A crab's hard shell contains a large amount of chitin.
Tips
usage
壳质 (chitin) is the scientific/biology term - a long-chain polysaccharide found in insect exoskeletons, crustacean shells, and fungal cell walls. The everyday word 壳 just means 'shell', without the chemistry.
mistakes
The 壳 in 壳质 is read qiào, not ké. The qiào reading appears in technical/literary compounds (地壳 'earth's crust', 甲壳 'carapace'); ké is the colloquial reading (蛋壳 'eggshell').
