nerves, which are the principal seats of combustion, through the
external surface. It has, therefore, no special respiratory organs.
But the waste matter of the muscles cannot escape so easily, for
these are becoming deeper seated. Hence we find an excretory system
consisting of two tubes with many branches in the parenchym, and
discharging at the rear end of the body. This again is a sign that
the muscles are becoming more important, for the excretory system is
needed mainly to remove their waste. These tubes maybe only greatly
enlarged glands of the skin.

[Illustration: 5. TURBELLARIAN. LANG.
_va_ and _ha_, front and rear branches of gastro-vascular cavity;
_ph_, pharynx. The dark oval with fine branches represents the
nervous system.]

The nervous system consists of a plexus of fibres and cells, the
cells originating impulses and the fibres conveying them. But this
much was present in hydra also. Here the front end of the body goes
foremost and is continually coming in contact with new conditions.
Here the lookout for food and danger must be kept. Hence, as a
result of constant exercise, or selection, or both, the
nerve-plexus has thickened at this point into a little compact mass
of cells and fibres called a ganglion. And because this ganglion
throughout higher forms usually lies over the oesophagus, it is
called the supra-oesophogeal ganglion. This is the first faint and
dim prophecy of a brain, and it sends its nerves to the front end of
the body. But there run from it to the rear end of the body four to
eight nerve-cords, consisting of bundles of nerve-threads like our
nerves, but overlaid with a coating of ganglion cells capable of
originating impulses. These cords are, therefore, like the plexus