cellular being. Because most cetaceans have some vestigial structures such as under-developed os coxae, the former of these two theories remains prevalent in mostresearchers minds.The possibility that cetaceans evolved from a goat-like being is not at all farcical. The similarities between the penurious systems are more than coincidental. The only difference is that the cetaceans adapted to a ocean lifestyle. The front limbs became modified as paddle-shaped flippers, the bones of which are still remindful of jointed limbs and digits, but the hind limbs were lost. The broad horizontal tail flukes that succeed the main propulsive thrust bear no anatomi cal conjunctive 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 tendinous elastic tissue. The body is enveloped in a thick layer of avoirdupois that aids in bouyancy, helps to preserve body heat, and is a source of stored energy. A cetaceans skin is free of sweat g dry lands, 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 touch of nostrils, located on the top of the head but contrary to a popular image, they do not spout water when they exhale. The visible spout, the surface and shape of which is unique to many species, is simply water vapor in the lungs and a small amount of water present in the embossmentaround the blowhole, which is blown into the air as th cetacean exhales.A number of physiological adaptations enable whales 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 increased 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, equivalence 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, or else than the lack of oxygen, triggers the involuntary breathing response of most mammals.