. The biology of marine animals. Marine animals; Physiology, Comparative. 346 THE BIOLOGY OF MARINE ANIMALS GRAVITY SENSE Many organisms orient towards or away from the source of the force of gravity by a form of response termed geotaxis. The specialized gravity receptor for geotaxis is the statocyst. This is a fluid-filled chamber lined by a sensory epithelium bearing hair cells, and containing a solid or semi- solid body known as a statolith (Fig. ). The latter rests on part of the epithelium, or hangs from the wall, and mechanically stimulates the sense cells. Any displacement of the st
. The biology of marine animals. Marine animals; Physiology, Comparative. 346 THE BIOLOGY OF MARINE ANIMALS GRAVITY SENSE Many organisms orient towards or away from the source of the force of gravity by a form of response termed geotaxis. The specialized gravity receptor for geotaxis is the statocyst. This is a fluid-filled chamber lined by a sensory epithelium bearing hair cells, and containing a solid or semi- solid body known as a statolith (Fig. ). The latter rests on part of the epithelium, or hangs from the wall, and mechanically stimulates the sense cells. Any displacement of the statocyst from its resting position, owing to movement of the animal, alters the pattern and force of stimulation and leads to appropriate adjusting reactions. These cease when the statolith has returned to its original position. A beautiful experiment demonstrating statocyst functioning was per- formed by Kreidl over half a century ago. In shrimps the statoliths are lost. Fig. Section through the Statocysts of Pecten inflexus , , left and right statocysts; A, external duct; N, nerve; and , connec- tive tissue. (After von Buddenbrock.) at each moult and are replaced anew with sand grains from the environ- ment. When iron dust was provided, it became incorporated in the stato- cysts in place of sand particles. These shrimps now became sensitive to the pull of a magnet: when, for example, a magnet was held above the animal, the pull on the statoliths towards the top of the statocyst caused the animal to turn over on its back. Statocysts found in scyphomedusae and ctenophores are concerned with orientation reflexes. Medusae have eight statocysts symmetrically arranged around the margin. When the animal is tilted, stimulation of the statocysts causes the musculature of the lowered portion of the umbrella to contract more strongly than the upper portion and the animal soon rights itself. Extirpation of several neighbouring statocysts causes permanent disorient
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