placental mammals

例句：

1. 1. Placental mammals are the only animals that give birth to live young.
   胎盘哺乳动物是唯一能产活仔的动物。
2. 2. The placental mammals have a unique adaptation for carrying and feeding their young.
   胎盘哺乳动物具有独特的适应性，用于携带和喂养幼崽。
3. 3. Researchers are studying the genetic differences between placental mammals and marsupials.
   研究人员正在研究胎盘哺乳动物和有袋类动物之间的遗传差异。