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Cetaceans And Evolution Essay, Research Paper

Cetaceans

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, and

not 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

researchers 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-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 bouyancy, helps to preserve body heat, and is a source of stored energy. A cetacean s 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 popular 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 small amount of water present in the depression

around 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 whale s lung, compare with only 10 to 20

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.

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.

There are many differences between the cetacean s skeleton and our own. Again, the cervical vertebrae in most species have been fused, which allows them to stay focused on the direction they are going, while humans do not have this hindrance, which allows us to rotate our heads, and peer in all directions. The ribs are also a major difference between cetaceans and

humans. Our rib cage does not need to collapse, therefore, most of them are attached firmly to both the thoracic vertebrae and the costal cartilage. The elongated phlanges are also a major difference between us. Theirs have been elongated so the frontal flippers maintain their shape, while we do not need such long, flat paddles to help us steer. The positionin of the nares is also a

major help in the life of a cetacean. It allows them to breathe without pausing to dip the tip of their snouts above the water. If I were to list anymore differences between the skeletal system of the cetaceans and our own, I would only be repeating what I said earlier on in the paper.

The cetaceans of the world are marvelously built. Designed perfectly for their underwater life. Their sleek, stream-line shape allows speed, and the skeletal an physiological make-up of their bodies only adds to their already amazing abilities.


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