282 Annals Entomological Society of America [Vel Ve 
Jennings (1906, pp. 92-102). The following interesting quotation 
(pp. 92-94) is taken from the latter’s observations upon Para- 
mecia: ‘“‘If the animal is at rest against a mass of vegetable mat- 
ter or a bit of paper under the action of contact stimulus and it is 
then struck with the tip of a-glass rod, we find that at first it may 
not. react to*the latter stimulus at alky*-* 2" -* eFinally 
a strong blow on the anterior end causes the animal to leave 
the solid and give the typical avoiding reaction. *. * * * 
If specimens showing the contact reaction are heated, it is 
found that they do not react to the heat until a higher temper- 
ature is reached than that necessary to cause a definite reaction 
in free-swimming specimens. * -* * * On the other hand, 
both heat and cold interfere with the contact reaction. Para- 
mecia much above or below the usual temperature do not settle 
against solids with which they come in contact, but respond 
instead by a pronounced avoiding reaction. * * * * Speci- 
mens in contact with a solid react less readily to chemicals than 
do free specimens. * * * * Qn the other hand, immersion 
ue strong chemicals prevents the positive contact reaction 
ie Sie eral e contact reaction may completely prevent 
the reaction to gravity.”’ 
The inhibition of one stimulus by another is a somewhat 
puzzling matter. Why should a dragon-fly nymph reacting 
negatively to light, as it comes in contact with another nymph, 
-cease this function and display its thigmotactic proclivities? 
The stimulus from the electric arc of a projection lantern is so 
strong that we might expect the organism to continue to react 
to the light rather than to respond to contact. However, this 
is not the case for the stimul are sufficiently powerful to over- 
come the response to light. The explanation seems to le with 
certain changes which take place within the animal. The 
external conditions are the same—the stimuli from the electric 
arc are still present—but the nymphs no longer react; the organ- 
isms now respond to contact stimuli. (However, some of the 
nymphs continue to react to the light.) One form of response 
gives way to another. This is probably due to certain changes 
in the bodily state of the organisms. We are unable to witness 
these changes as they occur within the animals themselves, 
but they can be inferred from the difference in the external 
response. Jennings (1904, p. 120), in connection with his 
discussion of the reactions of Stentors, has stated that, ‘‘We 
