d that, they are "defined more by what they do not have than by any particular series of specializations."。不同于其他两侧对称动物，扁形动物并没有体腔，所以被称为无体腔动物。牠们也没有特化的循环系统及唿吸系统器官，而这些都是为扁形动物分类时的解剖特征。牠们的身体是柔软且没有分节的。
Features common to all subgroups.The lack of c