logical meaning of the generic name Limacina is "snail-like".
These little snails are pelagic, and they are marine gastropod mollusks. They swim by flapping their parapodia, and that flapping action is the reason why they were given the common name sea butterflies.
The sea butterflies are part of the clade Thecosomata. The sea angels, which look a little similar and are also pelagic swimming snails, are in a different order, the Gymnosomata. Both of these orders are still sometimes called "pteropods"; the sea butterflies of the order Thecosomata have a shell, while the sea angels in the order Gymnosomata lack a shell.
Read more about Limacina: Shell Description, Description of The Soft Parts, Life Habits, Species