The South American boutu and Chinese beiji, for instance, appear to have very limited vision, and the Indian susus are blind, their eyes reduced to slits that probably allow them to sense only the direction and intensity of light.

例如，南美江豚和中国白鳍豚的视觉似乎非常有限，而印度江豚则完全没有视觉，它们的眼睛已经退化成窄缝，可能只能感知方向和光线强度。