CH. II] 



THE CRAYFISH 



61 



that had just moulted. The animals were kept in water 

 carefully filtered so as to exclude all other solid particles. 

 When they had been observed to place iron particles in 

 their otocysts (statocysts) an electro-magnet was brought 

 to bear upon them. (This instrument is preferable to a 

 permanent magnet, for it can be placed in the desired 

 position near the tank while the electric circuit is open. 

 On closing the circuit it at once becomes a magnet and 

 the behaviour of the animals can be noticed without the 

 possibility of disturbances and excitement that might arise 

 from the movements necessary in bringing a permanent 

 magnet within range.) Under these conditions the animals 

 adjusted the positions of their bodies in obedience to the 

 influence of the magnet upon the particles of iron. In 

 the normal positions the plane (BA, BC, Fig. 8) in which 



Fig. 8. 



the statoliths lie upon the sensory setae makes an angle of 

 about 30 with the horizontal. Hence the force of gravity 

 acting upon the particles can be resolved into two com- 

 ponents ; one which presses at right angles to the supports 



