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).

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.

[7] Occasionally, thecosomans swarm in large numbers, and can sometimes be found washed ashore in flotsam, especially along the coastline of eastern Australia.

[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.