下列属于腔肠动物的是（　　）

阅读下面一首诗，完成下面题目。
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                                                                                                                               李商隐
                                                                                                      相见时难别亦难，东风无力百花残。
                                                                                                      春蚕到死丝方尽，蜡炬成灰泪始干。
                                                                                                      晓镜但愁云鬓改，夜吟应觉月光寒。
                                                                                                       蓬山此去无多路，青鸟殷勤为探看。