其中一些表皮很坚硬——就像坚果的外壳——可以提供额外的保护。
Some of these coats are hard—like the shell of a nut—for extra protection.
在这种情况下，骨骼或外壳的原始成分会保留下来，但化石会变得更坚硬、更耐用。
In such cases, the original composition of the bone or shell remains, but the fossil is made harder and more durable.
海龟颈部和四肢的肌肉及骨骼的特殊设计使它能把暴露在外的部分缩回去，这样攻击者只能去咬坚硬的外壳。
The particular design of muscles and bones in the neck and limbs of the turtle allow that it can draw in its exposed parts such that an attacker can find nothing but hard shell to bite.
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