252 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



longtime; but the aniinal gradually becomes less 

 and less energetic, till at last it will only move in 

 a bout of feeble pulsations v.-hen 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 IS, the Medusa will swim about for days. 



" It 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 ph3'siological 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 



