"If the exposure to sea-water has only lasted two minutes, a similar series of phenomena is presented, except that the spontaneous twitching movements supervene in much less time than twenty minutes. But an exposure of one minute may determine a fatal result a few hours after the Medusa has been restored to fresh water.
"Contact with sea-water causes an opalescence and eventual disintegration of the tissues, which precisely resemble the effects of fresh water upon the marine Medusæ. When immersed in sea-water this Medusa floats upon the surface, owing to its smaller specific gravity.
"In diluted sea-water (fifty per cent.) the preliminary tonic spasms do not occur, but all the other phases are the same, though extended through a longer period. In sea-water still more diluted (1 in 4 or 6) there is a gradual loss of spontaneity, till all movement ceases, shortly after which irritability also disappears; manubrium and tentacles expanded. After an hour's continued exposure, intense rigor mortis slowly and progressively developes itself, so that at last the bell has shrivelled almost to nothing. An exposure of a few minutes to this strength places the animal past recovery when restored to fresh water. In still weaker mixtures (1 in 8, or 1 in 10) spontaneity persists for a long time; but the animal gradually becomes less and less energetic, till at last it will only move in a bout of feeble pulsations when 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 18, 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. Moreover, 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 manubrium 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 profound physiological 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 than is fresh water to the unmodified. There can be no doubt that the modification was gradual—probably brought about by the ancestors of the fresh-water Medusa penetrating higher and higher through the brackish waters of estuaries into the fresh water of rivers—and it would, I think, be hard to point to a more remarkable case of profound physiological modification in adaptation to changed conditions of life. If an animal so exceedingly intolerant of fresh water as is a marine jelly-fish may yet have all its tissues changed so as to adapt them to thrive in fresh water, and even die after an exposure of one minute to their ancestral element, assuredly we can see no reason why any animal in earth or sea or anywhere else may not in time become fitted to change its element."[38]
CHAPTER X.
STAR-FISH AND SEA-URCHINS.
Structure of Star-fish and Sea-Urchins.
We shall now proceed to consider in the organization of the Echinodermata a type of nervous system which is more highly developed than that of the Medusæ. In conducting this research, I was joined by my friend Professor J. Cossar Ewart, to whose unusual skill and untiring patience the anatomical part of the inquiry is due. But here, as formerly, I shall devote myself to the physiology of the subject, as it is not possible within the limits assigned to this volume to travel further into morphology than is necessary for the purpose of rendering the experiments intelligible. I shall therefore begin by seeking to give merely such a general idea of the structure of the Echinodermata as is necessary for this purpose.
