SECTION OF COVERED-EYED MEDUSZ. 97 
In conclusion, I may mention an interesting fact 
which is probabiy connected with the summation 
of stimuli before explained. When a contractile 
strip is allowed to rest for a minute or more, and 
when a wave is then made to traverse it, careful 
observation will show that the passage of the first 
wave is slower than that of its successor, provided 
the latter follows the former after not too great an 
interval of time. The difference, however, is exceed- 
ingly slight, so that to render it apparent at all the 
longest possible strips must be used, and even then 
the experimenter may fail to detect the difference, 
unless he has been accustomed to signalling, by 
which method all these observations on rate have 
to be made. 
Stimulus-waves. 
The rate of transmission of tentacular waves 
is only one-half that of contraction-waves, viz. 
nine inches a second. This fact appeared to me 
very remarkable in view of the consideration that 
the tentacular wave is the optical expression of a 
stimulus-wave, and that there can be no conceivable 
use in a stimulus-wave being able to pass through 
contractile tissue independently of a contraction- 
wave, unless the former is able to travel more 
rapidly than the latter; for the only conceivable use 
of the stimulus-wave is to establish physiological 
harmony between different parts of the organism, 
and if this wave cannot travel more rapidly than 
a contraction-wave which starts from the same 
point, it would clearly fail to perform this function. 
