They are sometimes known as sea angels.

Many free-swimming sea slugs, such as sea angels, flap fin-like structures.

This pelagic snail is not at all closely related to the pelagic opistobranchs such as the sea angels and sea butterflies.

As is the case in all gymnosome pteropods, these sea angels lack a shell except during an early embryonic stage.

The other suborder of pteropods, Thecosomata, is superficially similar to sea angels, but are not closely related.

Their "wings" allow sea angels to swim much faster than the larger (usually fused) wings of sea butterflies.

Another large polar species of sea angel, Clione antarctica, defends itself from predators by synthesizing a previously unknown molecule, pteroenone.

Video of a sea angel in motion: http://www.biol.sc.edu/ vogt/courses/neuro/neurobehavior.

Sea butterflies of the order Thecosomata have a shell, while sea angels in the order Gymnosomata do not.

The Clionidae are a family of sea angels, which are a group of pelagic marine gastropods.