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Atlantidae is a family of sea snails, holoplanktonic gastropod molluscs in the clade Littorinimorpha. According to taxonomy of the Gastropoda by Bouchet & Rocroi (2005) the family Atlantidae has no subfamilies. == Description == The Atlantidae is a group of holoplanktonic gastropods, which all demonstrate a strong adaptation to pelagic life, in the form of a lenticular, laterally flattened, aragonitic shell, the surface of which is further enlarged by the presence of a wide, and very thin and fragile, double-walled keel.〔 The apex is on the right side of the shell, the umbilicus on the left.〔 For the living animal the enlargement of the shell’s surface serves ‘to increase stabilization during swimming and sinking’.〔 This phenomenon results in the occurrence of superficially very similar adult shells in separate species, whereas the larval shells may be utterly different.〔 Their body size is microscopic with a shell diameter of less than 1 cm. The foot has evolved into a muscular swimming fin. Their swimming fin extends anteriorly beneath their head. They swim with their ventral part upward through undulation of their swimming fin. The shell is coiled dextrally (i.e. clockwise). The thin, double-walled keel of the shell extends outward from the last shell whorl. The apex of the shell is found on the right side, the umbilicus on the left side. The shell and keel can be calcareous (genus ''Atlanta''), or composed exclusively of conchiolin (genus Oxygyrus), or the shell can be calcareous and its keel composed of conchiolin (genus ''Protatlanta''). The snails of this family can retract into their shell and close it off with a cartilaginous, flexible operculum. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Atlantidae」の詳細全文を読む スポンサード リンク
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