Countless animals, including us, bear external structures without blood supply or nerves - for example, our hair and fingernails, or the scales, claws, and ho rns of other animals.

 
  • 包括人在内的无 数动物都有一些没有血液供应和神经的体外构造,例如,我们的头发和指甲,或者鳞片、爪 子和其它一些动物的角。
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