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|Referát vhodný pre:||Stredná odborná škola||Počet A4:||2.3|
|Priemerná známka:||2.97||Rýchle čítanie:||3m 50s|
|Pomalé čítanie:||5m 45s|
Then, shutting its cavernous mouth and pressing its tongue against the back of the baleen bristles, the whale forces the water out of its mouth, trapping the plankton on a mat of overlapping baleen plates.
Probably the largest animal ever to have lived is a baleen whale, the blue whale, which has been measured up to 30.5 m (100 ft) in length, with a weight of about 190 tonnes. Baleen whales tend to spend the summer in polar seas, where plankton blooms provide abundant food. After months of heavy feeding they migrate to temperate or tropical zones, often fasting there over the winter.
The dramatic streamlining of whales in the course of their evolution resulted in an animal that appears remarkably fish-like. Thus, 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 separate and distinct development. They contain no bone, and owe their firm and yet flexible shape to underlying fibrous and elastic tissue. The body is enveloped in a thick layer of blubber that aids in buoyancy, helps to preserve body heat, and is a source of stored energy. A whale's skin is free from sweat glands, oil glands, has almost no hair, and feels much like smooth, wet rubber to the touch.
Whales, like other mammals, have lungs. They breathe air through a single nostril, or pair of nostrils, situated on the top of the head (the blowhole); 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 vapour in the lungs and a small amount of water present in the depression around the blowhole, which is blown into the air as the whale comes to the surface of water (breaches) and exhales.