  In this paper we characterize the irreducible curves lying in $C^{(2)}$. We
prove that a curve $B$ has a degree one morphism to $C^{(2)}$ with image a
curve of degree $d$ with irreducible preimage in $C\times C$ if and only if
there exists an irreducible smooth curve $D$ and morphisms from $D$ to $C$ and
$B$ of degrees $d$ and $2$ respectively forming a diagram which does not
reduce.
