744 
ME, G. J. EOMANES ON THE LOCOMOTOE SYSTEM OE MEDUSAE. 
in the poisoned water for an hour, they may not move for one or two hours after their 
restoration to unpoisoned water. In consequence of such great variations occurring 
from these causes, I was not able to compare the action of one poison with that of 
another in respect of the time required for effects of poisoning to pass away. 
§ 12. Fresh Water . — As fresh water exerts a very deadly influence on the Medusae, 
this seems the most appropriate place for describing its action. Such a description has 
already been given by Professor L. Agassiz, but it is erroneous. He writes : — “ Taking 
up in a spoonful of sea-water a fresh Sarsia in full activity, when swimming most ener- 
getically, and emptying it into a tumbler full of fresh water of the same temperature, 
the little animal will at once drop like a ball to the bottom of the glass and remain for 
ever motionless — killed instantaneously by the mere difference of the density of the two 
media” *. As regards the appearance presented by Sarsia when subjected to “ this little 
experiment,” the account just quoted is partly correct ; but Professor Agassiz must have 
been over-hasty in concluding that, because the animals seemed to be thus “ killed 
instantaneously,” such was really the case. Nothing, indeed, could be more natural than 
his conclusion ; for not only is the contrast between the active swimming-motions of the 
Sarsia in the sea-water and their sudden cessation of all motion in the fresh water very 
suggestive of instantaneous death, but, a short time after immersion in the latter, their 
contractile tissues, as Professor Agassiz observed, become opalescent and whitish. 
Nevertheless, if he had taken the precaution of again transferring the Sarsia to sea- 
water, he would have found that the previous exposure to fresh water had not had the 
effect which he ascribes to it. After a variable time his specimens would have resumed 
their swimming-motions ; and although these might have had their vigour somewhat 
impaired, the animals would have continued to live for an indefinite time — in fact quite 
as long as other specimens which had never been removed from the sea-water. Even 
after five minutes’ immersion in fresh water, Sarsia will revive feebly on being again 
restored to sea-water, although it may be two or three hours before they do so ; they 
may then, however, live as long as other specimens. In many cases Sarsia will revive 
even after ten minutes’ exposure ; but the time required for recovery is then very long, 
and the subsequent pulsations are of an exceedingly feeble character. I never knew a 
specimen survive an exposure of fifteen minutes'! . In not a few cases, after immersion 
in fresh water, the animal continues to pulsate feebly for some little time ; and, in all 
cases, irritability of the contractile tissues persists for a little while after spontaneity has 
ceased. The opalescence above referred to principally affects the polypite, tentacles, 
and margin of the nectocalyx. While in fresh water the polypite and tentacles of Sarsia 
are strongly retracted. 
* Mem. American Acad. Arts and Sciences, 1850, page 229. 
t The covered-eyed Medusae survive a longer immersion than the naked-eyed — Aurelia aurita, for instance, 
requiring from a quarter to half an hour’s exposure before being placed beyond recovery. Moreover the ces- 
sation of spontaneity on the first immersion is not so sudden as it is in the case of the nakc d-eyed Medusae — the 
pulsations continuing for about five minutes, during -which time they become weaker and weaker in so gradual 
a manner that it is hard to tell exactly when they first cease. 
