Sensory Discrimination : the Chemical Sense 83 



for it will not investigate a glass tube or ball; the smooth 

 surface seems not to be the adequate stimulus for beginning 

 the movements involved in exploring and entering a shell (41). 

 The responses of Crustacea to food stimulation vary, as 

 might be expected, with different genera and species. Nagel 

 finds the role of the food sense in aquatic Crustacea very in- 

 significant; they occasionally show antennal movements in 

 the presence of food, he says, but are not guided to it (292). 

 That general restlessness is shown by various Crustacea in the 

 neighborhood of food, but not in contact with it, has been ob- 

 served by Bell in the crayfish (22), by Holmes in the amphipod 

 Amphithoe longimana (180), by Bateson in shrimps and 

 prawns (n), and by Bethe in the green crab (28). Bethe 

 arranged a series of aquaria one above the other, with a con- 

 nection between them, and found that when food was placed 

 in the uppermost compartment the crabs in the lower ones 

 were successively excited as the food juices diffused themselves 

 from each compartment to the one below. In the amphipod, 

 the small antennae and the mouth parts appeared to be the 

 regions especially sensitive to food stimulation; if the food 

 touched one of the former, the animal instantly made a dart 

 for it. Touching the antennule with a needle very rarely 

 caused such a reaction (180). Bateson 's shrimps and prawns 

 had their food sensibility located chiefly in the antennules, 

 though if food was placed very near them they would show 

 disturbance even though deprived of antennules (n). This 

 was the case also with Holmes's amphipod. Bell, on the other 

 hand, found the whole body of the crayfish sensitive to chemi- 

 cal stimulation, and no evidence that the small antennae were 

 especially concerned. The crayfish's reactions to contact with 

 food were such as to direct the stimulus toward the mouth ; 

 negative reactions of rubbing, scratching, and pulling at the 

 affected part were obtained by stimulation with acids, salts, 



