252 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
long time; but the animal gradually becomes less 
and less energetic, till at last it wiil only move in 
a bout of feeble pulsations when irritated. In still 
weaker solutions (1 in 12, or 1 in 15) spontaneity 
continues for hours, and in solutions of from 1 in 15, 
or 1 in 18, the Medusa will swim about for days. 
“Tt will be seen from this account that the fresh- 
water Medusa is even more intolerant of sea-water 
than are the marine species of fresh water. More- 
over, the fresh-water Medusa is beyond all comparison 
more intolerant of sea-water than are the marine 
species of brine; for I have previously found 
that the marine species will survive many hours’ 
immersion in a saturated solution of salt. While 
in such a solution they are motionless, with manu- 
brium and tentacles relaxed, so resembling the 
fresh-water Medusa shortly after being immersed 
in a mixture of one part sea-water to five of fresh; 
but there is the great difference that, while this 
small amount of salt is very quickly fatal to the 
fresh-water species, the large addition of salt exerts 
no permanently deleterious influence on the marine 
species. 
“We have thus altogether a curious set of cross 
relations. It would appear that a much less pro- 
found physiological change would be required to 
transmute a sea-water jelly-fish into a jelly-fish 
adapted to inhabit brine, than would be required to 
enable it to inhabit fresh water. Yet the latter is 
the direction in which the modification has taken 
place, and taken place so completely that the sea- 
water is now more poisonous to the modified species 
