38 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
Different species of Medusz exhibit different 
degrees of irritability in responding to stimuli; but 
in all the cases I have met with the degree 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 response to thermal stimulation, from 
the fact that 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 response 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 elevations of temperature. 
