在1758年,瑞典生物學家卡爾·林奈建立二名法後幾年,他根據部分蛇、蜥蜴生存於水中,而建立兩棲綱,以包含兩棲動物、爬行動物[3]。在1768年,奥地利自然學家约瑟夫斯·尼古劳斯·劳伦蒂建立爬行綱(Reptilia),但當時兩棲動物、爬行動物的界定仍然模糊[4];Repere在拉丁語中意為「爬行」[5]。直到18世紀末期,兩棲動物、爬行動物才被視為不同的動物。在1825年,拉特雷耶·皮埃爾·安德烈(Pierre André Latreille)建立「Batracia」綱,以取代兩棲綱,並將四足動物分成四大類:兩棲動物、爬行動物、鳥類、哺乳動物,已經相當接近現代傳統分類法[6]。
在1916年,Edwin Stephen Goodrich建立了蜥形綱與獸形綱,用來區別蜥蜴、鳥類與其近親,以及哺乳類與其已滅絕近親。兩個演化支的心臟與血管差異,以及前腦結構等特徵,證實這個分類法。根據Goodrich宣稱,蜥形綱與獸形綱共同從一個較早期的原始動物演化而來,名為「Protosauria」,該分類單元也包含某些古生代的兩棲類以及早期爬行動物[8]。
爬行動物的排泄系統主要藉由兩顆腎臟。雙孔動物所排泄的主要含氮廢物是尿酸;澤龜龜主要排泄尿素,陸龜如蘇卡達、星龜排尿酸,而部分則排尿液,類似哺乳類。不像哺乳類與鳥類的腎臟,爬行動物的腎臟不能夠製造尿液,尿液可以儲藏更多的身體廢物。這是因為牠們缺乏鳥類與哺乳類所擁有的腎元,稱為亨利氏環(Loop of Henle)。因此許多爬行動物使用牠們的結腸與排泄腔來再度吸收水分。有些爬行動物甚至可利用膀胱來取得水分。有些爬行動物的鼻部與舌部具有鹽腺,可以儲存額外的鹽分。
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^Latreielle, P.A. (1804): Nouveau Dictionnaire à Histoire Naturelle, xxiv., cited in Latreille's Familles naturelles du règne animal, exposés succinctement et dans un ordre analytique, 1825
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