33 JELLY-FISH, STAR-FISH, AND SEA-URCHl\-S. 



Different species of Medusye exhibit different 

 degrees of irritability in responding to stimuli ; but 

 in all the cases I have met with the decree of 

 irritability is remarkably high. Thus, I have seen 

 responsive contractions of the whole umbrella follow 

 upon the exceedingly slight stimulus caused by 

 a single drop of sea- water let fall upon the irritable 

 surface from the height of one inch. As regards 

 chemical stimulation, dilute spirit or other irritant, 

 when dropped on the paralyzed swimming organ of 

 Aurelia aurita, often gives rise to a whole series 

 of rhythmical pulsations, the systoles and diastoles 

 following one another at about the same rate as is 

 observable in the normal swimming motions of the 

 unmutilated animal. 



It is somewhat difficult, in the case of paralyzed 

 swimming organs, to prove the occurrence of a 

 contraction in respon e to thermal stimulation, from 

 the fact th.^.t while these tissues are not nearly so 

 sensitive to this mode of excitation as might be 

 anticipated, they are, as just observed, extraordi- 

 narily sensitive to mechanical excitation. It there- 

 fore becomes difficult to administer the appropriate 

 thermal stimulus without at the same time causing 

 a sufficient mechanical disturbance to render it 

 doubtful to which of the stimuli the resj)onse is 

 due. This may be done, however, by allowing 

 a few drops of heated sea- water to run over the 

 excitable surface while it is exposed to the air. In 

 this and in other ways I have satisfied myself that 

 the paralyzed tissues of swimming organs respond 

 to sudden cl<^^^ations of temperature. 



