REFLEX ACTION 13 
In the simplest cases there is movement in the single part 
affected, as when a single tentacle of a polyp is withdrawn 
upon being stimulated. In other cases a single stimulus 
may produce combined movements of several parts, as when 
the various appendages of a brainless crayfish are set in mo- 
tion when but one of them is stimulated. A stimulus on the 
right side of a frog may not only cause coérdinated action in 
the various muscles of the hind leg, but the right fore leg 
may be applied to the stimulated part as well. And if 
the animal is rendered especially responsive by the injection 
of a solution of strychnine, contraction may take place in 
most of the muscles of the body. 
One factor which is concerned in the complexity of the 
response is the strength of the stimulus. A light stimulus 
applied to the tentacle of a jelly fish usually causes a contrac- 
tion of that tentacle only, but if the stimulus is stronger, 
the neighboring tentacles will also contract, and if still 
stronger, movements may be set up in the umbrella or disk 
which cause the animal to swim away. Complex responses 
are also produced if a stimulus affects many sense organs at 
once. 
A further method of complication occurs in the so-called 
chain reflexes in which one action serves as the stimulus 
for a second, and this for a third, and so on. A frog with 
its cerebral hemispheres removed will respond to the move- 
ments of a fly in its vicinity by snapping at it. This com- 
plicated reflex causes the fly to be seized by the jaws; the 
stimuli caused by the juices of the fly acting on the taste 
buds of the frog’s mouth set up the swallowing reflex, which 
involves complex and coérdinated movements of the tongue 
and throat. When in the stomach the fly reflexly excites 
this organ to activity, resulting finally in the extrusion of the 
digested mass into the intestine, which in turn is reflexly 
