$x=-\sqrt{64-y^{2}}$
$x=\sqrt{64-y^{2}}$

$y=-\sqrt{64-x^{2}}$
$y=\sqrt{64-x^{2}}$

$x=\sqrt{64-y^{2}}$
$x=-\sqrt{64-y^{2}}\text{, }|y|\leq 8$

$y=\sqrt{64-x^{2}}$
$y=-\sqrt{64-x^{2}}\text{, }|x|\leq 8$