ditions.
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
from which they have condensed, both nerves and centres;
differentiation has not gone so far as at the front of the body.
Sense organs are still very rudimentary. Special cells of the skin
have been modified into neuro-epithelial cells, having sensory hairs
protruding from them and nerve-fibrils running from their bases.

[Illustration: 6. CROSS-SECTION OF TURBELLARIAN. HATSCHEK, FROM
JIJIMA.
_e_, external skin; _rm_, lateral muscles; _la_ and _li_,
longitudinal muscles; _mdv_, dorso-ventral muscles; _pa_,
parenchyma; _h_, testicle; _ov_, oviduct; _dt_, yolk-gland; _n_,
ventral nerve; _i_, gastro-vascular cavity.]

In a very few turbellaria we find otolith vesicles. These are
little sacks in the skin, lined with neuro-epithelial cells and
having in the middle a little concretion of carbonate of lime hung
on rather a stiffer hair, like a clapper in a bell. Such organs
serve in higher animals as organs of hearing, for the sensory hairs
are set in vibration by the sound-waves. It is quite as probable
that they here serve as organs for feeling the slightest vibrations
in the surrounding water, and thus giving warning of approaching
food or danger. The animal has also eyes, and these may be very
numerous. They are not able to form images of external objects, but
only of perceiving light and the direction of its source. A little
group of these eyes lies directly over the brain, near the front end
of the body; the others are distributed around the front or nearly
the whole margin of the body.

The turbellaria, doubtless, have the sense of smell, although we can
discover no special olfactory organ. This sense would seem to be as
old as protoplasm itself.

This distribution of the eyes around a la