192 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



sufficient access to the excitable tissues to serve as 

 an adequate stimulus. 



During the soaking stage of the ex|)eriment — i.e. 

 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. 



