ME. G. J. ROMANES ON THE LOCOMOTOR SYSTEM OE MEDUSAE. 
747 
after having temporarily quickened it. But water between 50° and 60° permanently 
quickens the rhythm during the time that Medusae, which have been removed from 
colder water, are exposed to its influence. In very cold water the loss of spontaneity is 
a gradual though rapid process, as is also its return in warmer water. After having 
been frozen solid, Aurelia will recover on being thawed out, but the original rate of 
rhythm was not observed fully to return. 
Oxygen accelerates the rhythm, while carbonic acid retards it, and in strong doses 
destroys both spontaneity and irritability. Deficient aeration of the water containing 
the Medusse causes irregularity of their rhythm, as well as the occurrence of pauses. 
The latter become more and more pronounced the longer the water is left without being 
changed, till at last spontaneity altogether ceases ; but on now restoring the animals to 
fresh sea-water, their recovery is surprisingly sudden. 
As regards stimulation, I have proved that a few drops of hot water allowed to run 
over the excitable tissues of Medusae cause a responsive contraction. Single mechanical 
or chemical stimuli applied to paralyzed swimming-bells of covered-eyed Medusae 
frequently produces in response a small series of rhythmical contractions. 
Light acts as a powerful stimulus to some species of Medusas. Vigorous Sarsice 
respond to a single flash by entering on a bout of swimming; but less vigorous 
specimens only give a single contraction. The stimulus has been proved to be light 
'per se, and not the sudden transition from darkness to light. Moreover a constant flood 
of light seems to act towards Scirsia as a constant stimulus. After removal of their 
marginal bodies, Sarsice no longer respond to luminous stimulation, while, on the other 
hand, any contractile tissue left adhering to the excised marginal bodies will continue 
to do so. Tiaropsis polydiademata responds to luminous stimulation as it responds to 
all other kinds of stimulation, viz. by going into a spasm ; but the time that elapses 
between the occurrence of the stimulus and the occurrence of the response is very much 
longer in the case of luminous than in that of any other kind of stimulation. I find that 
this time is not, properly speaking, a period of latent stimulation ; for a single flash 
requires to be of the same duration as a continuous flood of light in order to procure a 
response. Hence the period represents the time during which a certain summation of 
stimulating influence is taking place in the ganglia. 
The period of latent stimulation in the case of Aurelia is greatly modified by certain 
conditions. Of these, temperature exerts the greatest influence ; but, as a novel fact of 
great interest, the most important influence from a physiological point of view is that of 
the summation of stimuli, which, however, I need not again describe. At the bottom of 
a staircase the latent period is ■§• second, while at the top of a staircase it is only f second. 
Summation of stimuli also greatly increases the amplitude of the contractions ; so that it 
both develops in the tissue a state of expectancy and arouses it into a state of increased 
activity. My results in this connexion differ from those of Dr. Bowditch in that (a) the 
medusoid tissue has a shorter memory for the occurrence of a previous stimulation than 
has the cardiac ; and (d) in the medusoid tissue a stimulus which at the bottom of a stair- 
MDCCCLXXVII. 5 M 
