What adaptations to life in the aquatic environment were formed in different classes of animals in the process of evolution?

The aquatic environment is very large, as water covers most of our planet. It was in the water that life was born, so evolution in this environment took much longer than in other habitats. Aquatic inhabitants are able to move around thanks to special body parts. These can be fins. Jellyfish do not have fins, but they are able to move in water. Each living organism lives at a certain depth, since the deeper you go, the more pressure will act on the body. Special body colors also help to survive.