BEHAVIOUR 89 



statoliths are relayed to the brain, and the crustacean is thus aware 

 of its position in relation to the pull of gravity. 



In some prawns the place of the statolith is taken by small sand 

 grains, which are placed in the statocyst by the prawn after it has 

 moulted. This allows an interesting experiment. If iron filings are 

 offered to the prawn it uses them instead of sand grains. It is then 

 possible to make the prawn turn upside down by placing a magnet 

 above it. 



In some species, such as the crayfish, Potamobius. it seem that 

 two statocvsts are necessary for the creature to maintain its balance. 

 If one is removed and the body is held in its normal position with 

 respect to gravity the crayfish acts as if it is tilted towards the intact 

 side, and pushes with the legs of that side as if to right itself. 

 Leander xiphias seems able to manage with just one statocyst; its 

 balance is not disturbed when one is removed. 



Normally a crustacean will be subjected to light from above and 

 the pull of gravity from below. What happens if the two stimuli 

 are in opposition? In those species with statocysts the pull of gravity 

 takes precedence. Although lit from below a prawn such as Leander 

 will remain standing in its normal position. If its statocysts are 

 removed it will then turn its back towards the light. We have 

 already seen that Processa caniculata performs a dorsal light re- 

 action when it is intact; this is because it has no statocysts; this is 

 also true of the other Crustacea which show a dorsal light reaction 

 when intact. 



A statocyst can also give information about the rate of turning 

 when a crab changes direction. In the statocysts of crabs there are 

 two types of hair-like processes coming from the sense cells. One 

 type is hooked and is frequently in contact with the numerous 

 minute statoliths; this type gives information about the position 

 of the body in relation to gravity. The other type of hair is long 

 and straight, projecting into the fluid within the statocyst. These 

 hairs are affected by the movement of the fluid when the crab 

 turns, and the sense cells from which they arise relay information 

 to the brain. 



There is also evidence that crabs can hear, at least in the wide 

 sense of the term hearing. Pumphrey (1950) has defined hearing in 

 the following way. " An animal hears when it behaves as if it has 

 located a moving object (a sound source) not in contact with it." 

 Experiments to demonstrate hearing ability must of course be 

 designed to exclude the possibility of other senses locating the 



