As fresh water exerts a very deadly influence on the Medusæ, this seems the most appropriate place for describing its action. Such a description has already been given by Professor L. Agassiz, but it is erroneous. He writes, "Taking up in a spoonful of sea-water a fresh Sarsia in full activity, when swimming most energetically, and emptying it into a tumbler full of fresh water of the same temperature, the little animal will at once drop like a ball to the bottom of the glass and remain for ever motionless--killed instantaneously by the mere difference of the density of the two media."[34] As regards the appearance presented by Sarsia when subjected to "this little experiment," the account just quoted is partly correct; but Professor Agassiz must have been over-hasty in concluding that, because the animals seemed to be thus "killed instantaneously," such was really the case. Nothing, indeed, could be more natural than his conclusion; for not only is the contrast between the active swimming motions of the Sarsia in the sea-water and their cessation of all motion in the fresh water very suggestive of instantaneous death; but a short time after immersion in the latter their contractile tissues, as Professor Agassiz observed, become opalescent and whitish. Nevertheless, if he had taken the precaution of again transferring the Sarsia to sea-water, he would have found that the previous exposure to fresh water had not had the effect which he ascribes to it. After a variable time his specimens would have resumed their swimming motions; and although these might have had their vigour somewhat impaired, the animals would have continued to live for an indefinite time--in fact, quite as long as other specimens which had never been removed from the sea-water. Even after five minutes' immersion in fresh water, Sarsia will revive feebly on being again restored to sea-water, although it may be two or three hours before they do so; they may then, however, live as long as other specimens. In many cases Sarsia will revive even after ten minutes' exposure; but the time required for recovery is then very long, and the subsequent pulsations are of an exceedingly feeble character. I never knew a specimen survive an exposure of fifteen minutes.[35] In not a few cases, after immersion in fresh water, the animal continues to pulsate feebly for some little time; and, in all cases, irritability of the contractile tissues persists for a little while after spontaneity has ceased. The opalescence above referred to principally affects the manubrium, tentacles, and margin of the nectocalyx. While in fresh water the manubrium and tentacles of Sarsia are strongly retracted. [34] "Mem. American Acad. Arts and Sciences," 1850, p. 229.