Amphibians are a fascinating group of vertebrates that bridge the gap between aquatic and terrestrial life. They are characterized by their unique life cycle, which involves metamorphosis from water-breathing larvae to air-breathing adults. Most amphibians have moist, scaleless skin and are cold-blooded animals that depend on their environment for temperature regulation.
The most remarkable feature of amphibians is their metamorphosis. This transformation begins with eggs laid in water. The larvae, such as tadpoles, live entirely in water and breathe through gills. As they develop, they gradually grow limbs and develop lungs. Eventually, they lose their gills and tail, becoming adults capable of living on land while still returning to water for reproduction.
Amphibians have evolved unique breathing mechanisms that change throughout their lives. Larvae like tadpoles breathe through gills while living underwater. Adult amphibians develop lungs for air breathing, but they also rely heavily on their moist skin for gas exchange. This dual breathing system requires them to live in humid environments near water sources. Their permeable skin makes them sensitive to environmental changes and pollution.
Adult amphibians are primarily carnivorous, feeding on insects, worms, and other small invertebrates. They are skilled hunters, often using their sticky tongues to catch prey. In contrast, many larvae are herbivorous or omnivorous. For reproduction, most amphibians return to water where they engage in external fertilization. Females lay soft, jelly-covered eggs in water, and males fertilize them externally. Most species provide no parental care, relying on large numbers of offspring for species survival.
两栖动物是一类非常特殊的脊椎动物，它们的名字来源于希腊语，意思是双重生活。两栖动物最显著的特点是能够在水中和陆地上都能生存。它们具有湿润的皮肤，可以通过皮肤进行气体交换，是变温动物，体温会随环境温度变化。大多数两栖动物都会经历从水生幼体到陆生成体的变态发育过程。
现存的两栖动物主要分为三个目。无尾目包括我们熟悉的蛙和蟾蜍，它们成体没有尾巴，后肢发达善于跳跃，幼体叫做蝌蚪。有尾目包括蝾螈和娃娃鱼，它们终生保持尾巴，四肢等长，主要生活在潮湿的环境中。无足目是蚓螈，它们没有四肢，外形像蚯蚓，大多数生活在地下，种类相对较少。
两栖动物对栖息地有特殊要求。它们既需要水生环境，如淡水湖泊、池塘、溪流等，用于繁殖和幼体发育；也需要陆生环境，如森林地面、潮湿土壤、石缝洞穴等，供成体生活。有些种类甚至生活在树上。由于皮肤湿润且透气性强，两栖动物对环境变化非常敏感，需要相对稳定的湿润环境和良好的水质。
两栖动物的变态发育是一个奇妙的过程。首先是卵期，卵产在水中或潮湿环境，外面有胶质膜保护。接着是幼体期，以蝌蚪为例，它们完全水生，用鳃呼吸，有尾巴游泳，主要吃植物。然后进入变态过程，长出四肢，尾巴逐渐消失，鳃退化而肺发育，消化系统也发生改变。最后成为成体，可以在陆地生活，用肺和皮肤呼吸，变成肉食性动物。
作为冷血动物，两栖动物无法内部调节体温，必须依靠行为策略。白天它们经常晒太阳取暖，而温度过高时会寻找阴凉处或水源。大多数两栖动物在黎明、黄昏或夜间最为活跃，这时温度适中，湿度较高。在寒冷季节，许多种类会进入冬眠或夏眠状态来度过极端温度期。