The radulas of at least some chitons and perhaps some limpets are
impregnated with magnetite, which is a very hard and abrasive substance
in your gut.  It helps them scrape algae off rocks.

On Fri, 1 Feb 2002 15:22:18 -0500 bivalve
<[log in to unmask]> writes:
> >Are limpet radulae more likely to cause health problems (apparently
> gastro-intestinal problems, from what I can gather) than other
> radulae?  Are ingested radulae a known cause of health problems for
> people who consume Littorina, for example?  Or Helix?<
>
> The problem is that the radulae are relatively large and hard to
> digest.  As a result, eating lots of whole limpets produces a ball
> of radulae in the stomach and a tummyache.
> I found a pile of chitons and radulae in the Bahamas, which suggest
> that the locals avoid those as well.  My guess is that anythign with
> a relatively large radula is better eaten without it.
>
>     Dr. David Campbell
>     Old Seashells
>     46860 Hilton Dr #1113
>     Lexington Park MD 20653 USA
>     [log in to unmask]
>
> That is Uncle Joe, taken in the masonic regalia of a Grand Exalted
> Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at
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>