MR. G. J. ROMANES ON THE LOCOMOTOR SYSTEM OE MEDUSAE. 
679 
mechanical or other irritation always originated a fresh swimming-bout. Next only 
one very feeble contraction was given at long and irregular intervals — a contraction so 
feeble that it was restricted to the immediate vicinity of the lithocyst in which it 
originated. Soon after this stage irritability towards all kinds of stimuli entirely ceased, 
including even strong spirit dropped on the under surface of the animal when taken 
momentarily out of the water. All these stages thus described were passed through 
rapidly, the whole series occupying rather less than five minutes. On now leaving 
the specimen for ten minutes and then restoring it to its original water at 45°, all the 
above-mentioned stages were passed through in reverse order. The first faint mar- 
ginal contraction occurred four minutes after restoration to the warmer water. This 
contraction was confined to the immediate vicinity of the prepotent lithocyst, and all 
subsequent contractions continued to be so for the next three minutes, llhythm very 
slow. Contractions now began to penetrate round the margin, and in eight minutes 
from the restoration had gone all the way round, the rate of their rhythm meanwhile 
increasing. In two minutes more all the umbrella was contracting at the rate of 
15 per minute.” 
In another specimen, subjected to the same conditions, the rate of recovery was even 
more rapid, occupying only two minutes altogether ; but in every case the process of 
recovery is a gradual one, and differs only in the time it occupies in passing through 
the various stages. 
(c) Effects of freezing Medusae . — In conclusion I will describe some rather inter- 
esting experiments that consisted in freezing some specimens of Aurelia into a solid 
block of ice. Of course, as sea-Avater had to be employed, the cold required was very 
considerable ; but I succeeded in turning out the Medusae encased on all sides in 
a continuous block of frozen sea-water. By now immersing this block in warm 
water, I was able to release the contained specimens, which then presented a very extra- 
ordinary appearance. The thick and massive gelatinous bell of a Medusa is, as every one 
knows, chiefly composed of sea-water, which everywhere enters very intimately into the 
structure of the tissue. Now all this sea-water was, of course, frozen in situ-, so that 
the animals were everywhere and in all directions pierced through by an innumerable 
multitude of ice crystals, which formed a very beautiful meshwork pervading the whole 
substance of their transparent tissues. 
These experiments were made in order to ascertain whether the Medusae, after having 
been thus completely frozen, would survive on being again thawed out ; and, if so, whether 
the freezing process would exert any permanent influence on the rate of their rhythm. 
Now in all the cases the Medusae after having been thawed out presented a ragged appear- 
ance, which was due to the disintegrating effect exerted by the ice crystals while forming 
in the tissues; yet, notwithstanding this mechanical injury superimposed on the physio- 
logical effects of such extreme cold, all the Medusae recovered on being restored to sea- 
water of the normal temperature. The time occupied by the process of recovery varied 
in different individuals from a few minutes to half an hour or more ; and it was observable 
