In the annelid especially, and to some extent in the schematic worm, the supra-oesophageal ganglion is relieved in part of the direct control of the muscular fibrils and has become an organ of perception and the seat of government of lower nervous centres.  In all higher forms it innervates directly only the principal sense-organs of the head.  And at this stage the light-perceiving directive eye has developed into a form-perceiving, eidoscopic organ.  The eye was short of range and its images were perhaps rude and imperfect, but it was a visual eye and had vast possibilities.  The animal is taking cognizance of ever more subtle elements in its environment.  Perhaps it is not too much to say that the eidoscopic eye first awakened the slumbering animal mind, for its reflex effect upon the supra-oesophageal ganglion cannot be over-estimated.  The animal will very soon begin to think.

Between the turbellarian and the annelid many aberrant lines diverged.  Some of these attained a comparatively high level and then seemed to meet insuperable obstacles, while others came to an end or turned downward very early.  Three of these demanded attention, those leading to mollusks, insects, and vertebrates.  And it is interesting to notice that the fundamental difference between these three lines was the skeleton, or perhaps we ought to say it was the habit of life which led to the development of such a skeleton.

The mollusk took to a sluggish, creeping mode of life, under an external purely protective skeleton; the insect to a creeping mode of life, with an external but almost purely locomotive skeleton; the vertebrate kept on swimming and developed an internal locomotive skeleton.  And it must already have become clear to you that the destiny of these different lines was fixed not so much directly by the skeleton itself as by its reflex effect in moulding the muscular, and ultimately the nervous, system.

The insects formed their skeleton by thickening the horny cuticle of the annelid.  They transformed the annelid parapodia into legs and developed wings.  They attained life in the air.  They devoted the muscles of the body largely to the extremities and gained swift locomotion.  They have a fair circulatory and an excellent respiratory system.  Best of all, they developed a head and a brain by fusing the three anterior ganglia of the body.  The insect could and does think.  Such a structure ought to lead to great and high results.  But actually their possibilities were very limited.  They have not progressed markedly during the last geological period.  Their external skeleton was easily attained and brought speedy advantages, which for a time placed them far above all competitors.  But it limited their size and length of life and opportunities, and finally their intelligence.  They remained largely the slaves of instinct.  They followed an attractive and exceedingly promising path, but it led to the bottom of a cliff, not to the summit.