240 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
ever, live as long as other specimens. In many 
cases Sarsize 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 manubrium, tentacles, and 
margin of the nectocalyx. While in fresh water 
the manubrium and tentacles of Sarsia are strongly 
retracted. 
Thinking it a curious circumstance that the mere 
absence of the few mineral substances which occur in 
sea-water should exert so profound and deadly an 
influence on the neuro-muscular tissues of the 
Medusz, I was led to try some further experiments 
to ascertain whether it is, as Agassiz affirms, to the 
mere difference in density between the fresh and 
the sea water, or to the absence of the particular 
mineral substances in question, that the deleterious 
influence of fresh water is to be ascribed. Although 
* The covered-eyed Medusz 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 cessation of spontaneity on the first 
immersion is not so sudden as it is in the case of the naked-eyed 
Medusze—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. 
