被称为城市的肾的杭州湾湿地景观带，湿地内土地肥沃、潮湿，是野生动植物的天然场所，湿地内有几百种动物和植物。常见的有白鹭、螃蟹、蛇、芦苇、青蛙，有同学为它们编了一个检索表，请帮助他继续完成。

1  a 有叶绿体，能进行光合作用························ P

1  b 无叶绿体，不能进行光合作用······················· 2

2 a 无脊椎骨······························· Q

2 b 有脊椎骨······························· 3

3 a 体温恒定，外被羽毛························· R

3 b 体温不恒定，体表裸露························ S

请选择上述五种生物中的四种填在后面四个空格：PQRS