ME. G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSAE. 
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marked exhibition of rhythmic response under the influence of constant stimulation of 
low intensity. 
§ 2. Luminous stimulation of the covered-eyed Medusce. —In my previous papers 
I have detailed the results of various experiments on the luminous stimulation of the 
naked-eyed Medusae. Since the publication of these papers I have tried whether the 
covered-eyed Medusae are likewise susceptible to this mode of stimulation, and I have 
found that, although the results are not of so marked a character as they are in the 
case of the naked-eyed Medusae, nevertheless, by a little care, it does admit of being 
proved that the lithocysts of the covered-eyecl Medusae resemble the marginal bodies 
of the naked-eyed Medusae in being rudimentary organs of vision. 
The best way that I have found of proving this fact is to divide an Aurelia into 
segments, leaving one or more lithocysts in each segment. On then choosing a 
segment which is not very active, and leaving it for some time in the dark, it may be 
observed, on approaching it with a candle, that its activity is much less pronounced 
than it was in the daylight. But after the candle has been allowed to shine upon it 
for a few seconds, the segment begins to become gradually aroused into a state of 
greater activity. If the lithocyst, or lithocysts, be now removed, the approach of the 
candle, or the access of daylight, produces no stimulating effect. 
§ 3. Localising movements of the polypite of Aurelia aurita.-—In my previous paper 
I have fully described certain movements of localisation which are performed by the 
polypite of a certain naked-eyed Medusa ( Tiaropsis indicans), whenever any portion 
either of the polypite itself or of the concave surface of the nectocalyx is stimulated. 
I have now ascertained that somewhat similar movements are performed, under similar 
circumstances, by the polypite of Aurelia aurita. For if an Aurelia be suspended in 
a large bell-jar of sea-water, so as to admit of the free movement of the polypite in all 
directions, and if any part of the concave surface of the umbrella be then touched, or 
otherwise irritated, the two lobes of the polypite which are nearest to the seat of 
irritation will slowly approximate and close over that point, as if attempting to 
embrace the irritating body. Sometimes only one lobe will move over to the seat of 
irritation, but it is more usual for two, or even more, of the four lobes to take part in 
these localising movements. These movements, however, are not performed with 
nearly so much activity and precision as they are in the case of Tiaropsis indicans; 
they are, comparatively speaking, of a very sluggish and somewhat uncertain character, 
never beginning till a number of seconds, perhaps even half a minute, after the 
application of the stimulus, and the lobes of the polypite then sweeping slowly back¬ 
wards and forwards over a large area of the umbrella, instead of being quickly and 
invariably applied to the precise spot that was stimulated. 
§ 4. Effects of alternating the direction of the constant current on the excitable tissues 
of the Medusce. —It is a fact known to physiologists that, in the muscular tissues of the 
higher animals, the stimulus which is supplied by make and break of the constant 
current becomes less and less effective as a stimulus the more frequently the current is 
