William Golding's novel, Lord of the Flies, has been approached critically in differing ways. One such way is the psychoanalytical approach, particularly that of Sigmund Freud. According to Freud, gods and devils are basically human processes projected onto the human world. Thus, according to critic Claire Rosenfield, Ralph represents a projection of man's good impulses, from which people derive authority figures, and Jack, on the contrary, represents the evil, instinctual forces of the unconscious.
Working with this dichotomy, Piggy, who is associated with Ralph represents the rational side of man, the mature man of society who has an acceptance of rules and order. As such, he looks older with his thinning hair, myopic eyes that need glasses, and heavy body. It is Piggy who suggests using the conch to call the boys to order when they need to meet and discuss important issues:
How can you expect to be rescued if you don't put first things first and act proper?
The conch is much like the gavel that is used by judges who call the court to order. Near the end of Golding's narrative, the conch crashes against the rock, "that token of preposterous time," symbolizing the end of an societal order on the island. Also, according to Rosenfield, Piggy, a father figure, is also a symbol of the degeneration of boys from adults to animalistic savages when he, too, is dashed against the large, sharp rocks and the conch
exploded into a thousand white fragments and ceased to exist....Piggy fell forty feet....Then the sea breathed again in a long, slow sigh, the water boiled white and pink over the rock; and when it went, sucking back again, the body of Piggy was gone.
Because Golding based his narrative upon the Victorian novel, Coral Island, in which boys were stranded on an island, but were able to fight the natives there and be victorious over savagery, the island is used in Lord of the Flies without any natives to represent a type of Garden of Eden so that Golding can demonstrate that man, on his own, releases his savage impulses when the controls of society are not present; and, these savage impulses symbolized in the character of Jack, and his sadistic counterpart, Roger, "who carried death in his hands."
In the first part of your question, the conch is considered a symbol of society and in some ways democracy because it is the object that confers some power upon its holder. For a time, the boy that holds the conch will be able to get everyone to listen to him.
Piggy serves in some way as a symbol of civilization as well, or at least the thoughtful application of science and reason to problems as he is the one who plays that role for the boys but is eventually destroyed by the anti-civilization group represented by Jack.