例如，科学家知道没有四肢的蛇钻洞更有效率，但是它们的四肢究竟是如何退化的，依然是个谜。

For instance, although scientists know snakes can burrow more efficiently without arms and legs, exactly how they lost their limbs remains a mystery.

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也有科学家认为，800种小蜥蜴的短小的有些甚至了退化的四肢提供了蛇起源的线索。

Still others suggest the short or sometimes nonexistent legs of the 800 species of skinks are clues to where limbless snakes originated.

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骨关节炎，又名退行性关节炎，是一种常见的犬病。特点是四肢关节软骨逐步退化。

Osteoarthritis, also known as degenerative arthritis, is a common condition in dogs characterized by progressive deterioration of articular cartilage in the joints of the limbs.

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