  We show that the algebraic curve $a_0(x)(y-r(x))+p_2(x)a'(x)=0,$ where $r(x)$
and $p_2(x)$ are polynomial of degree 1 and 2 respectively and $a_0(x)$ is a
polynomial solution of the convenient Fucsh's equation, is an invariant curve
of the quadratic planar differential system. We study the particular case when
$a_0(x)$ is an orthogonal polynomials. We prove that that in this case the
quadratic differential system is Liouvillian integrable.
