Since this word is made up of a single kanji, it probably uses the kun'yomi reading. You did not learn this reading, so here's a mnemonic to help you:
Shells come from (から) living animals. You can't have a shell — or a carapace (から) — unless something was once in there. They have to come from something.
This word isn't limited to animal shells though! It covers seed shells, or husks, too.
Please put crab shells on this plate.
What do we do after removing the husks from wheat?
I watched a video that captured the moment when a seed breaks through its shell and sprouts.