178 JLLLY-FISH, STAR-FISH, AND SEA-URCHINS. 
more pronounced up to a certain point, when it 
would gradually have begun to pass into more 
persistent tetanus. But as in this case, instead of 
strengthening the current still further, I again 
weakened it to its original intensity, the rhythm 
immediately returned to its original rate and regu- 
larity. 
Such being the facts, the question arises as to 
their interpretation. At first I was naturally in- 
clined to suppose that the artificial rhythm was 
due to a periodic variation in the strength of the 
stimulus, caused by some slight breach of contact 
between the terminals and the tissue on each con- 
traction of the latter. This supposition, of course, 
would divest the phenomena in question of all 
physiological meaning, and I therefore took pains 
in the first instance to exclude it. This I did in 
two ways: first, by observing that in many cases 
(and especially in Cyanzea capillata) the rate of the 
rhythm is so slow that the contractions do not 
follow one another till a considerable interval of 
total relaxation has intervened; and second, by 
placing the terminals close together, so as to include 
only a small piece of tissue between, and then 
firmly pinning the tissue all round the electrodes to 
a piece of wood placed beneath the Medusa. In 
this way the small portion of tissue which served 
as the seat of stimulation was itself prevented from 
moving, and therefore the rhythmic motions which 
the rest of the Medusa presented cannot have been 
due to any variations in the quality of the contact 
between the electrodes and this stationary seat of 
stimulation, 
