Mechanoreceptors - Receptors
Mechanoreceptors involve those receptors involved in perception of touch, pressure, tension, hearing, vibration, gravity, muscle tension etc. These would show to us at first sight as rather heterogeneous assemblage of sensations, but the basic mechanism included in the perception of all these is similar, they are all sensitive to contact with objects and are mechanically deformed temporarily by all the above stimuli. In the simplest case, they involve free nerve endings on the body surface.
Figure: Gravitation and equilibrium receptors. A. A statocyh of a mollusc.
But frequently mechanoreceptors are more complex. A mechanoreceptor of wide occurrence in the non-chordate metazoans is the statocyst. It is mainly an organ concerned with perception of gravitational force and is a balancing organ found in approximately all phyla, including coelenterates. It consists necessarily of a particle of calcium carbonate called statolith. The statolith is carried in a vesicle lined by a layer of sense cells that have hair-like processes. The statolith rests on the sense cells. The vesicle is of course filled along with a fluid. The animal can appreciate the direction of gravity or tilt, from the sense cells on which the statolith comes to rest.