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The nervous system consists of a large supra-oesophageal ganglion in the first segment; then of a chain of ganglia, one to each segment, on the ventral side of the body. With one ganglion in each segment there is far more controlling, perceptive, ganglionic material than in lower worms.
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.
But a ganglion cell sends an impulse to a muscle because it has received one along a sensory nerve from one or more of these sensory cells. Hence the ganglion cells will increase in number. The old cobweb-like plexus condenses into a little knot, the supra-oesophageal ganglion.
The snail represents the average result of the experiment. It can crawl, but that is about all; it is neither swift nor energetic. Even the earthworm can outcrawl it. It has feelers and eyes, and is thus better provided with sense-organs than almost any worm. It has a supra-oesophageal ganglion of fair size.
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.
The little knot of nervous material which forms the supra-oesophageal ganglion is so small that it might easily escape our notice; but it is the promise of an infinite future. The atom of nervous power shall increase until it subdues and dominates the whole mass.
The highest forms gave up the purely defensive campaign, developed a powerful beak, led a life like that of the old Norse pirates, and were for a time the rulers and terrors of the sea. With their more rapid locomotion the supra-oesophageal ganglion reached a higher degree of development, and it was served by sense-organs of great efficiency.
Furthermore the supra-oesophageal ganglion is relieved of a large part of the direct control of the muscles of each segment, and is becoming more a centre of control and perception for the body as a whole. It is more like our brain, commander-in-chief, the other ganglia constituting its staff. The sense-organs have improved greatly.
And the high development of their supra-oesophageal ganglion and sense-organs, and their powerful jaws and arms, or tentacles, show to what good purpose they have struggled. But the struggle was in vain, as far as the supremacy of the animal kingdom was concerned. Their ancestors had taken a course which rendered it impossible for their descendants to reach the goal.
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