. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. Mechano- and Equilibrium-Reception 515 position. This kind of annulling reaction is widespread throughout the animal kingdom and is the result of a coordinated nervous activity called "negative feedback.'"-'^ The function of statocysts in animal orientation is strikingly illustrated in the experiments of ; The statocysts of crabs open to the exterior by slits. At each molt, the statoliths are lost and are replaced de novo from foreign bodies in the environment. Iron or nickel dust in the aq
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. Mechano- and Equilibrium-Reception 515 position. This kind of annulling reaction is widespread throughout the animal kingdom and is the result of a coordinated nervous activity called "negative feedback.'"-'^ The function of statocysts in animal orientation is strikingly illustrated in the experiments of ; The statocysts of crabs open to the exterior by slits. At each molt, the statoliths are lost and are replaced de novo from foreign bodies in the environment. Iron or nickel dust in the aquarium forms statoliths with magnetic properties. The introduction of a magnet above a crab immersed in water causes the statoliths to be pulled to the top of the statocyst and the animal promptly turns over on its back. Orientation to Gravity: Geotaxis. Well differentiated statocysts first make their appearance in the coelenterates, although well defined gravity reactions appear in certain Protozoa, , Paramecium.^^ Among the coelenterates, the Scyphomedusae have eight statocysts radially arranged around the margin of the mande. In the normal position the axis of symmetry is kept vertical. When the animal is tilted, the lower portion of the mande. Fig. 185. Diagram showing terms used in the description of orientation of animals on the inclined plane. From Crozier and Pincus.^" musculature contracts more strongly than the upper portion, and the animal rights itself.^^- Surgical removal of several of the statocysts com- pletely disorients the animal. Ctenophores likewise are well equipped with functional statocysts. Many organisms (turbellarians, polychaetes, holothurians, brachipods, isopods, dipteran larvae and branchiate snails) exhibit an orientation toward the source of the force of gravity, called positive geotaxis. Many of these organisms burrow in mud or sand. In some cases this positive geotaxis is statocyst controlled, as evidenced by the lack of this orientati
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