What is an order of pelagic Gastropoda, having the foot developed into a median fin. Some of the species are naked; others, as Carinaria and Atlanta, have thin glassy shells called?

heteropoda — An order of pelagic Gastropoda, having the foot developed into a median fin. Some of the species are naked; others, as Carinaria and Atlanta, have thin glassy shells. (Webster's 1913.)

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