192 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
sufficient access to the excitable tissues to scrve as 
an adequate stimulus. 
During the soaking stage of the experiment—ze. 
before the artificial rhythm begins—the excitability 
of the tissues may be observed progressively and 
abnormally to increase ; for soon after the soaking 
stage begins, in response to a single nip with the 
forceps the bell may give two or three locomotor 
contractions, instead of a single one, as is invariably 
the case with a paralyzed bell of Sarsia in normal 
water. Later on during the soaking stage, four or 
five successive contractions may be yielded in 
response to a single mechanical stimulus, and shortly 
after this a whole bout of rhythmic contractions 
may be started by the same means. Indeed, in 
some cases the artificial rhythm in acidulated water 
requires such a single additional stimulus for its 
inauguration, the shivering movements failing to 
begin spontaneously, but beginning immediately 
upon the application of the additional stimulus. 
Similarly, after the shivering movements have 
ceased, a fresh bout may very often be started by 
again giving the motionless nectocalyx a single 
stimulation. The interpretation of these facts 
would seem to be that the general irritability of 
the excitable tissues is exalted by the universal and 
constant stimulus supplied by the acid to an extent 
that is just bordering on that which gives rise to 
rhythmic movement, so that when the violent con- 
traction is given in response to the mechanical 
stimulus, the disturbance serves to start the rhythmic 
movement. 
