The most conspicuous of these is
the little fleshy pad at the inner corner of each eye. It is a
common feature in mammals, and is always useless. When, however,
we look lower down in the animal scale we find that fishes and
reptiles (and birds) have a third eyelid, which is drawn across
the eye from this corner. There is little room to doubt that the
little fleshy vestige in the mammal's eye is the shrunken
remainder of the lateral eyelid of a remote fish-ancestor.
A similar reminiscence is found in the pineal body, a small and
useless object, about the size and shape of a hazel-nut, in the
centre of the brain. When we examine the reptile we find a third
eye in the top of the head. The skin has closed over it, but the
skull is still, in many cases, perforated as it is for the eyes
in front. I have seen it standing out like a ball on the head of
a dead crocodile, and in the living tuatara--the very primitive
New Zealand lizard--it still has a retina and optic nerve. As the
only animal in nature to-day with an eye in this position (the
Pyrosome, a little marine animal of the sea-squirt family) is not
in the line of reptile and mammal ancestry, it is difficult to
locate the third eye definitely.
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