On Sun, 2 Dec 2018 at 21:06, Lieven L. Litaer <levinius@gmx.de> wrote:
  - {-Du'} is a suffix for body parts and is generally used for body
parts. When non-body parts are named after body parts (like the teeth on
a gear or a cumb) then they are still associated with body parts, so the
suffix {-Du'} is used. But there are very few words that originally were
body parts, but the connection to the body part meaning has been lost.
So now, they use {-mey} for plural suffix.

I have a follow-up from Maltz for this about the suffix for body parts which are removable or transferable (e.g., in the case of hermit crabs).

--- begin quote ---
{nagh DIr} is considered a body part. The plural takes -{Du'}. That's the case whether the shell is still on the animal or not. But if the shell (off the animal) is broken up and a piece or pieces of it are used for something (like turtle shells here on Earth have been used for guitar picks), the plural takes {-mey}.
--- end quote --- 

--
De'vID