24 THE LIFE OF CRUSTACEA 



and a strong electro-magnet was brought near the 

 side of the vessel in which it was kept. It was 

 observed that the Prawn, which had been swimming 

 in the usual horizontal position, at once turned the 

 under-side of its body towards the magnet, and swam 

 about on its side as long as the magnet was in 

 action. When the current exciting the magnet was 

 cut off, the animal resumed its ordinary position. 

 This experiment shows that these organs, to which 

 we may now give their proper name of staiocysts, 

 are organs for perceiving the direction of the force 

 of gravity. The magnetic force acted on the 

 particles of iron in the same way that the force of 

 gravity acts on the sand-grains in normal conditions, 

 and the Prawn felt the weight of them, so to speak, 

 pulling towards the side instead of the bottom ot 

 the vessel, and turned its body accordingly, to swim, 

 as it supposed, right side up. It is now known that 

 those parts of the human ear called the " semi- 

 circular canals " have a somewhat similar function 

 as " organs of orientation," although to animals 

 walking on the solid ground this function is not so 

 important as it doubtless is to animals swimming in 

 water. 



The sense of smell is believed to have its seat 

 chiefly in the antennules. The outer branch of each 

 antennule bears tufts of peculiar hairs, in which the 

 chitinous covering is extremely delicate, so that sub- 

 stances dissolved in the water can easily pass through 



