EMBLEM

An emblem is similar to a symbol except that it is more limited in its meaning and often has a direct reference to just one object. Whereas the conch in Lord of the Flies has many significances: civilisation, order, comradeship, leadership etc., the daffodil, as emblem of Wales, means just that country. It also carries its reference with it: a daffodil indicates Wales to anyone, whereas a conch means civilsation only within the novel. A literary example would be the violet in the seventeenth century which was widely accepted as an emblem of faithful love and truth.

Symbol

A symbol is an object which develops deeper meanings, sometimes during the course of a literary work. In "Lord of the Flies" the conch becomes a symbol of civilisation and order. Yet it is a "real" shell present in the narrative also.
In simile or metaphor the compared object is not real or present. If we speak of her "shell-like ears" there is no shell present in the discourse. Similarly in: "He retreated into his shell", there is no real shell.