Editing Sea butterfly (section)

==Morphology==
Unlike other [[Sea snail|sea snails]], or even [[Land snail|land snails]], sea butterflies float and swim freely through the ocean, traveling along with the [[Ocean current|currents]]. This has led to a number of evolutionary [[Adaptation|adaptations]] in their bodies, including complete or near-complete loss of the shell and the [[gill]] in several families. Their [[gastropod]]al foot has also taken the form of two wing-like lobes, or ''[[parapodia]]'', which propel the animal through the sea by slow flapping movements.

[[file:sea butterfly.jpg|thumb|left|Unidentified thecosome]]
Most thecosomes have some form of calcified [[gastropod shell|shell]], although often very light.<ref name=Hunt-2007/>
They are rather difficult to see, since their shell, if present, is mostly transparent, fragile, and usually tiny (less than 1&nbsp;cm in length). Although their shell may be so fine as to be transparent, it is nevertheless [[calcareous]], and an important part of the oceanic [[calcium cycle]].<ref name=Comeau-Gorsky-etal-2009>
{{cite journal
 | last1 = Comeau  | first1 = S.    | last2 = Gorsky  | first2 = G.
 | last3 = Jeffree | first3 = R.    | last4 = Teyssié | first4 = J.-L.
 | last5 = Gattuso | first5 = J.-P.
 | year = 2009
 | title = Impact of ocean acidification on a key Arctic pelagic mollusc (''Limacina helicina'')
 | journal = Biogeosciences
 | volume = 6  | issue = 9  | page = 1877
 | doi = 10.5194/bg-6-1877-2009  | bibcode = 2009BGeo....6.1877C | doi-access = free 
| hdl = 10453/14721| hdl-access = free}}
</ref>
Their shells are [[symmetry (biology)#Bilateral symmetry|bilaterally symmetric]] and can vary widely in shape, ranging from coiled or needle-like to triangular or globular.

The shell is present in all life cycle stages of the [[Cavolinioidea]] (euthecosomata). In the [[Cymbulioidea]] (pseudothecosomata), adult Peraclididae also bear shells; the [[Cymbuliidae]] shed their larval shells and develop a cartilaginous ''pseudoconch'' in adulthood. Only the [[Desmopteridae]] lack any rigid covering when mature.