No. 389.] STIMULI IN UNICELLULAR ORGANISMS. 387 
researches of Nageli, to be true of the flagellate swarm spores 
of plants. According to this investigator, such swarm spores 
swim in a spiral course, with the same side always toward the 
outside of the spiral— exactly as I have described for Para- 
mecium, Spirostomum, and Stentor. Another observation of 
Nageli renders it extremely probable that the mechanism of the 
reactions of these organisms is essentially the same as in the 
three ciliates named. When these flagellate swarm spores 
strike in their forward course against an object, they cease 
the forward motion for a time, but continue to turn on the 
long axis; then “there ensues a backward motion, with poste- 
rior end in advance, while at the same time they rotate in the 
Opposite direction. This backward course usually lasts but a 
short time and becomes gradually slower; it is soon exchanged 
for the forward motion, which takes place, as a rule, in a some- 
what different direction from the original one.” 1 This descrip- 
tion would apply without the slightest change to the reactions 
of Paramecium, Spirostomum, and Stentor under the same cir- 
cumstances. It may be predicted with much confidence that 
the reactions would be found to be similar under other circum- 
stances; that the “somewhat different direction ” of the swarm 
spores would be found to be always toward the side which faces 
the outside of the spiral course, and that the same reaction 
would be given whatever the nature and position of the stim- 
ulus. If this is true, then the conclusions which I have drawn 
for Paramecium in my previous paper would apply equally to 
the Flagellata. Certainly until the mechanism of the reactions 
of these. and other unicellular organisms is determined by 
observation, it is hardly worth while to base any conclusions 
on theoretical schemes of the character usually given for such 
reactions. Theories of the reactions of unicellular organisms 
may be placed in two general classes: on the one hand are 
those theories which look upon the activities of unicellular 
organisms as determined in a manner similar to those of 
human beings, by a play of desires, motives, etc.; while at the 
other extreme are theories in which the movements are looked 
upon as of a character essentially similar to those taking place 
1 The above quotation from Nägeli is translated from Hertwig, Die Zelle, p. 66. 
