Pteropoda (common name pteropods, from the Greek meaning "wing-foot") are specialized free-swimming pelagic sea snails and sea slugs, marine opisthobranch gastropods. Most live in the top 10 m of the ocean and are less than 1 cm long. The monophyly of Pteropoda is the subject of a lengthy debate; they have even been considered as paraphyletic with respect to cephalopods. Current consensus, guided by molecular studies, leans towards interpreting the group as monophyletic. Pteropoda encompasses the two clades Thecosomata, the sea butterflies, and Gymnosomata, the sea angels. The Thecosomata ( "case-body") have a shell, while the Gymnosomata ("naked body") do not. The two clades may or may not be sister taxa; if not, their similarity (in that they are both pelagic, small, and transparent, and both groups swim using wing-like flaps (parapodia) which protrude from their bodies) may reflect adaptation to their particular lifestyle. Taxonomy The group Pteropoda was established by Georges Cuvier as "" in 1804. François Péron and Charles Alexandre Lesueur thought the group to be larger, and so they also included the opisthobranch taxa (Phyllirhoë and Glaucus), the heteropoda taxa (Carinaria and Firola), and even the Ctenophora (Callianira). In 1810, these authors divided the whole group in two separate groups: those with a shell and those without a shell. In 1824, Henri Marie Ducrotay de Blainville named these two groups Gymnosomata and Thecosomata and named the combining order Aporobranchia instead of Pteropoda. He rejected the additional genera, except Phyllirhoë which he upgraded to a third group that he called Psilosomata. Only much later was Phyllirhoë classified within the order Nudibranchia. Other attempts were made to describe the Pteropoda. John Edward Gray divided the Pteropoda into Dactylobranchia (with just the genus Cavolinia) and Pterobranchia (including all the other genera). Cuvier (and his followers) did not accept the classification by de Blainville; they preferred the original classification as described in . In 1829, Paul Rang followed the Cuvierian classification but tried to include the character of having a distinct head or not. The German naturalist Lorenz Oken went one step further and, for the sake of symmetry, wanted each order to contain four families and each family to contain four genera. Pierre André Latreille divided the Pteropoda according to the size of their fins: "" (including only Pneumonoderma) and "" (including all the others). In 1851, William Bullock Clark treated the Pteropoda as a family and emended the spelling