6 NERVOUS SYSTEM AND SENSE. ORGANS 
Il. Protozoa 
In Amoeba, there seems to be no portion of the surface more 
sensitive than others. The exoplasm is a general sensory organ. 
Experiments by Hyman ’17, with toxic substance show that a 
local region of increased susceptibility exists along the axes of each 
pseudopodium from its distal to its proximal end, the distal end 
being more susceptible. The youngest and most vigorous forms 
are most susceptible. 
According to several investigators, the exoplasm of Amoeba is 
like a tough skin and this in part at least acts as a sensory area. 
The more fluid endoplasm may become quite rigid under stimulation. 
The changes in Amoeba which are the causes of amoeboid 
movement and behavior originate within the Amoeba and external 
stimuli do not act directly to produce those physical alterations 
which result in movement, but they act through the protoplasm of 
the Amoeba. The reactions of Amoeba are similar to the reflexes 
of more complex forms involving reception of stimuli, and the con- 
duction of internal changes leading to response, but sensation, con- 
duction and movement are not differentiated. 
If one side of an Amoeba touches some object it may move 
away from the source of stimulus. Jennings has found that when 
touched the animal does not usually move directly away from the 
side stimulated, but merely in some other direction. If the anterior 
edge is touched this part stops and contracts while the current 
turns to one side at this point, so that the animal moves at an angle 
with its former direction. If the advancing edge of an Amoeba is 
touched it withdraws and a new pseudopodium is sent out else- 
where. Sometimes Amoebae react positively to solid bodies, they 
may also under various sorts of stimuli thrust out many pseu- 
dopodia at once or draw all into a compact mass. Amoeba reacts 
not only to mechanical*but also to chemical, temperature, light and 
electrical stimuli. The direction of movement in negative reac- 
tions is not determined entirely by the position of the stimulating 
agent. Other stimuli may have already altered the character of the 
protoplasm, for example the moving Amoeba is temporarily dif- 
ferentiated, having two ends different and the sides differing from 
the ends. These and perhaps other internal factors have a large 
part in the determination of movement. 
It is impossible to explain how Amoeba alters its own metabolic 
process. If Amoeba is capable of self stimulation then this might 
suggest that living substance has a psychic quality which is pos- 
sessed by all protoplasm. If this is not accepted for simpler organ- 
isms it would be hard to accept it for the cells of the cerebral cortex 
of man and all would be referred to present or past conditions of 
external or internal environment. 
There is no clear evidence that Amoeba has memory. The 
