Let us return to Aeolidina.
Flabelinna affinis on a chocolate-coloured Ircinia felix sponge demonstrates it's ribbed rhinophore horns.
"Rhino" in classic Greek means "nose", "phore" is "carrier" or "owner".
Correspondingly, rhinophore are the organs of smell, which appears to be slugs' most important sense along with the sense of touch.
That is why the rhinophore is ribbed — this increases it's surface and the quantity of receptors contacting with chemicals of odour.