Fish live in the water all their lives, breathing with gills and swimming with fins. Most of them have scales on their bodies. They are fertilized outside the body and lay eggs. Their body temperature is not constant. Their bodies are streamlined and have lateral lines. The seal's body is covered with fur, it can swim in the water, breathe with its lungs, and has the characteristics of mammalian viviparous and lactating animals, so it is not a fish but a mammal.