When Does a Hermit Crab Change its Shell ?
A hermit crab changes its shell when it has grown large enough to need a bigger home. This type of crab has a soft abdomen or "tail", which is folded up under the body, but it is not protected by a shell of its own as are most other crabs. Instead, the hermit uses empty snail shells as portable shelters, often having to fight with another crab for possession of an attractive home. Sometimes the homeseeker pulls out the original occupier, eats it and then takes over the shell.
One of the claws of the hermit is larger than the other. The crab uses this to stop up the entrance after withdrawing into the shell. The last two legs on its abdomen have roughened pads which grip the inside of the shell and hold the body in position. The crab has a spiral-shaped abdomen and moves in and out of its shell with a spiral movement.
One kind of hermit occupies a sponge which conveniently grows at the same pace as the crab. Sometimes sea-anemones enter into partnership with hermit crabs and take up residence on top of the shells. The crab provides the sea-anemone with transport and in return, receives an extra shield against attack.
寄居蟹什么时候换壳？
寄居蟹长大到一定程度，需要更大的居住场所时，就要换壳了。这种寄居蟹的腹部或称“尾部”很柔软，卷曲于身下。与大多数蟹不同的是，寄居蟹并非用自己的壳来保护身体，而是把空的蜗牛壳用作可移动的居所。因而，为了抢占一个舒适的居所，它不得不经常与其他寄居蟹搏斗。有时，寻找住处的寄居蟹干脆把原来的主人从壳里拉出来吃掉，然后把壳占为已有。
寄居蟹的两只蟹钳中有一只稍大。寄居蟹缩进壳内后便用大蟹钳将壳口封住。寄居蟹腹部最后面的两只脚有粗糙的肉趾，可钳住壳的内壁，固定住身体的位置。寄居蟹腹部呈螺旋形，它以螺旋动作进出于自己的壳。
有一种寄居蟹寄居在海绵里，海绵会自然地与它同速生长。有时海葵会与寄居蟹结伴，寄居在壳顶上。寄居蟹为海葵提供了行走的工具。而海葵则给寄居蟹提供了防御外敌进攻的额外保护作为回报。
摘自 1000 Questions and Answers
整理：鲁蒙海
编辑：海 新
审核：张 扬
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