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Updated: October 3, 2026
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.
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.
Remember that these neuro-epithelial sensory cells are suited to respond not merely to pressure, but to a variety of the stimuli, chemical, molecular, and of vibration, which excite our organs of smell, taste, and hearing. Such organs and the directive eyes appear mainly at this anterior end.