They are found all over the world's oceans, most of them near the surface with less and less as you go deeper. Most of them have a thin, transparent shell, which I guess is very lightweight and is only about 1cm across. These are usually still calcified for whatever strength they can manage, but some are more cartilaginous and other Sea Butterflies don't have much of a shell at all. Regardless, they are all very delicate and even dainty little animals.
The foot of the Sea Butterflies has become a large pair of wings which are used to flap or soar in the water. They are very slow moving but it really does look like they are doing all the fluttering and gliding of their insect namesakes; from vigorous wing thrusts to the serenity of floating with the currents, it all looks wonderful! I'm not so sure about their table manners, though.
Not much more seems to be known about Sea Butterflies. They are so small and difficult to see, so fragile and difficult to catch alive - although quite a lot of animals aren't that bothered about the "alive" bit. These humble snails can sometimes form swarms that provide food for fish, birds and even whales. All the more unfortunate then that ocean acidification may lead to the extinction of the shell forming ones. I really hope that doesn't happen. They show that even snails can be beautiful and mesmerizing, even if their eating habits, while probably equally mesmerizing, are just a bit wrong.