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.
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 is also a general term, like terminal nodule.
Image Credit: Paula Furey
This image is a scanning electron micrograph of the internal distal valve of Eunotia macroglossa, showing the helictoglossa positioned on the valve mantle.
Image Credit: Pat Kociolek
Light micrograph of the footpole of Gomphoneis olivaceum. The helictoglossae is somewhat removed from the valve apex.
Image Credit: Marina Potapova
SEM of the footpole of Gomphoneis geitleri, showing the internal view. The distal raphe terminates in the raised, and thickened helictoglossa.