海底世界之鲸
鲸的体型很大，是世界上最大的哺乳动物，不是鱼。眼小，尾呈水平鳍状；用肺呼吸，最大的体长可达30多米，最小只有1米。目前，已知最大的鲸是蓝鲸，约有190吨重，最小的有40千克。鲸的体型是世界上现在存在的动物中最大的，鲸是终生生活在水中的哺乳动物；对水的依赖程度很大，以致它们一旦离开了水便无法生活；鲸形状像鱼，鼻孔在头的上部，用肺呼吸。
鲸为适应水中生活，减少阻力，它们的后肢退化；前肢变成划水的浆板。身体成为流线型，酷似鱼。鲸的潜水能力很强，小型齿鲸可潜至100-300米的水深处，停留4-5分钟；长须鲸可在水下300-500米处呆上1小时，抹香鲸能潜至千米以下，并在水中持续2小时之久。
鲸，世界上最大的哺乳动物，不是鱼；分为须鲸，虎鲸，伪虎鲸，座头鲸。鲸生活在海洋中。鲸的祖先原先生活在陆地上，因环境变化，后来生活在靠近陆地的浅海里。又经过了很长时间的进化，鲸的前肢和尾巴渐渐成了鳍，后肢完全退化，整个身子成了鱼的样子（所以人们误认其为鱼），适应了海洋的生活。
鲸一般生活在南极、北太平洋、北大西洋，品种有须鲸亚目、齿鲸亚目、古鲸亚目（已灭绝）。
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