PRIMITIVE TYPES OF INTELLIGENCE 187 



the darkened part after the food was noted and the length of 

 time required for the crab to enter. On the first day only 

 three out of the thirty crabs used availed themselves of the 

 food. The number gradually increased on succeeding days, 

 and the average time of their response decreased i.atil, on 

 the eighth day, all the crabs but one entered the dark en- 

 closure, and most of them entered with little delay. 



"After a few days of this treatment immediately upon 

 the insertion of the screen the crabs became most agitated, 

 some hurrying and scurrying about, others making almost 

 directly for the openings." An association was evidently 

 formed between the appearance of the screen and the ex- 

 perience of being fed, and this association led the animals to 

 act counter to their natural proclivity to seek the light. 



The experiments of Drzewina on Pachygrapsus, hi which 

 it was shown that the crabs which were compelled to go 

 through a certain opening in order to get nearer the light 

 gradually learned the way and arrived more quickly hi the 

 compartment of their enclosure which was most illuminated, 

 show a similar power of forming associations. 



In the Mollusca we meet with indications of intelligence of 

 a primitive sort in the movements of gasteropods such as the 

 limpets, which have the faculty of making considerable 

 journeys from their accustomed stations on the rocks and 

 returning to their orginal position. Bethe has studied the 

 homing of limpets and has come to the conclusion that these 

 animals simply follow their own slimy trails and are guided 

 back to their resting place by a kind of chemotaxis. No 

 intelligence is required by the limpets; they simply obey 

 a blind tropism. 



Lloyd Morgan experimented with limpets by removing 

 them for some distance from their scars on the rock and 

 noting how many found their way back within a given time. 



