The tooth of an archosaur, left, which was a direct ancestor of many dinosaurs, and the complete skeleton, including soft tissue of a horseshoe crab, right.
左图，是古蜥蜴的牙齿，它是很多恐龙的直系祖先；右图，是一个完整的马蹄蟹的头盖骨。
2
Its abdomen is soft, so a hermit crab USES the empty shell of a sea snail to protect itself.