ME. G-. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSAE. 
673 
the operation and observing the rhythm. Hence it is that in my experience I have 
found but very few examples of Dr. Eimer’s rule. 
§ 4. Effects of other forms of Mutilation on the Rhythm . — The next point I have to 
dwell upon is one of some interest. If the polypite of Aurelia ,. or of any other covered- 
eyed Medusa, be suddenly cut off at its base, the swimming-motions of the umbrella 
immediately become accelerated*. This acceleration, however, only lasts for a few 
minutes, when it gradually begins to decline, the rate of the rhythm becoming slower 
and slower, until, finally, it comes to rest at a rate considerably less than was previously 
manifested by the unmutilated animal. If a circular piece be now cut out from the 
centre of the umbrella, the rhythm of the latter again becomes temporarily quickened, 
but, as before, gradual slowing next supervenes. This slowing, however, proceeds fur- 
ther than in the last case ; so that the rate at which the rhythm next becomes 
stationary is even less than before. If, now, another circular ring be cut from the cen- 
tral part of the umbrella, — i. e. if the previously open ring into which this organ had 
been reduced by the former operation be somewhat narrowed from within, — the same 
effects on the rhythm are again observable ; and so on with every repetition of the 
operation, the rate of the rhythm always being quickened in the first instance, but 
then gradually slowing down to a point somewhat below the rate it manifested before 
the previous operation. It will here suffice to quote one experiment among many I 
have made in this connexion : — 
An Aurelia manifested a regular and sustained rhythm of . . . . . 26 
Immediately after removal of polypite, rhythm rose to 36 
Rate then gradually fell for \ of an hour, and became stationary at . . 20 
Circular incision just including ovaries caused rhythm to rise to . . . 26 
After gradual fall during -j of an hour, rhythm became stationary at . . 17 
Another circular incision carried round midway between the former one 
and the margin caused rhythm to rise to 24 
Rate again gradually declined, and in \ of an hour was .12 
Another circular incision was carried round as close to the margin as was 
compatible with leaving the physiological continuity of all the litho- 
cysts intact. Rhythm rose to 14 
Within a few minutes it fell to 6 
Excepting the cases where the effects of shock are apparent, some such series of phe- 
nomena as those just recorded are always sure to ensue when a covered-eyed Medusa is 
mutilated in the way describedf. But this kind of mutilation, besides producing such 
marked effects on the rate of the rhythm, also produces an effect in impairing the 
regularity of the rhythm. In some specimens the latter effect is more marked than it 
* In some cases this effect is obliterated by that of shock, the swimming-motions ceasing altogether for a 
time, or becoming more feeble. 
t I have not tried any experiments with the naked-eyed Medusse in this connexion. 
