Cetaceans and evolution
There are many forms of cetacean life living in the worlds waters. How an why they came into existence is uncertain, but there have 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 being. Because most cetaceans have some vestigial structures such as under-developed os coxae, the former of these two theories remains prevalent in most 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-shaped 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 bear 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 bouyan
The basic mammalian skeleton has undergone a number of specialized changes as well. Not only have the forelimbs been modified into flippers, but the bones of the hind limbs have disappeared altogether. The pelvis girdle remains as a mere vestige buried in the ventral musculature. A large number of the cetacean's ribs are "floating", i.e. not attached to the sternum; ribs that are attached are often jointed, enabling the rib-cage to collapse under the extreme pressure of a deep dive without being damaged. The skull has become tilted upwards in line with the spinal column, and the foramen magnum has been repositioned in the middle of the occipital bone. The cervical vertebrae have become fused together in most species, which inhibits the rotation of the head. percent 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. Finally, cetaceans are able to restrict blood flow to vital organs during a deep dive, so that essential organs such as heart and brain do not suffer injurious oxygen deprivation. A number of ph
Some common words found in the essay are:
, hind limbs, cervical vertebrae, land mammals, major difference, deep dive, difference cetaceans, flippers bones,
Approximate Word count = 811
Approximate Pages = 3 (250 words per page double spaced)
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