The boys use the conch shell to call meetings. The boy who held the conch had the power to talk.
At the end of the novel, the conch is shattered, representing the unravelling of the boys' humanity and civility.
At the beginning of the novel, the boys make a point to keep their signal fire burning, showing their desire to be rescued. As time goes on, the forget about the fire and about being rescued.
When Piggy is killed, he is washed out to sea, representing his purity and goodness.
The ocean is the source of salvation for the boys. It takes Piggy away from a situation where he was miserable and it brings them the naval officer that saves them.
Piggy is the smartest boy on the island. His glasses represent the knowledge humans have gained as a result of civilization. When the crack, it shows the breaking of the boys' humanity.
When the boys steal Piggy's glasses, they get the power of fire, allowing them to eat meat, stay warm, and survive.
The lord of the flies comes to life in Simon's vision as the fly-covered sow head on a stick. It tells him that he cannot escape. This evil shows itself when Simon goes back to tell the others what he has seen, and they kill him with their bare hands and teeth.
The fact that the lord of the flies manifests itself as a decomposing sow head shows that it represents the animals instinct within every human.