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The internal, distal termination of the raphe in the shape of a pair of lips or a rolled tongue. The helictoglossa occurs in many raphid diatoms. If present, it can be distinguished in the light microscope in valve view by optical dissection (focusing through many narrow, optical planes). It may also be visible in girdle view as a thickened lip of silica. Plural form is helictoglossae. From Greek for rolled tongue.

Alternative Term

terminal nodule

Terminal nodule is a more general term than helictoglossa, because it has been used to refer to the thicking of silica at the apices of raphid diatoms rather than a specific structure.


Some of the older literature uses the term infundibulum to describe the helictoglossa.

polar nodule

Polar nodule is also a general term, like terminal nodule.