1. The curve corresponding to the equation $x \sqrt{1-y^{2}}-y \sqrt{1-x^{2}}= \mathrm{~ m ~ is ~}$ ( ).

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-,1.L.
From the given, we have
$\left(x-\sqrt{1-y^{2}}\right)^{2}+\left(y+\sqrt{1-x^{2}}\right)^{2}$
$=2 \cdots 2\left(, \sqrt{1 \cdots y^{2}} \cdots \sqrt{1-x^{2}}\right)=2-2=0$.
Thus, $x \sqrt{1-y^{2}}=0, y+\sqrt{1-x^{2}}=0$,
which implies $x^{2}+y^{4}=1(x \geqslant 0, y \leqslant 0)$.