62 



THE CRAYFISH 



[CH. II 



of the statoliths (viz. the setae) which may be termed the 

 ' pressure-component ' (OD, Fig. 8), the other which 

 exercises an outward pull upon the statoliths parallel to 

 the planes BA, BC : this may be termed the 'pull- 

 component' (OZ). The animal regulates its position in 

 accordance with the pressure-component. 



When the magnetic force is brought into action it is, 

 of course, in addition to the existing force of gravity and 

 the total effect is, so to speak, a compromise (or alliance) 

 between the two forces, or rather between their two 

 pressure-components. Dealing now with the right side of 

 the body only, in Fig. 9, where the magnet M is placed 

 above and to the right of the animal, OS, the direction of 

 the force of gravity upon 0, the centre of gravity of the 



Fig. 9. 



otolith (statolith) may be resolved into OD (pressure- 

 component) and OZ (pull-component), while OM, the 

 direction of magnetic attraction, may be resolved into 

 OD 1 and OZ 1 . It will be seen that OD 1 is in the contrary 

 direction to OD and therefore diminishes the effect of the 



