Rays have gill slits on the ventral side of their bodies, while sharks have gill slits on the side of their bodies.

Penguins are flightless birds, or avifauna. They used to fly in the past, but it is thought that they evolved and stopped flying. Instead, they became good at swimming & diving.

They stay still in the cold deep sea to conserve energy as much as possible.

Penguins live only in the Southern Hemisphere. Penguins are thought to be found only in cold areas, but they also live in warm areas.

Members of the toothed " Toothed whale" family, such as the Common bottlenose dolphin and Pacific white-sided dolphin, have almost completely lost their olfactory structures, such as the nerves that sense smell.
This is because dolphins live in water and do not need it during the evolutionary process.