The old cobweb-like plexus
condenses into a little knot, the supra-oesophageal ganglion. This
ganglion cannot do much, if any, thinking; it is rather a steering
organ to control the muscles and guide the animal. It is the servant
of the locomotive system. Yet it is the beginning of the brain of
higher animals, and probably still persists as an infinitesimal
portion of our human brain. And all this is the prophecy of a head
soon to be developed. An excretory system has appeared to carry off
the waste of the muscles and nerves.
In the schematic worm and annelid the reproductive system is
simpler, though perhaps equally effective. It takes the excess of
nutriment of the body. The muscular system has taken the form of a
sack composed of longitudinal and transverse fibres. The
perivisceral cavity, formed perhaps by cutting off and enlarging the
lateral pouches of the turbellarian digestive system, serves as a
very simple but serviceable circulatory system. But in the annelid
and all higher forms a special system of tubes has developed to
carry the nutriment, and usually oxygen also, needed to keep up the
combustion required to furnish the energy in these active organs.
The digestive system has attained its definite form with the
appearance of an anal opening and the accompanying division of labor
and differentiation into fore-, mid-, and hind-intestine.
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