The pig’s head and the conch are two very different symbols of power in the novel. Discuss the theme of power and how it is explored in the novel.

If the pig's head is a symbol of one type of power, it is the incredible power of human nature, in this case mostly for evil.  The beast even reveals this side of power to Simon.  The boys start to exhibit some aspects of this type of power as they give themselves over to Jack's tribe, submitting to their more carnal desires to hunt, to be anonymous through the use of paint and filth, to hurt and to kill and to feed.  This power becomes irresistable to all but Ralph and Piggy and Samneric, even as some of the boys under its spell object to the use of that power by Jack.

The conch stands for the power of civilization, the power of rules and order and, in this case, the very thin veneer of goodness that it lends to people.  It only takes a few days for this power to be overcome by the power of disorder, of inner desire, of the animalistic tendencies of the boys.

Through the boys, Golding has crafted a commentary on the influence of both kinds of powers and perhaps what holds it at bay some of the time in "civilized" society.