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The mummies never change, even in death. It's change that Holden fears, not death and the mummies show us that. You could also go in another (darker) direction, and say the mummies are about lifeless shells – bodies without spirit, frozen physical forms, much like the fish stuck in the frozen lake and absorbing nutrients through their pores. Maybe this is how Holden sees himself?
The graffiti offends him so much because these are places that remind him of his own childhood, that would made him feel secure and comfortable and maybe happy? Yet in Holden's mind, this isn't possible; everything has been corrupted by vulgarities. Even his own death, Holden says, couldn't be sacred or peaceful, because someone would probably write a "flutter you" on his very tombstone.
any other questions, I would read this: