Feeding Ramshorns to Puffs - What's that Black Thing?

[Pufftastic]

I'm maintaining a 10 gallon tank of ramshorns to feed my F8s.

When I feed the snails to my puffers, I crush them first. The puffs attack the snails immediately, but there's this, like, harder part of the snail's insides, kind of black in color, that doesn't get eaten right away and just falls to the tank floor. (They do go back for it later, though, once they're satisfied that I'm not dropping in any more snails.)

What IS that black thing? My knowledge of snail anatomy is about zero.

[RTR]

Snail poop in process.

[Pufftastic]

Thanks! Ewwww.

[RTR]

Yup!

[zookey]

Now I know why it is not eaten! I wouldn't be in a hurry to eat it either!