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 Kreidl." 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, i.e., 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 orientation 

 after surgical removal of the statocysts CArenicola marina, A. gruhei, 



