例20　Studies of the Weddell seal in the laboratory have described the physiological mechanisms that allow the seals to cope with the extreme oxygen deprivation that occurs during its longest dives which can extend 500 meters below the ocean's surface and last for over 70 minutes.

【结构分析】主干是Studies of the Weddell seal in the laboratory have described the physiological mechanisms，后面是个连环定语从句，定语从句that allow the seals to cope with the extreme oxygen deprivation，修饰前面的physiological mechanisms，后面加上定语从句that occurs during its longest dives，修饰前面的名词，定语从句which can extend 500 meters below the ocean's surface and last for over 70 minutes修饰前面的名词。

【亮点回放】此句话的亮点是三个连环定语从句，气势磅礴，层层修饰前面，是定语从句中少有的高级句型，作文也可以使用，很经典。

【核心词汇】

Weddell seal　威德尔海豹

physiological mechanisms　生理机能

oxygen deprivation　缺氧

longest dives　最长时间的潜游

extend　延展

【参考译文】在实验室中研究威德尔海豹的实验描述它们的生理特征，这些特征允许海豹应对极度的缺氧，这种缺氧出现在最长时间的潜游中，这种潜游可以持续到海平面500米以下而且维持超过七十分钟。