A hard bony or chitinous outer covering,such as the fused dorsal plates of a turtle or the portion of the exoskeleton covering the head and thorax of a crustacean.

• 硬壳，背甲，头胸甲一种硬骨质或甲壳质外部覆盖物，如海龟融为一体的背部板形器官，或甲壳纲动物覆盖头胸部的外骨骼部分。