REFLEX ACTION 
471 
The afferent and efferent nerves, whether in reflex or 
in general nervous action, never vary in the work which 
they do. The sensory afferent nerves form the only paths 
over which our information of the outside world travels to 
the brain. The stimuli which cause the different sensations, 
such as taste, sight, etc., have their individual paths and re¬ 
ceiving organs. This is indicated by the fact that no other 
nerves than those of the ear are ever affected when we hear. 
Reflex Action in the Frog . — The frog, like man, is able 
to act in a definite way. If any one approaches a frog 
while it is sitting on the edge of a pond, it jumps into the 
water, stirs up the mud, and then returns to the shallow 
water near the place where it entered. The frog, in this 
case, acts as if it, or its ancestors, had learned that this is 
the best way to escape enemies. While this series of acts 
is called a habit, it is really a series of reflet acts which are 
similar to the reflex action described for man, and require 
the same nerve structures. 
Reflex Action in the Earthworm. —: If a light is flashed 
on an earthworm at night, the worm will quickly with¬ 
draw to its burrow, before it can be seized. The earth¬ 
worm has no eyes, but it is able to respond to light and 
can tell the difference between night and day. It is believed 
that special nerve cells in the skin, which are connected 
with the nerve ganglia, help the earthworm to become 
aware of the light stimulus. 
Reflex Action in Hydra. — Hydra is a minute water 
animal which has no definite nervous system, but only 
a few nerve cells scattered through the body. As the 
hydra waves its arms about in the water, there seems to 
be no purpose in its motions. But if a water flea swims 
against one of the tentacles, a part or all of the tentacles 
at once begin to carry the flea to the mouth of the hydra. 
The hydra, then, without a definite nervous system, can 
carry out a definite reflex action. 
