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Keel
A keel is a thickening of silica that contains the raphe within a canal. The keel elevates the raphe from the surface of the valve, allowing maximal contact of the raphe with surfaces for motility.
A number of genera possess a keel, including Nitzschia, Iconella and Entomoneis.
Image Credit: Ditmar Metzeltin, Marina Potapova, Pat Kociolek
A scanning electron micrograph of a frustule of Rhopalodia. The keel of each valve is indicated by an arrow. The raphe is visible as a faint line along the edge of each keel.
Image Credit: Marina Potapova
External surface of Surirella amphioxys (SEM). The raised portion of the undulation is a porca. The raised canal (keel) containing the raphe is at the top of the image.
Image Credit: Jonathan English
Surirella ovalis possesses fibulae that support the marginal raphe canal. Note that fibulae should not be confused with costae.