Porca

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Image Credit: Jana Veselá
External surface of a species of Surirella (SEM). A porca is outlined in white. Two porcae are visible in the image.
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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.