ME. G-. 3. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSiE. 
681 
to normal sea-water, they recover themselves completely, the rate and regularity of 
their rhythm being then quite natural. The suddenness with which this return to the 
normal state of things is effected cannot but strike the observer as very remarkable ; 
and, I may observe, it takes place with equal suddenness at whatever stage in the 
above-described process of asphyxiation the transference to normal sea-water is 
accomplished *. 
III. STIMULATION. 
Under this heading I have a great number of fresh results to communicate ; and as 
these chiefly have reference to electrical stimulation, I think it will be convenient to 
modify the order observed in my last paper, and to consider the other modes of stimu- 
lation first. 
§ 1. Mechanical stimulation. — The only remark I have to make in this connexion is, 
that when the paralyzed swimming-organ of Aurelia aurita is stimulated by a single 
mechanical irritation, it often responds with two, and more rarely with three contractions, 
which are separated from one another by an interval of about the same length as the 
normal diastole of the unmutilated animal. 
§ 2. 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. 
§ 3. Thermal stimulation . — It is somewhat difficult, in the case of paralyzed swim- 
ming-organs, to prove the occurrence of a contraction in response to thermal stimula- 
tion, from the fact that while these tissues are not nearly so sensitive to this mode of 
excitation as might be anticipated, they are, as observed in my former paper, extraor- 
dinarily sensitive to mechanical excitation. It therefore becomes difficult to administer 
the appropriate thermal stimulus without at the same time causing a sufficient mecha- 
nical 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 eleva- 
tions of temperature. 
§ 4. Luminous stimulation. — It is interesting to note that, in the case of some 
of the naked-eyed Medusae, the action of light as a stimulus is most marked and 
unfailing. In the case of Sarsia, for instance, a flash of light let fall upon a living 
specimen almost invariably causes it to respond with one or more contractions. If 
the animal is vigorous and swimming freely in water, the effect of a momentary flash 
thrown upon it during one of the natural pauses is immediately to originate a bout 
of swimming. But if the animal is non-vigorous, or if it is removed from the water and 
spread flat upon an object-glass, it usually gives only one contraction in response to 
* Hr. Eimee Las also observed the facts mentioned in this paragraph (d). 
