s
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.

The digestive and reproductive systems have thus nearly attained
their final form. From the higher worms upward the digestive system
will improve greatly. Its lining will fold and flex and vastly
increase the digestive and absorptive surfaces. The layer of cells
which now secrete the digestive fluids will in part be replaced by
massive glands. Far better means of grasping food than the horny
teeth of annelids will yet appear. But all these changes are
inconsiderable compared with the vast advance made by the muscular
and nervous systems. Reproduction and digestion are losing their
supremacy in the animal body. Their advance and improvement will
require but little further attention.

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