164 NOTES AND MEMORANDA. 
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 instantencously by the mere difference or 
the density of the two media.”! As regards the appearance pre- 
sented by Sarsia wheu 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 this con- 
clusion ; for not only is the contrast between the active swimming 
motions of the Sarsia in the sea water and their sudden cessa- 
tion of all motion in the fresh water very suggestive of instan- 
taneous death, but, a short time after immersion in the latter, 
their contractile tissues, as Professor Agassiz observed, became 
opalescent and whitish. Nevertheless, if he had taken the pre- 
caution of again transferring the Swrsia to sea water, he wonld 
have found that the previous exposure to fresh water had not 
had the effects 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 have never been 
removed from the sea water. Even after five minutes’ immer- 
sion in fresh water, Sarséa 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. 
Thinking it a curious circumstance that the mere absence of 
the few mineral substances that occur in sea water should exert 
1 ¢Mem. American Acad. Arts and Sciences,’ 1850, p. 229. 
2 The covered-eye Meduse survive a longer immersion than the naked- 
eyed—Aurelia aurita, for instance, requiring from a quarter to half an 
hour’s exposure before bemg placed beyond recovery. Moreover the 
cessation of spontaneity on the first immersion is not so sudden as it is in 
the case of the naked-eyed Meduse—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. 
