成语“鹬蚌相争”中的鹬蚌两种动物，体内与外界气体交换的场所是（　　）

CO₂和CO的混合气体3.6g与足量灼热的氧化铁充分反应后，将生成的气体全部通入80克质量分数为20%的氢氧化钠溶液中充分吸收，测得最终所得溶液的质量为84.4克，则原混合气体中CO质量为（　　）

    I'm Lucy. I have a happy family. There are four people in my family. My birthday is on January 1st. It's New Year's Day, too. My family and I will have a big dinner on that day. I have a little brother. He is four years old. His birthday is in winter. It's very cold but we can make a snowman. My mother's birthday is in March. My father's birthday is in March，too. We can plant trees and fly kites.

根据下列图示的实验基本操作，回答问题．

结构简单、有口无肛门是腔肠动物的特征，以下不属于该动物类群的是（  ）