鹦鹉螺壳对称斐波那契半横截面螺旋金比结构生长鹦鹉螺特写后亮母珍珠特写股票，照片，照片，图像，图片, Nautilus shell pearl Fibonacci sequence symmetry cross section spiral shell structure golden ratio background nature pattern mollusk shell (nautilus pompilius) copy space half split stock photo