But the annelids were probably the first animals to develop an eye capable of forming an image of external objects.  The importance of this organ in the pursuit of food or the escape from enemies can scarcely be over-estimated.  The lining of the mouth and pharynx can be protruded as a proboscis, and drawn back by powerful muscles, and is armed with two or more horny claws.  Eyes and claws gave them a great advantage over their not quite blind but really visionless and comparatively defenceless neighbors, and they must have wrought terrible extinction of lower and older forms.  But while we cannot over-estimate the importance of these eyes, we can easily exaggerate their perfectness.  They were of short range, fitted for seeing objects only a few inches distant, and the image was very imperfect in detail.  But the plan or fundamental scheme of these eyes is correct and capable of indefinitely greater development than the organs of touch or smell, perhaps greater even than the otolith vesicle.

And the reflex influence of the eye on the brain was the greatest advantage of all.  Hitherto with feeble muscles and sense-organs it has hardly paid the animal to devote more material to building a larger brain.  It was better to build more muscle.  But now with stronger muscles at its command, and better sense-organs to report to it, every grain of added brain material is beginning to be worth ten devoted to muscle.  The muscular system will still continue to develop, but the brain has begun an almost endless march of progress.  The eye becomes of continually increasing advantage and importance because it has a capable brain to use it; and brain is a more and more profitable investment, because it is served by an ever-improving eye.

  [Illustration:  9.  MYRMELEO FORMICARIUS.  ANT-LION.  HERTWIG, FROM
  SCHMARDA.
  1, adult; 2, larva; 3, cocoon.]

The annelid had hit upon a most advantageous line of development, which led ultimately to the insect.  The study of the insect will show us clearly the advantages and defects of the annelid plan.  First of all, the insect, like the mollusk, has an external skeleton.  But the skeleton of the mollusk was purely protective, a hindrance to locomotion.  That of the insect is still somewhat protective, but is mainly, almost purely, locomotive.  It is never allowed to become so heavy as to interfere with locomotion.  In the second place, the insect has three body regions, having each its own special functions or work.  And one of these is a head.  The annelid had two anterior segments differing from those of the rest of the body; these may, perhaps, be considered as the foreshadowings of a structure not yet realized; they can only by courtesy be called a head.  Thirdly, the insect has legs.  The annelid had fin-like parapodia, approaching the legs of insects about as closely as the fins of a fish approach the legs of a mammal.  The reproductive and digestive systems, while somewhat improved, are not very markedly higher than those of annelids.  The excretory system has more work to perform and reaches a rather higher development.