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 i)resented by 

 Tiaropsis indicans, I sliould say that the two chiefly 

 ditfer 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 maroin and radial 



o 



tubes, instead of with reference to the whole ex- 

 citable surface of the animal. 



"All marine Medusse 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-eved Medusa which is swimmincr 

 actively in sea-water is suddenly transferred to 

 fresh water, it will instantaneously collapse, become 

 motionless, and sink to the bottom of the containina 

 vessel. There it will remain motionless until it 

 dies ; but if it be again transferred to sea- water it 

 \7ill 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 

 Medusa?, it is much more probable that the fi-esh-water species 

 was derived from them tlian that they were derived from a 

 fresh -water ancestry. 



t For full account, see P^uL Trans., voL clxvii. pp. 744, 745. 



