POISONS. 249 
and probably has a direct reference to the distribu- 
tion of the principal nerve-tracts. 
“On the whole, therefore, contrasting this case of 
localization with the closely parallel case presented by 
Tiaropsis indicans, I should say that the two chiefly 
differ in the fresh-water Medusa, even when unmuti- 
lated, not being able to localize so promptly or so 
certainly, and in the localization being only per- 
formed with reference to the margin and radial 
tubes, instead of with reference to the whole ex- 
citable surface of the animal. 
“All marine Medusze are very intolerant of fresh 
water, and, therefore, as the fresh-water species must 
presumably have had marine ancestors,* it seemed 
an interesting question to determine how far this 
species would prove tolerant of sea-water. For the 
sake of comparison, I shall first briefly describe 
the effects of fresh water upon the marine species.f 
If a naked-eyed Medusa which is swimming 
actively in sea-water is suddenly transferred to 
fresh water, it will instantaneously collapse, become 
motionless, and sink to the bottom of the containing 
vessel. There it will remain motionless until it 
dies; but if it be again transferred to sea-water it 
will recover, provided that its exposure to the fresh 
water has not been of too long duration. I have 
never known a naked-eyed Medusa survive an ex- 
posure of fifteen minutes; but they may survive 
* Looking to the enormous number of marine species of 
Medusz, it is much more probable that the fresh-water species 
wus derived from them than that they were derived from a 
fresh-water ancestry. 
t For full account, see Phil. Trans., vol. clxvii. pp. 744, 745. 
