BRAIN PHYSIOLOGY OF WorMS 865 
in this paper on stereotropism in brainless Planarians, and 
the reaction of brainless animals toward light. 
3. Even though the brain, or more accurately that part of 
the body containing the brain, determines in the main the bio- 
logical or psychic reactions of worms which are typical for 
each species in the same way as in higher animals, there never- 
theless exists a specific difference between the brain of worms 
and that of many of the higher animals. The worms lack asso- 
ciative memory and consequently also consciousness, which is 
only a function of the former. By associative memory we 
understand that arrangement of the brain by virtue of which 
a stimulus brings about not only the effects corresponding 
with its nature and the specific structure of the irritable 
tissue, but also the stimulating effects of other causes which 
at a previous time once affected the organism at the same or 
almost the same time with the stimulus. No trace of such an 
associative effect of stimuli can be proved to exist in worms, 
and consequently also no trace of consciousness. 
One might go farther and speak of memory, when the 
effect of a stimulus depends upon previous effects of stimula- 
tion at all. Under these circumstances, for example, we 
would have to designate it as memory when a plant which 
was originally cultivated in the tropics does not withstand 
low temperatures as well as a plant of the same species culti- 
vated in the North. We would also have to call it memory 
when a Medusa of the temperate zone is moved to the regions 
of the midnight sun and here continues its depth migrations 
in a period corresponding with the changes of day and night 
in its home. No objection could be made to this, only I do 
not believe that this kind of memory can be looked upon as 
a lower form of associative memory. And I am convinced 
that it is something entirely different from associative mem- 
ory. When the plant which has been cultivated in the South 
does not do as well in the North as the same species of plant 
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