190 
MR, G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSAE. 
passed in the same direction, and that on reversing the direction of the current the 
stimulations which are supplied by make and break again rise to their original value. 
(See especially my papers “ On the Modification of Excitability produced by Injury,” 
‘ Journal of Anatomy and Physiology/ vol. x., and in ‘ Proceedings of Poyal Society/ 
Nos. 171 and 811.) I have therefore tried w'hether these effects admit of being pro¬ 
duced in the case of the excitable tissues of the Medusae, and I find that they are so. 
I find, further, that these effects admit of being equally well produced wdiether the 
current be passed in the direction of the length or in that of the thickness of the 
muscular fibres. In other words, the effects in question occur equally well, wdiether 
the electrodes be placed on the same radius of the umbrella—so causing the current to 
traverse the muscle-fibres for the most part transversely—or placed on different, 
though not widely separated radii, at points equi-distant from the circumference of 
the umbrella—so causing the current to traverse the muscle-fibres for the most part 
longitudinally. 
IT. SECTION. 
§ 1.—Allusion has already been made to the fact that in the contractile tissues of 
Aurelia there are frequently to be observed immense differences in the force of the 
contractions, according as the latter-are originated by a strong or by a weak stimula¬ 
tion ; that is to say, while a stimulus which is not below some certain degree of inten¬ 
sity will always start a wave of strong contraction from any point of the tissue area 
which may be chosen as the seat of stimulation, a weaker stimulus may start a wave 
of contraction so feeble as to be scarcely perceptible. In some specimens of Aurelia 
these waves of feeble contraction are more easily produced than in other specimens, 
and occasionally it happens that they are so feeble as to be only rendered perceptible 
by raising the tissue above the water and inclining the head at such an angle to their 
level as admits of the eye receiving the reflection of light which falls upon their 
glancing surface; as a response to each feeble stimulus a slight change in the glancing^ 
indicative of a slight contraction, may then be observed. 
It might reasonably be supposed that such feeble waves of contraction would not 
present much power of radiating through extensive areas of tissue; but, as a matter of 
fact, they often travel quite as far and well as do the stronger waves. Indeed, it not 
unfrequently happens that they gain strength as they advance; so that in a long strip 
of tissue—especially if a large piece of the umbrella be left attached to the end remote 
from stimulation-—it may happen that a contractile-wave, when started by a feeble 
stimulus at one end of the strip, is barely perceptible at that end, while it grows into 
a tolerably strong contraction towards the other end. 
When I first observed the occurrence of these almost imperceptible waves of con¬ 
traction it appeared to me that they might possibly serve to explain the occurrence of 
what in my previous paper I called waves of stimulation. But before I published my 
previous paper I satisfied myself that such was not the case; and as the establishment 
