70 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



heliotropism must drive Asterina tenuispina to the 

 surface of the ocean, just as Asterina gibbosa is driven 

 there by negative geotropism. 



Preyer mentions briefly in his extensive work on 

 The Movements of the Starfish the " tendency of these 

 animals to move upwards." " The strong tendency of 

 starfish and brittlestars to go upward cannot be traced 

 back to lack of air, lack of food, changes in tempera- 

 ture or current, or to a desire for light, for they climb 

 up just the same when these conditions are eliminated. 

 Probably some peculiarity of the bottom, or of just 

 that part of the bottom where the animal is, makes it 

 unsuitable for the suction of the tube-feet. The ani- 

 mals can remain there no longer, so they move up- 

 wards. But it is possible that parasites, which I have 

 often found in the ambulacral furrows, may cause this 

 upward migration, for as the stimuli produced by 

 them come from below, they might seem to belong to 

 the bottom." 



The first sentence in this generalisation is wrong ; 

 the light attracts Asterina tenuispina upwards. Sec- 

 ond, the character of the bottom does not determine 

 the phenomenon. If Asterina gibbosa be placed in a 

 cubical box with glass sides, the animals leave the 

 basal horizontal side and crawl up the vertical sides. 

 If the box then be turned 90 around a horizontal 

 axis, the side which is now basal is deserted by the 

 animals. They crawl up and remain on the side 

 which, while horizontal, they had left. Finally, if 

 Preyer believed that parasites force the animals to 



