1a体外长有皮毛,胎生...................................................................................................哺乳类 1b体外没有皮毛,卵生............................................................................................................2 2a体外长有羽毛,体温恒定..................................................................................................P类 2b体外没有羽毛,体温不恒定...............................................................................................3 3a表皮干燥,在陆地上产卵...................................................................................................Q类 3b皮肤湿润,在水中产卵.........................................................................................................4 4a幼体生活在水中,用鳃呼吸:成体生活在水中或上陆生活,用肺呼吸,有四肢.........R类 4b幼体、成体都生活在水中,用鳃呼吸,无四肢,用鳍划水.........................................S类 |
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