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.
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