Secondary aquatic animals are those whose ancestors, in the process of evolution, first came out of water to land

Secondary aquatic animals are those whose ancestors, in the process of evolution, first came out of water to land, and then returned back to water. They belong to the most different taxonomic groups, these are insects and vertebrates, and molluscs. What is the common feature of all these animals, according to which is it safe to say that they are secondary water?

Secondary aquatic animals in the process of evolution returned back to the water element, despite the fact that they initially adapted to existence on land. For example: reptiles live in the water – sea snakes, crocodiles, sea snakes, turtles and mammals – various types of whales. In the process of evolution, they have formed such organs that allow them to breathe under water and on land. This is their feature.