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鱼是生长在水中的动物。生活在水中的动物,也和陆地上的动物一样,概括起来可以分为两大类:一类是背部有一条脊椎骨,称为脊椎动物;另一类是背部没有脊椎骨的,叫做无脊椎动物。一般说来,“鱼”是适应于水中生活的脊椎动物。但是这样回答尚不完善。诚然,在水中生活的一些无脊椎动物,如:虾、蟹、螺、蚌,各种浮游动物……,这些都不属于“鱼”,人们很容易把它们和鱼类区分开来。不过,生活在水中的动物,许多是背部具有脊椎骨,同时还有鱼的称呼。如:甲鱼、娃娃鱼、鲸鱼、鳄鱼……。而这些种类又不能算作“鱼”,它们和真正的鱼类又有什么区别呢?
Fish are animals that grow in the water. Animals that live in the water, like animals on land, can be summed up in two broad categories: one is a vertebra on the back called a vertebrate, and the other a vertebrate on the back without a vertebra, called an invertebrate. In general, “fish” is a vertebrate adapted to living in water. But this answer is not perfect. It is true that some invertebrates living in the water, such as shrimp, crabs, snails, mussels and various zooplankton ... are not “fish” and are easily distinguishable from fish. However, many of the animals that live in the water have vertebrae on the back, as well as fish. Such as: turtle, salamander, whale, crocodile ... .... And these species can not be counted as “fish”, what is the difference between them and real fish?