CO-ORDINATION, 135 
push the supposed case further, it is evident that 
if such reflex waves were to occur, the resulting 
confusion would very soon require to end in tetanus. 
As a matter of fact, these reflex waves do not occur ; 
and the question is, why do they not? Why is it 
that a wave is only reinforced in the direction in 
which it happens to be travelling—so that if, for 
- instance, it happens to start from A in the above 
series, it is successively propagated by B C in the 
direction A, B, C, D, and in that direction only ; 
whereas, if it happens to start from D, it is propa- 
gated by the same lithocysts in the opposite 
direction, D, C, B, A, and in that direction only—the 
wave in the one case terminating at the lithocyst D, 
and in the other case at the lithocyst A? Now, 
although this absence of reflex waves appears at 
first sight mysterious, it admits of an exceedingly 
simple explanation. JI find that the contractile 
tissues of the covered-eyed Medusze cannot be made 
to respond to two successive stimuli of minimal, or 
but slightly more than minimal intensity, unless 
such stimuli are separated from one another by 
a certain considerable interval of time. Now, when 
in the above illustration the contraction-wave starts 
from A, by the time it reaches B the portion of 
tissue included between A and B has just been in 
contraction in response to the stimulus from A, 
while the portion of tissue included between B and 
C has not been in contraction. Consequently, 
the stimulus resulting from a ganglionic discharge 
being presumably of minimal, or but slightly more 
than minimal intensity, the tissue included _be- 
