Porca

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Image Credit: Jana Veselá
Scanning electron micrograph of the external surface of a species within Surirella. A porca is outlined in white; a total of 2 porcae are visible in the image.
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Image Credit: Marina Potapova
Scanning electron micrograph of the external surface of Surirella amphioxys. The raised portion of the undulation is a porca. The raised canal containing the raphe is at the top of the image.