Vertebrate

The big breakthrough of the vertebrates appears to be the ability to swim around in a straight line and eat smaller species that are floating about.

Bones appear to help that a lot!

It is likely the most efficient design to travel long distances. Be thin and wiggle your tail around.

Perhaps smaller animals can skip the bone thing. Maybe a notable example are the lancets, which look a bit like small fish. But they only go up to 8 cm.