Limacinidae Cavoliniidae Cliidae Creseidae Cuvierinidae Praecuvierinidae Peraclididae Cymbuliidae Desmopteridae The Thecosomata (collective/plural: thecosomes,[1] meaning "case/shell-body"),[2] or sea butterflies, are a taxonomic suborder of small, pelagic, free-swimming sea snails known as holoplanktonic opisthobranch gastropod mollusks, in the order Pteropoda (also included within the informal group Opisthobranchia).

This has led to a number of evolutionary adaptations in their bodies, including complete or near-complete loss of the shell and the gill in several families.

Their gastropodal foot has also taken the form of two wing-like lobes, or parapodia, which propel the animal through the sea by slow flapping movements.

Thus, the high bending-angle supports the parapodia to diminish the drag forces generated by the classic "clap-and-fling" maneuver; additionally, it aids in carrying the extra weight of a shell and ascending the water column for the diel vertical migration.

In contrast, flying insects and shell-less pteropods encounter higher resistance forces that limit the body rotation.

[a] Thus, thecosomans are most common in the top 25 metres (82 ft) of the ocean—in terms of diversity, species richness, and abundance—and become scarcer with increased depth.

[3] However, the low stability of their aragonitic shells means that few end up being preserved within sediments as viable fossils; rather, they are mostly deposited in shallow, tropical seas.

[3] These creatures, which range from lentil- to orange-sized, are eaten by various marine species, including a wide variety of fish that are, in turn, consumed by penguins and polar bears.

[3] The validity of the pteropod order is not universally accepted; it fell out of favour for a number of years, but recent molecular evidence suggests that the taxon should be revived.