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 until, 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, in 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 in 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 
@ 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. 
