NOTES AND MEMORANDA. 
Meduse and Hydroid Polyps living in Fresh Water.— 
With reference to the interesting note on this subject in the 
October issue of this Journal I should like to make a few 
remarks. It is there said, ‘‘ Curiously enough, Mr. Romanes 
has found that marine Meduse are not so injuriously affected 
by brine as the Limnocodium is by sea water. The fact, how- 
ever, is less astonishing when we remember that the percentage 
of saline matter in solution in sea water is many hundred times 
what it is in average pond water, whilst the strongest brine has 
not a percentage of saline matter many times in excess of that 
of sea water” (p. 483). 
Now, this is certainly one way of looking at the matter, but 
I doubt whether it is the fairest way. The percentage of salt 
held in solution by “ average pond water” is really so small that 
it probably exerts no physiological influence of any kind on a 
Medusa, and, therefore, for purposes of physiological reasoning, 
ought not to be considered as a unit for comparison with higher 
percentages which do exert aphysiological influence. It ought 
rather to be considered as a vanishing quantity or zero, so far, 
at least, as the Medusz are concerned. Therefore, it seems to 
me that a fairer unit to takeis the one which I hadin my mind, 
although I did not explicitly state it, while writing my article to 
‘Nature’ of June 24th, to which you refer. This unit.is the 
percentage of salt which Limnocodium can tolerate for an in- 
definite time without manifesting a change in any of its phy- 
siological processes. What we want is a physiological, not a 
chemical, test of the percentage of salt that we are to consider 
as our unit, and this, it seems to me, can only be rendered by 
estimating the percentage of salt that first begins to exercise 
any perceptible influence upon the animal. This amount I found 
to be about + per cent. ‘Taking, therefore, ordinary sea water 
as having 3 per cent. of salt, and a saturated solution 36 per 
cent., we have as our proportions +: 3::3:36; or1:15:: 
15: 180; or1:15::1:12. This shows that, if we take the 
above as our unit, the estimated change of conditions which a 
freshwater Medusa undergoes on being transferred to the sea 
water is pretty nearly the same as that which a sea-water 
