But the ancestors of insects looked more like
thousand-legs or centipedes, and here head and thorax are much less
distinct. But in the annelid the mouth is on the second segment;
here it is on the fourth. It has evidently travelled backward. That
the mouth of an animal can migrate seems at first impossible, but if
we had time to examine the embryology of annelids and insects, it
would no longer appear inconceivable or improbable. And its backward
migration brought it among the legs which were grasping and chewing
the food. And in vertebrates the mouth has changed its position,
though not in exactly the same way. Our present mouth is probably
not at all the mouth of the primitive ancestor of vertebrates. Thus
in the insect three segments have fused around the mouth, and three,
possibly four, in front of it. This makes a head worthy of the name.
The ganglia of the three post-oral segments, which bear the jaws,
have fused in one compound ganglion innervating the mouth and jaws.
Those of the three prae-oral segments have fused to form a brain.
Eyes are well developed, giving images sometimes accurate in detail,
sometimes very rude. Ears are not uncommon. The sense of smell is
often keen.
Perhaps the greatest advance of the insect is its adaptation to land
life. This gives it a larger supply of oxygen than any aquatic
animal could ever obtain.
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