With one ganglion in each
segment there is far more controlling, perceptive, ganglionic
material than in lower worms. 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. There are tentacles and otolith
vesicles as very delicate organs of feeling, or possibly of hearing
also.
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.
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