a slow-moving land animal that can pull its head and legs into the hard round shell that covers its body.——朗文当代大辞典

A four-footed reptile of the order Chelonia, in which the trunk is enclosed between a carapace and plastron, formed by the dorsal vertebræ, ribs, and sternum; the skin being covered with large horny plates, commonly called the shell.——牛津当代大辞典  @en