[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 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.